Performance Monitoring of Remediation Technologies for Soil and Groundwater Contamination: Review

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ReCo. D.nologin techlt undntamlishedntaminntamintigateto enschemd themeterrounmajor component of soil and groundwater remediation projects. mance of the remediation technologies applied on meeting reme-Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.Based on monitoring data, the remediation technologies appliedto contaminated sites can be evaluated whether they perform asexpected to clean up the sites to meet site-specific remediationobjectives. The monitoring results are also crucial for deciding thetermination date of the remediation projects and the implementa-tion of contingency plans for further treatment. Another importantdiation objectives. The possible changes in contaminant zonesduring the remediation include redistribution of contaminants insubsurface, consequently creating a more extensive contaminantsource zone; change of microbial communities accordingly dis-rupting native natural attenuation capability in subsurface; andplugging of soil matrix which prevents further contaminantsource reduction by the delivery systems of the applied remedia-tion technologies ITRC 2004. For instance, thermal treatmentand surfactant/cosovlent flushing technologies may mobilizedense nonaqueous phase liquids DNAPLs beyond the treatmentzone. The chemicals injected during in situ chemical oxidationISCO may suppress native microorganisms in subsurface andsupport other microbial communities. Furthermore, gas evolvedfrom ISCO using hydrogen peroxide and biofilm formed duringbioremediation may lead to plugging of the soil matrix. Innova-tive technologies, such as monitored natural attenuation MNA,phytoremediation, and permeable reactive barriers PRBs requiremore extensive performance monitoring than the more acceptedremediation approaches Gavaskar et al. 2000; USEPA 2000,2004b.Performance monitoring of the remediation technologies is notonly a measure of technical success of the technologies to con-form state regulation but also involves the cost effectiveness ofthe technologies on soil and groundwater remediation. In assess-ing the performance of the remediation technologies, there aretwo primary criteria: remediation effectiveness and efficiency. Ef-fectiveness refers to the capability of the remediation technolo-gies to achieve remediation objectives at contaminated sites1Postdoctoral Fellow, Dept. of Civil, Architectural, and EnvironmentalEngineering, ECJ 8.210, Univ. of Texas at Austin, 301 E Dean Keeton,Austin, TX 78705.2Engineer Director and Adjunct Professor of EnvironmentalEngineering, Univ. of NebraskaLincoln, U.S. Environmental ProtectionAgency, P.O. Box 172141, Kansas City, KS 66117 correspondingauthor. E-mail: surampalli.rao@epa.gov3Professor, Water, Earth, and Environment Center, National Instituteof the Scientific Research, Univ. of Quebec, 490, de la Couronne, QubecQubec, Canada G1K 9A9.4Associate Professor, Dept. of Civil Engineering, Hong Kong Univ. ofScience and Technology, Clear Water Bay, Kowloon, Hong Kong.5Ph.D. Student, Water, Earth, and Environment Center, NationalInstitute of the Scientific Research, Univ. of Quebec, 490, de laCouronne, Qubec Qubec, Canada G1K 9A9.Note. Discussion open until December 1, 2007. Separate discussionsmust be submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on September 13, 2006; approved on October 20, 2006. Thispaper is part of the Practice Periodical of Hazardous, Toxic, andRadioactive Waste Management, Vol. 11, No. 3, July 1, 2007. ASCE,ISSN 1090-025X/2007/3-132157/$25.00.132 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007Performance Monitoring ofSoil and GroundwaterKeith C. K. Lai1; R. Y. Surampalli2; RAbstract: Performance monitoring of applied remediation techmeasurement of site parameters to evaluate whether the remediatioof remediation projects. Performance monitoring can be a difficuobjectives, such as a reduction in mass discharge rate from a comonitored is highly dependent on the remediation objectives estabnant concentration in soil, soil vapor, and groundwater within a coof a contaminant zone is the prime remediation objective, while coshould be the main monitoring parameter if the objective is to miis a unique monitoring parameter for in situ thermal remediationoxidant concentration is the monitoring parameter only for in situcriteria for establishing the site-specific remediation objectives anwhich help avoid waste of efforts to collect unnecessary site paraDOI: 10.1061/ASCE1090-025X200711:3132CE Database subject headings: Contamination; Efficiency; Gevaluation; Soil sampling; Water sampling.IntroductionMonitoring of the performance of remediation technologies is aPract. Period. Hazard. Toxic Radioact. Wmediation Technologies forntamination: ReviewTyagi3; Irene M. C. Lo4; and S. Yan5es is an important part of site remediation. It involves periodicnologies perform as expected or to determine the termination dateertaking if there are no well-defined and measurable remediationinant source. Besides, the selection of the site parameters to beand the remediation technologies applied. For instance, contami-ant zone should be monitored if reduction of the mass or volumeant concentration in groundwater just outside a contaminant zonethe contaminant migration. Furthermore, subsurface temperatureure the proper operation of the technology, whereas groundwaterical oxidation technologies. This paper mainly covers the generalkey monitoring parameters for various remediation technologies,s during the performance monitoring.d-water pollution; Monitoring; Parameters; Remedial action; Sitepoint is that performance monitoring can help site managers orengineers to examine whether the changes in contaminant zonesresulting from remediation activities can deteriorate the perfor-aste Manage. 2007.11:132-157.Table 1. Performance Metrics, Monitoring Methods, and Monitoring Parameters for the Performance Monitoring of the Remediation Technologies forDNAPL Source Zone TreatmentMponcentrnt conminantreakdooncentracersnt cononcentrant cont connt convitiesacers rnts retaseriesDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.ITRC 2004. For example, if the remediation objective is areduction in hexavalent chromium CrVI mobility, the effective-ness is defined whether the remediation technologies can haltthe continuous CrVI migration. Normally, assessment of theremediation effectiveness involves quantifying reductions in con-taminant mass, concentration, mobility, and/or toxicity by theremediation technologies NRC 1997.Efficiency of the remediation technologies refers to the opti-mization of the time, energy, and costs expended toward theachievement of remediation effectiveness. It is typically assessedby comparing system operating parameters to the relevant designspecifications. For example, ISCO systems are usually effective inreducing contaminant concentrations at the beginning stage butthe rate of reduction becomes lower and less efficient succes-sively with each injection in terms of time, energy, and moneyexpended. Thus optimization of the operating parameters to main-tain good remediation efficiency is required ITRC 2004.To assess the remediation effectiveness and efficiency, perfor-mance monitoring is conducted specifically to collect data fromcontaminated sites. By definition, performance monitoring in-volves periodic measurement of physical and/or chemical siteparameters to evaluate whether the remediation technologiesperform as expected. Generally, contaminant concentrations ingroundwater, soil, and soil vapor for volatile organic contami-nants are the parameters usually measured during the perfor-mance monitoring. However, each remediation technology has itsown specific measuring parameters. For instance, monitoring ofthe concentration of biological nutrients in subsurface is requiredin assessing the performance of in situ bioremediation. Normally,the selection of the monitoring parameters is dependent on theremediation objectives, performance metrics, and types of reme-diation technologies applied. Performance metrics are the envi-ronmental conditions and monitoring parameters measured toPerformancemetricsMonitoringmethodsAdsorbed-phase reduction Soil coring Soil contaminant cDissolved-phase reduction Groundwater sampling Aqueous contaminaSource mass extracted Effluent sampling Volumes and contaSource mass destroyed Groundwater sampling Concentration of bSource mass remaining Soil coring Soil contaminant cTracer tests Concentration of trGroundwater sampling Aqueous contaminaMobility reduction Soil coring Soil contaminant cProduct gauging NAPL thicknessToxicity reduction Soil coring Adsorbed contaminGroundwater sampling Aqueous contaminaMass flux/massdischarge reductionWell transect sampling Aqueous contaminaHydraulic conductiHydraulic gradientsFlux meter transects Mass of resident trMass of contaminaIntegral pumping tests ConcentrationtimePRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wevaluate remediation progress or to confirm attainment of reme-diation objectives ITRC 2004. Table 1 shows common perfor-mance metrics, monitoring methods, and monitoring parametersrequired for the monitoring of the performance of the remediationtechnologies for DNAPL source zone treatment.In light of the importance of the performance monitoring tosoil and groundwater remediation, and the difficulty of the selec-tion of appropriate and representative monitoring parameters, thischapter aims to introduce the remediation objectives, performancemetrics, and monitoring parameters generally involved during theperformance assessment of remediation technologies. Groundwa-ter and soil sampling systems commonly applied are also in-cluded. In the final part, key monitoring parameters of eachremediation technology, which includes pump-and-treat P&Tsystems, soil vapor extraction SVE, air sparging AS,surfactant/cosolvent flushing, in situ thermal remediation, ISCO,in situ bioremediation, MNA, PRBs and phytoremediation, arespecifically mentioned Bedient et al. 1999; USEPA 2004a; Loet al. 2006b.Remediation ObjectivesMeasuring the effectiveness of the remediation technologies is adifficult undertaking if clear remediation objectives are not speci-fied. Remediation objectives can be quantitative or qualitative innature but should at least be measurable so that the remediationprogress toward the remediation objectives can be monitored ob-jectively. In most contaminated sites, remediation objectives areusually designed based on the overriding goals of most state andfederal regulatory programs, namely protection of human healthand natural environment from contaminated sites. Generally, theonitoringarametersDerivedparametersations Sorbed concentrationcentrations Plume extentChange in NAPL compositionconcentrations of extracted fluids Contaminant mass removedwn products e.g., chloride Mass of DNAPL destroyed in situations and visual observation DNAPL distributionSorbed massNAPL volumeSaturationcentrations Aqueous concentrationations and visual observation NAPL presence and saturationNAPL thicknessncentrations DNAPL compositioncentrations Concentrations of toxic parameterscentrations Darcy fluxMass fluxMass dischargeetained on sorbents Darcy fluxined on sorbents Mass fluxMass dischargesampling from extraction well Mass dischargeAverage plume concentrationRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 133aste Manage. 2007.11:132-157.remediation objectives fall into three categories: short-term,intermediate-term, and long-term remediation objectives.nant concentrations in soil, groundwater and/or soil vapor. How-ever, these performance metrics of reduction in contaminant con-Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.Short-term remediation objectives typically involve the alle-viation of the immediate risks from contaminated sites to humansor natural resources through the prevention of further expansionof contaminant source zones. Therefore, short-term objectivescustomarily take account of the control of contaminant mobilityand mitigation of contaminant migration. Intermediate-term reme-diation objectives are established to guide the remediation activitywhen complete removal of a contaminant source in one aggres-sive remedial effort is not feasible and the contaminants remain-ing in contaminated sites still exceed the regulatory standardsNRC 1994. Intermediate-term objectives may include: 1depletion of a contaminant source adequately to allow for naturalattenuation; 2 reduction of dissolved phase contaminant concen-trations outside a contaminant source zone; 3 decrease in massdischarge rate or flux from a contaminant source; 4 reduction ofthe mass or volume of a contaminant source to a smaller extent;and 5 prevention of the migration of remediation fluids beyonda treatment zone. According to the information provided by U.S.Environmental Protection Agency USEPA, application of theinformation obtained from the interim remediation actions forlong-term risk management can accelerate the risk reduction fromcontaminant sites USEPA 1997. It may take a year or severalyears for contaminated sites to meet the intermediate-term objec-tives. Besides, long-term monitoring is required to ensure that theinterim treatment levels achieved are sustainable and are not sub-ject to a rebound in contaminant concentrations in groundwaterwhen posttreatment equilibrium is established in aquifers. Long-term remediation objectives focus on the compliance with theregulatory treatment standards applicable to all contaminatedmedia i.e., groundwater, soil, and soil vapor at the sites.Achievement of the regulatory standards leads to the terminationof remediation activities; but in analogy to intermediate-term ob-jectives, long-term monitoring is also required to ensure that thecompliance is sustainable. If long-term remediation objectives arenot met or achievable, a contingency plan is needed to implementand a secondary treatment may be required.Performance Metrics and Monitoring ParametersEach remediation objective should have its own set of perfor-mance metrics for assessing and monitoring the performance ofthe remediation technologies applied toward this objective. Thesemetrics are neither equivalent to nor interchangeable with oneanother. They range from qualitative indicators of remediationprogress to quantitative measures of specific factors followingremediation see Table 1. Currently, there are three main catego-ries of performance metrics: 1 qualitative estimation of thetreatment progress of a contaminant source; 2 quantitative esti-mation of contaminant source mass reduction; and 3 quantita-tive evaluation of the effect of contaminant source treatment onfactors, such as contaminant toxicity, mobility, and plumestrength.Qualitative Estimation of the Treatment Progress of aContaminant SourceOne of the common remediation objectives is the reduction of thesite contaminant concentrations below certain levels or to meetthe regulatory standards. The remediation activity is preliminaryviewed as success if there is a decrease in the average contami-134 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wcentrations in these subsurface media can only qualitativelyillustrate that progress is being made toward the contaminantsource treatment. They cannot quantify the degree of the reme-diation progress.Decrease in Contaminant Concentrations in SoilDecrease in the soil contaminant concentration can be monitoredthrough soil sample collection followed by chemical analyses ofcontaminant concentrations. Fraction of organic carbon foc ofthe soil samples is also the parameter being monitored if the siteis contaminated by organic contaminants Bedient et al. 1999.Pre- and posttreatment soil data are required to examine thechange of the soil concentration. To obtain reliable soil concen-tration data requires the collection of a large number of soilsamples to address site heterogeneities and temporal and spatialvariability of the contaminant concentrations. Soil samplingmethods may sometimes underestimate the actual soil concentra-tions because contaminated soils and the clean or less contami-nated soils at the top and bottom of the contaminated soils maycollect together during sampling, and then are analyzed as awhole sample. Another important point is that soil sampling isbasically destructive. When a sample is obtained from a location,subsequent sampling must be made in a new location that may ormay not contain contaminants, thereby resulting in an error inestimating the reduction in soil contaminant concentrations.Decrease in Contaminant Concentrations in GroundwaterMonitoring of the decrease in contaminant concentrations ingroundwater requires sampling and chemical analyses of contami-nated groundwater. Groundwater samples are collected via anetwork of monitoring wells or multilevel samplers installedupgradient, within, and downgradient, and in the periphery of thecontaminant plume. Before collecting groundwater samples, purg-ing of monitoring wells or multilevel samplers is required to re-move stagnant water inside the wells or the samplers, therebyensuring that the groundwater samples collected are representa-tive of the groundwater condition in aquifers. However, excessivepurging of the wells and the samplers may dilute or increase thecontaminant concentration at the sampling points because of driv-ing the less contaminated or more contaminated groundwaternearby to the sampling points. In analogy to the soil data, therepresentativeness of the groundwater data is prone to be affectedby the temporal and spatial variability of the groundwater con-taminant concentrations. Besides, it is also subject to the slowrelease or dissolution of contaminants into groundwater, whichmay lead to a rebound of groundwater contaminant concentra-tions after groundwater sampling.Decrease in Contaminant Concentrations in Soil VaporMonitoring of the reduction in the contaminant concentrations insoil vapor is only applicable when treating volatile contaminantsfrom the vadose zone. Contaminant concentrations in extractedvapor steam and the vapor remaining in subsurface are monitored.During the remediation, soil vapor concentration in subsurfacemust be monitored over the area of a whole soil gas plume tocheck whether the remediation is addressing the entire contami-nant plume. Similar to the soil and groundwater data, influencefrom site heterogeneities, spatial, and temporal variability of con-taminant concentrations on the soil vapor data should be takeninto account, and a rebound in soil vapor concentrations may alsooccur.E WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Quantitative Estimation of Mass Reduction of aContaminant Sourceeter for the estimation of the mass of chlorinated contaminantsDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.For the sake of estimating the reduction of contaminant mass inthe sites during the remediation, three types of performance met-rics, including contaminant mass extracted from subsurface, con-taminant mass destroyed, and contaminant mass remaining insubsurface after each treatment, must be monitored. The informa-tion is then compared to the baseline information obtained beforethe remediation i.e., total contaminant mass to compute themass removal percentage or the fraction of contaminant massremaining.Contaminant Mass Extracted from SubsurfaceThis performance metric is applied to contaminated sites in whichthe remediation technologies applied involve extraction of con-taminated groundwater and/or soil vapor for ex situ treatment.These kinds of technologies include P&T systems, SVE, AS, insitu thermal remediation, and surfactant/cosolvent flushing. Bymeasuring the parameters of contaminant concentrations in theextracted groundwater and/or soil vapor, estimation of the con-taminant mass extracted can be readily made by integrating a plotof the contaminant concentrations against the cumulative volumeof extracted groundwater or soil vapor. The area under the curverepresents the mass of contaminants removed from the contami-nant plume through the extraction of contaminated groundwaterand soil vapor.Contaminant Mass Destroyed in SubsurfaceThis performance metric focuses on the amount of contaminantmass destroyed in situ by the remediation technologies, such asMNA, ISCO, bioremediation, and PRB, through either oxidation,reduction, or biodegradation processes. Generally, estimation ofthe contaminant mass destroyed is much more complicated thanthe estimation of contaminant mass extracted, in which the formeris usually determined based on the measurement of specific indi-cating parameters in groundwater. For example, if chemical oxi-dant e.g., potassium permanganate, KMnO4 is injected intoaquifers for the remediation of dissolved chlorinated solvent e.g.,trichloroethylene, or TCE, the amount of TCE destroyed byKMnO4 in aquifers can be calculated by monitoring the parameterof chloride concentration before and after the injection. This isbecause chloride ions are released when TCE is oxidized byKMnO4 to form carbon dioxide Eq. 1. According to thefollowing stoichiometric equation, it is known that each gramincrease in chloride ions in groundwater is ascribed to the degra-dation of 1.23 gram of TCE in aquifers2KMnO4 + C2HCl3 2CO2 + 2MnO2 + 3Cl + H+ + 2K+ 1In addition to chloride ions, a ratio of carbon isotopes of contami-nants is another indicating parameter for examining the mass ofchlorinated solvents being destroyed Song et al. 2002. The prin-ciple is that molecules of chlorinated solvents or organic contami-nants are composed of both light 12C and heavy carbon 13Catoms. However, there is slight different in the oxidation, reduc-tion, and biodegradation rates between 12C and 13C, in which theformer tends to be transformed more quickly, thereby resulting inenrichment of 13C in residual reactants and 12C in products. Bymeasuring the ratio of 12C/ 13C of contaminants before and afterthe treatment, the amount of contaminant destroyed can then becalculated using Rayleigh model Dayan et al. 1999; VanStoneet al. 2004. In a similar manner, the ratio of chlorine isotope37Cl and 35Cl of contaminants is also another indicating param-PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wdestroyed Sturchio et al. 1998; Heraty et al. 1999.Contaminant Mass Remaining in SubsurfaceEstimation of contaminant mass remaining in subsurface after thetreatment is of overriding importance if the remediation objectiveis specifically linked to the fraction of contaminant mass remain-ing. Contaminant mass remaining in the soil matrix can be calcu-lated by multiplying the contaminant concentrations in soilsamples by the bulk density of soil and the volume of the con-taminant zone. Mass of contaminants remaining in groundwatercan be estimated from the contaminant concentrations in the col-lected groundwater samples. With the application of the relevanthydrologic and hydrogeologic data, such as porosity and ground-water velocity, the contaminant mass in groundwater is computedfrom a numerical model Garabedian et al. 1991; Lai et al.2006b. To estimate the mass of DNAPLs remaining in subsur-face, a partitioning interwell tracer test PITT can be applied,which involves the injection of conservative and nonconservativetracers into one or more wells, and the subsequent recovery of thetracers from the nearby extraction wells. The conservative tracerscan pass through the DNAPLs source zone freely without retar-dation, whereas the transport of nonconservative tracers is notice-ably retarded by the interaction with the DNAPLs. The tracerresponses observed at the monitoring wells or extraction wellscan then be used to estimate the average DNAPL saturation andthe total volume of DNAPLs remaining in subsurface Jin 1995;Jin et al. 1995; Dwarakanath 1997.Quantitative Evaluation of the Remediation Effects onthe Toxicity, Mobility, and Mass Flux of a ContaminantSourceDecrease in the Toxicity of a Contaminant SourceIn the situation where the sites are contaminated by more than onetype of contaminants, monitoring of the overall reduction of thetotal contaminant mass may not be a suitable performance metric.This is because the reduction of the contaminant mass may be dueto the removal of less toxic contaminants, while highly toxiccontaminants still remain in the sites. Hence, the remediation ob-jective should target the concentration of the highly toxic con-taminants so as to lower the overall toxicity of the contaminantsource. Virtually, even though the sites still contain high concen-tration of less toxic contaminants, the remediation objective maystill be met if the concentration of the highly toxic contaminantsis substantially reduced. To monitor the decrease in the toxicity,the concentration levels of highly toxic contaminants should bethe monitoring parameter to be measured before and after thetreatment.Decrease in the Mobility of a Contaminant SourceAs previously mentioned, short-term remediation objectives cus-tomarily focus on the mitigation of the further spread of thecontaminant zone. To accomplish this goal, remediation technolo-gies applied should be able to deplete the contaminant sourcesufficiently to reduce the concentration level to a point that thecontaminant source is relatively stable. For example, DNAPLsaturation, which can be determined by analyzing the totalDNAPL concentration in soil matrix adsorbed, dissolved, vapor,and NAPL phase or by PITT, is a good measuring parameterRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 135aste Manage. 2007.11:132-157.lers loDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.indicating the mobility of DNAPL source. When the DNAPLsaturation drops to a residual saturation level, DNAPL sourcemobility reduces considerably.Decrease in the Mass Flux of a Contaminant SourceFrom a risk perspective, the most important characteristic of thecontaminant source is the mass, which is able to contribute to alarger dissolved phase plume. It is called plume load and isdefined as the discharge rate of contaminant mass from thecontaminant source to surrounding groundwater. By integrating alocalized mass flux, which is defined as the rate of dissolvedcontaminant in groundwater flowing per unit area of the planeperpendicular to the groundwater flow direction, total mass dis-charge rate across the entire source zone can be obtained. Signifi-cant reduction in the contaminant mass may not correspondinglylead to substantial decrease in plume load since it is also a func-tion of contaminant distribution and hydrodynamic structure ofsubsurface. Sometimes termination of remediation activities canbe considered if the total mass discharge rate from the sourcezone is smaller than the natural attenuation capacity of aquifersRao et al. 2001.Currently, there are four methods available for the estimationof the total mass discharge rate or the mass flux across the planeperpendicular to the groundwater flow direction. The first methodis related to a continuous pumping from extraction wells. Thismethod involves capturing of the whole contaminant plume byone or more extraction wells pumping at a continuous rate. Byknowing the pumping rate and measuring the steady contaminantconcentration in the extracted groundwater, the total dischargerate of the contaminant mass can be calculated directly. This ap-proach will be more cost effective if there is a hydraulic contain-ment system surrounding the contaminant plume.Another estimation method involves integrated pump testsbased on short-term and active pumping of wells located in atransect across the contaminant plume Fig. 1a. This techniquewas developed by the researchers at the University of Tbingen,Germany, for calculating the natural attenuation rate constant at aformer gasworks site Bockelmann et al. 2001; Teutsch et al.2001. Well positions, pumping rates, and pumping times are op-timized to allow the well capture zones to cover the entiregroundwater flow downstream of the contaminant site. During theoperation, the wells are pumped and contaminant concentrationsin the effluents of the wells are measured as a function of timeuntil the entire mass discharge at the transect location is known orassumed to be extracted.The most common method used to estimate the mass flux isthrough the groundwater sampling from multilevel samplers. Un-like the extraction wells, multilevel samplers allow collection ofFig. 1. a Extraction wells; b multilevel samp136 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wcontaminated groundwater from numerous closely spaced sam-pling points along a transect of wells intersecting the plume andaligned perpendicular to the groundwater flow direction Fig.1b. Measurement of the contaminant concentrations in thegroundwater samples collected from the transect of multilevelsamplers gives the geostatistical average concentration. By know-ing the velocity of groundwater flowing through the transect ofmultilevel samplers, the mass flux can be computed.Passive borehole flux meter is an innovative technology fordirect in situ measurement of both cumulative groundwater andcontaminant fluxes. During the contaminant mass flux estimation,the meter is inserted into a well or borehole so that it interceptsgroundwater flow. The hydrophobic and hydrophilic permeablesorbents inside the permeable unit of the meter can retain dis-solved organic and inorganic contaminants in groundwater inter-cepted by the meter. Moreover, the permeable part of the meteralso contains a known amount of soluble tracers, which areleached from the meter at a rate proportional to the groundwaterflux. After exposing the flux meter to the contaminated ground-water for a period of time, the meter is then removed, and themass of contaminants sorbed by the sorbents and the tracers re-maining are quantified. The contaminant mass sorbed can be usedto calculate the time-average contaminant mass flux, whereas themass of the residual tracers remaining can be used to computecumulative groundwater flux ESTCP 2003.Groundwater Sampling for PerformanceMonitoringThe focus of this section is primarily on the sampling of ground-water for performance monitoring since groundwater is the expo-sure pathway of significant concern. A monitoring networkinstalled at contaminated sites for the groundwater samplingshould be capable of providing data to demonstrate an attainmentof all remediation objectives. Specification of the monitoring net-work design should be based on all available information con-cerning the processes and factors expected to control contaminantdistribution. For instance, original contaminant source distribu-tion, site geology, and hydrology can result in a spatial and tem-poral variability of plume shapes, which consequently affects theselection of monitoring locations and frequencies, and necessi-tates continual reevaluation of the monitoring network. In addi-tion, the density of the sampling points in the monitoring networkdepends on the spatial scale of horizontal and vertical variabilityof contaminant distribution in subsurface. The distance betweenwell transects is a function of the changes in contaminant concen-cated in a transect across the contaminant plumeE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.tration along the plume, and the location of the contaminantsource and distal portion of the plume. Besides, subsurfacestratigraphy, the vertical component of hydraulic gradients, andvertical contaminant distribution are the general factors affectingthe elevation of sampling intervals. Ideally, the elevation of thesampling intervals should allow groundwater sampling from dif-ferent stratigraphic layers, from the core of the contaminantplume, and above and below the plume USEPA 2004b. Cur-rently, transect-based monitoring network are widely applied forthe performance monitoring because this type of the monitoringnetwork can give a better delineation of contaminant distributionand its spatial and temporal variability Bockelmann et al. 2001;Kao and Wang 2001; Lai et al. 2006c. The transect approach canalso help to locate groundwater flow lines and contaminant mi-gration paths. Fig. 2 illustrates the plan and the side and frontviews of a series of well transects installed over a contaminantplume.Fig. 2. a Plan; b side; and c front views of the monitoring netwof natural attenuation adapted from USEPA 2004bPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. WDetermination of Groundwater Sampling LocationsGroundwater sampling locations for monitoring the performanceof the remediation technologies generally include: 1 contami-nant source area; 2 transmissive zone with the highest contami-nant concentration or hydraulic conductivity; 3 distal or fringeportions of the plume; 4 plume boundaries; and 5 recalcitrantzone see Fig. 2. Transects of monitoring wells installed withinand immediately downgradient of the contaminant source areascan provide data showing if there is any further releases of con-taminants to groundwater, and enable estimation of the reductionof contaminant concentration over time. In the case in which con-tainment technologies have already been applied to contain thesource areas, these well transects can provide data illustratingwhether there is an increase in contaminant concentration in thedowngradient groundwater caused by the failure of slurry wall,stalled over a contaminant plume for monitoring of the performanceork inRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 137aste Manage. 2007.11:132-157.grout curtain, or sheet piling, or by a rise in water table bringingadditional contaminants from vadose zone to aquifers.Generally, more frequent sampling is needed at the early stage ofthe remediation processes, whereas less frequent sampling is re-Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.Groundwater sampling from transmissive zones with the high-est contaminant concentration or hydraulic conductivity is ofparamount importance. This is because any change in conditionsin these zones, such as an increase in contaminant mass releasedfrom contaminant source areas or groundwater velocity, maybring a relatively rapid impact to downgradient receptors. In re-sponse to the remedial action, the distal or fringe portion of thecontaminant plume is the plume area where reduction of contami-nant concentration to a level required by remediation objectivesmay be attained most rapidly. Moreover, observation of continu-ous increase in contaminant concentration in these zones can in-dicate a possible plume expansion. Sampling points installedaround plume boundaries include the points placed at the sidegra-dient and downgradient locations of the plume, around verticalplume boundaries i.e., above and below the plume, and the lo-cation between plume boundaries and possible receptors. Moni-toring data obtained from these sampling points can show anyunacceptable plume expansion and change in groundwater flowdirection. In addition, monitoring wells should also be installed ina recalcitrant zone in which contaminant reduction rates appear tobe lower than the rate required to meet remediation objectives.This is because attainment of remediation concentration levels inthese areas within accepted time frames may be impeded by siteconditions e.g., presence of previously undetected contaminantsource. Therefore, monitoring data from these areas is needed todetermine whether additional remedial action is required.Determination of Screen Lengths of Monitoring WellsThe length of the well screen should be sized to sample the inter-val of interest, which is determined by subsurface stratigraphyand contaminant loadings in subsurface. Well screens can matchto stratigraphic intervals if the intervals are relatively small andcontaminant concentrations are similar throughout the vertical ex-tent of the interval intersected by the well screen. For example, arelatively homogeneous sand layer with 1 m of thickness anduniform contaminant concentration can be sampled by a 1-m-longscreen to intersect the whole interval. Intervals with significantlydifferent hydraulic conductivities can be sampled by the screenswith specific screen lengths. In addition to the stratigraphic inter-vals, well screens should also be sized to match with the contami-nant loadings in subsurface. They should be sized to sample themost contaminated part of the plume. If the plume is highly het-erogeneous i.e., highly spatial variation in contaminant concen-tration, application of long screens for groundwater samplingshould be avoided. Because of mixing with groundwater possess-ing low contaminant concentration, the use of the long screensmay erroneously result in lower contaminant concentration.Determination of Groundwater Sampling FrequenciesIn designing the sampling frequency for the performance moni-toring program, it should be carefully checked whether the de-signed sampling frequency can Give timely warning of impact to receptors; Detect contaminant releases to groundwater warning possibleplume expansion; Detect changes in plume size; Illustrate temporal variability of monitoring data; and Provide sufficient data to reliably evaluate remediationprogress toward the remediation objectives.138 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wquired as the remediation systems stabilize at close-to-optimumconditions ITRC 2004. Factors customarily taken into accountfor designing sampling frequency include: 1 the possiblecontaminant travel time to receptors; 2 cyclical changes in con-taminant concentrations and plume boundaries; 3 relevance ofmonitoring parameters; and 4 stability of the monitoring data.Short groundwater travel time from downgradient plume bound-aries to receptors implies the requirement of more frequent sam-pling. Moreover, groundwater sampling should be more frequentfor the wells near and downgradient of the plume boundaries thanthose located in upgradient and core of the plume.Climatic changes in recharge rates and groundwater flow char-acteristics during wet and dry seasons may lead to noticeablycyclical trends in contaminant concentrations and plume bound-aries. This is because the climatic changes may cause seasonalinput of contaminants from vadose zones to aquifers. Under thesecircumstances, a more frequent groundwater sampling is required,which can be designed based on the historical variability ingroundwater level at the sites and the recorded climatic variabilityi.e., drought frequencies or periods of above average rainfall.Therefore, sufficient data can be obtained during the cyclicalchanges to prevent unmonitored expansion of the plume, and col-lection of unnecessary data outside the period of cyclical changescan be avoided. Generally, monitoring data gathered over severalyears are needed to evaluate the cyclical changes and determinethe suitable sampling frequency for capturing the changes.Measuring frequency of monitoring parameters is highly de-pendent on their significance on assessment of the performance ofthe applied remediation technologies. If the parameters are notexpected to significantly influence the evaluation of remedy per-formance, the relevant monitoring frequency can be substantiallyreduced. However, the entire suite of contaminant concentrations,site geochemical parameters, and hydrogeological parametersshould be measured at all sampling points during the remedialactions if there is no specific reason for excluding these param-eters. Besides, observation of a stable data trend over a period ofseveral years allows reduction in the monitoring frequency of therelevant parameters. Furthermore, if two or more wells samplingthe same zone are located closely together and consistently pro-duce a similar trend of data, reduction in the sampling frequencyof these wells can be considered. However, if sudden change inthese parameters is observed, an increase in the monitoring fre-quency may be required to obtain enough information for under-standing the changes and provide earlier warning of furtherchanges.Soil Sampling for Performance MonitoringThe objective of soil sampling is to obtain information about aparticular soil, such as contaminant concentration, thereby evalu-ating whether the applied soil remediation effort has been suc-cessful. In comparison to groundwater sampling, soil sampling isgenerally more expensive. However, if the boundaries of a con-taminant zone and soil contaminant concentration can be pre-cisely established and evaluated, soil sampling can save vastamount of remediation works. Since detailed introduction of soilcoring techniques has been widely covered elsewhere Tan 1996;ASCE 2000; Nielsen 2006, the focus of this section is on thedesign of remedial verification soil sampling plans, including asimple random sampling plan, a systematic or grid sampling plan,E WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.ampliDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.a stratified sampling plan, and a composite sampling planUSEPA 1989.Design of Soil Sampling PlansSimple Random Sampling PlanA random sampling plan is a probability sampling in which prob-ability or confidence statements about the sampling results can beobtained. The major feature of the simple random sampling is thatselection of particular sampling locations at any stage of samplingmust not be influenced by other sampling locations that havealready been selected Fig. 3a. Determination of sampling lo-cations for the random sampling plan requires a detailed map of acontaminated site containing well-defined site boundaries, sampleareas, coordinate system, and all important features e.g., trees,boulders, or other landmarks that are useful in identifying sam-pling locations in the site. The random sampling locations withina sample area are generated by creating a series of random coor-dinates X ,Y through the following steps i.e., Steps 1 to 4. It isimportant to note that the determination of the random samplinglocations should be performed prior to a field visit so as to avoidall personal bias relating to landscape position USEPA 1989.Step 1: Generate a set of coordinates X ,Y using Eqs. 2 and3X = Xmin + Xmax Xmin RND 2Y = Ymin + Ymax Ymin RND 3where Xmax, Xmin, Ymax, and Ymincoordinates of arectangle on the map that can completely cover thesample area; and RND is an unused random numberbetween 0 and 1 in a sequence of random number.Step 2: If generated X ,Y-coordinate is outside the sample area,return to step 1 to generate another random coordinate;otherwise go to Step 3.Step 3: Round the generated X ,Y-coordinate to the nearest unitthat can be located easily in the field. Then set thiscoordinate as Xi and Yi.Fig. 3. The maps showing a random sampling plan; b systematic ssite adapted from USEPA 1989PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. WStep 4: Repeat Steps 1 to 3 to generate the next random coor-dinate i.e., Xi+1, Yi+1.Systematic or Grid Sampling PlanA systematic sampling plan involves sampling of soil from thepoints that are at regular distance from each other. This samplingplan can guarantee the complete coverage of a whole sample area.Although the sampling pattern is predetermined, the first sam-pling location should be selected at random manner, as illustratedin Fig. 3b. Since the sampling locations in systematic samplingplan follow a simple pattern and are separated by a fixed distance,locating the sampling points in a contaminated site may be easierusing systematic sampling than using random sampling. Besides,a systematic sampling plan generally results in a gain in precisionof sampling results and thereby is preferable to mapping proce-dure. Currently, a square grid and triangular grid Fig. 4 are thecommon patterns of sampling locations usually used in systematicsampling plan. To calculate the fixed distance between the adja-cent sampling points L, Eqs. 4 and 5 should be applied forsquare grid pattern and triangular grid pattern, respectivelyL =Anf4ng plan; and c stratified random sampling plan over a contaminatedFig. 4. Square grid pattern and triangular grid pattern of samplinglocations for systematic sampling plan adapted from USEPA 1989RADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 139aste Manage. 2007.11:132-157.Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.L = A0.866nf5where Lfixed distance between the adjacent sampling points;Asize of the sample area; and nfnumber of sampling pointsrequired.After determining the fixed distance, the location of the firstsampling point in the sample area is generated via Steps 13mentioned in the previous section and then marked on the mapFig. 5a. This randomly created location is then used as oneintersection point of two gridlines Fig. 5b. Afterward, gridlinesrunning parallel to coordinate axes and separated by a distance Lare constructed on the map Figs. 5c and d in which the sam-Fig. 5. Steps used to locate square grid sampling locatioFig. 6. Another approach for positioning systematic sampl140 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wpling locations are the intersection points within the sample areaboundaries. Another approach for locating sampling locations forsystematic sampling plan is shown in Fig. 6 in which randomstarting position is determined by choosing a random angle aand a random distance on XX line i.e., point Y from point X.Then a line with endpoints Y and Y which is a degree from XXline is constructed on the map. Another line starting from point Ywith 90 +a from XX line is also constructed. By repeatingthese steps, a new set of lines is constructed from a new point onthe XX line, which is at a distance D from point Y. Other newsets of lines are continuously constructed from other new pointson XX line, which are separated at a fixed interval of D untilthere is an intersection point within the boundary of the samplesystematic sampling plan adapted from USEPA 1989ations in a contaminated site adapted from USEPA 1989ing locns forE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.te depDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.area. This intersection point is the first sampling location, and theother sampling locations within the sample area are then deter-mined using the procedure delineated in Fig. 5.Stratified Sampling PlanIn stratified sampling, the total sample area is divided into a num-ber of strata or subpopulation of soil in which a random samplingplan or a systematic sampling plan is applied to each stratum Fig.3c. It is interesting to note that the sampling plan chosen forone stratum does not need to be used in another stratum. Themain merit points of implementing a stratified sampling plan isthat evaluation of the performance of remediation technologiescan be made separately in each stratum. In addition, it is morehomogenous within the strata in comparison to the whole samplearea so that the stratified sampling plan may ameliorate the pre-cision of contaminant levels estimated. In analogy to systematicsampling, stratified sampling can also ensure that all importantareas of a contaminated site are covered in the sampling plan.Another advantage of stratification is that it encourages anefficient allocation of sampling resources since it allows dispro-portionate allocation of few resources to some strata and moreresources to other strata. Criteria used to define strata includesampling depth, contaminant concentration levels, physiography/topography, type of contaminants, the history and sources of con-tamination, and previous clean up attempts. During stratification,the strata must not overlap and the sum of size of the strata mustbe equal to the total sample area.Fig. 7. Different approaches for soil sampling across depth: a discresampling; and e stratified random depth sampling adapted from USPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. WComposite Sampling PlanA composite sampling plan is applied when average soil contami-nant concentrations are needed. A number of soil samples repre-senting the soil population under performance monitoring arecollected and mixed thoroughly to form a composite for labora-tory analyses. Since this sampling plan can only give an averagecontaminant concentration, there is no estimation of the varianceof the average value. However, in the case where composite sam-pling is used in conjunction with stratification, a compositesample is obtained from each stratum. The average contaminantconcentration obtained from each stratum can then be used tocalculate the mean concentration, standard deviation, and otherrequired statistics for the whole sample area.Sampling across DepthIn addition to the planar distribution of the sampling locationsover the sample area, the sampling plan should also include theapproach for soil sampling across the depth of a contaminatedsite. Discrete depth sampling, compositing across depth, randomdepth sampling, and stratified random depth sampling are the ap-proaches frequently applied USEPA 1989; Carter 2003.Discrete Depth SamplingThis approach of soil sampling involves collection of soil fromexact positions across depth. For instance, as illustrated in Fig.7a, soil samples are obtained from a soil core at elevation 1.5 toth sampling; b and c compositing across depth; d random depth1989EPARADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 141aste Manage. 2007.11:132-157.x situadapteDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.1.4 m, 0.5 to 0.6 m, and 4.5 to 4.6 m for separate laboratoryanalyses. The size of an interval is dependent on the sample sizerequired for analyses. Since soil samples are collected at differ-ently discrete depths, conclusions on the attainment of clean upstandard can be made independently for each soil horizon.Compositing across DepthAs mentioned previously, compositing approach can only providethe mean contaminant concentration in soil. Evaluation of theproportion of the soil samples above clean up standard is impos-sible because of thorough mixing of the collected soil samples.Currently, there are two compositing approaches. The first oneinvolves the sampling of an entire soil core from a randomly orsystematically identified location. The entire soil core is thencompletely mixed and subsampled for laboratory analyses Fig.7b. The second approach involves random sampling of differ-ent segments of a soil core. These segments are then thoroughlymixed together and subsampled for analyses Fig. 7c. The com-positing approach is not appropriate for depth sampling if thecontaminant mass in the soil samples, such as mass of volatileorganics within a soil matrix, are susceptibly influenced by themixing process.Random Depth SamplingThis approach of depth sampling is usually applied to the situa-tion in which the subsurface does not show visible or knownhorizon along the depth or the objective of soil sampling is toevaluate the proportion of soil samples above clean up standard. Itinvolves random sampling of a single location within each soilcore, as shown in Fig. 7d. By applying this approach, the soilsamples collected from different soil cores can represent manydepths in a contaminated site. Another advantage of random depthsampling is that the sampling cost may be low because at manylocations, the auger may not need to drill to bedrock. Some sam-pling depths may be even close to the ground surface.Stratified Random Depth SamplingUnlike random depth sampling, this approach is customarily usedwhen there are visible horizons along the depth or horizons aredefined for purpose of study. During stratified random depth sam-pling, a soil sample is collected at a random depth in each stratumFig. 7e.Fig. 8. Schematic diagrams showing a the extraction wells and ehydraulic containment of contaminant plume by the extraction well 142 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. WKey Monitoring Parameters for Specific Soil andGroundwater Remediation TechnologiesSuccessful evaluation of the effectiveness and efficiency of theapplied remediation technologies for soil and groundwater treat-ment is highly dependent on the parameters to be monitored.Monitoring of appropriate parameters not only indicates theprogress toward the attainment of remediation objectives, but alsocan point out the possible culprits leading to the failure of theremediation technologies. Criteria for the selection of the moni-toring parameters are based on the remediation objectives, spe-cific site conditions, performance metrics to be evaluated, andtypes of remediation technologies. In this section, key monitoringparameters of each soil and groundwater remediation technolo-gies will be comprehensively introduced.Pump-and-Treat SystemsHeretofore, P&T systems are the most common remediation tech-nology used to contain groundwater contaminants, and/or reme-diate contaminated groundwater i.e., aquifer restoration. InUnited States, 72% of superfund site Records of Decisions chooseP&T systems as a prime groundwater remediation approachUSEPA 1992, 1994. The operating principles of the P&Tsystems involve capturing and pumping of contaminated ground-water by extraction wells for ex situ treatment, as illustrated inFig. 8a. After the treatment, water either is reinfiltrated back toaquifers or pumped into water sources nearby. The inward hy-draulic gradient created by the extraction wells inside the plume,as demonstrated in Fig. 8b, also helps control the downgradientmovement of contaminated groundwater, thereby preventing con-tinued expansion of the plume Eldho 2003. Besides, by install-ing physical containment systems and/or fluid injection systemsflushing the contaminated groundwater toward the capture zonesof the extraction wells, the performance of P&T systems can beenhanced.Key Monitoring ParametersPerformance on the Hydraulic Containment. Monitoring ofthe performance of P&T systems on hydraulic containment ofcontaminant plume involves monitoring of hydraulic gradient,measuring of pumping rate and/or injection rate if necessary,contaminant concentrations, and tracer movement. Table 2 sum-treatment unit of a P&T system adapted from USEPA 2001b; bd from USEPA 1996E WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Table 2. Monitoring Parameters of P&T Systems for the Hydraulic Containment of Contaminant Plume and Aquifer RestorationParaydraulhydraun rateratenant coaterovemec contratec gradovemeater flater cs of coDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.marizes these monitoring parameters, and the correspondingmonitoring locations and purposes. Monitoring of an inward hy-draulic gradient across the boundary of and/or within the contain-ment zone can indicate that contaminated groundwater flow isinward, thereby ensuring the successful capture or contain of thecontaminant plume by P&T systems. Inward hydraulic gradientcan be estimated by comparing hydraulic head in piezometersnear the containment perimeter and located in the downgradientof the extraction wells. It can also be evaluated by interpretingpotentiometric surface maps determined from the hydraulic headdata measured in the wells within and outside of containmentarea. Furthermore, the hydraulic gradient of more permeable por-tions of the aquifer should be monitored to prevent preferentialflow of contaminated groundwater and migration of contaminantsacross the containment boundary. Besides, at the base of the con-taminant plume or containment volume, the inward hydraulic gra-dient toward the extraction wells may be specified as upwardhydraulic gradient. Monitoring of the upward hydraulic gradientcan prevent the possible downward contaminant migration. It canbe monitored by comparing the hydraulic head differences mea-sured at different depths or comparing the potentiometric surfacesobtained at different elevations and stratigraphic layers.Monitoring of the pumping rate and injection rate are requiredto maintain the inward and upward hydraulic gradient toward theextraction wells. Thus, contaminated groundwater in the contain-ment zone can be determined to be following pathlines to theP&T systems. Another important point is that the data of hydrau-lic head/gradient and pumping rates are the information requiredLocationsHydraulic containment of contaminant plumeContainment perimeter and primarily downgradientlocations of the extraction wellsInward hWithin and outside the containment areaPermeable portion of the aquiferBase of the contaminant plume or containment volume UpwardExtraction wells ExtractioInjection wells InjectionMonitoring points along or near the potentialdowngradient containment boundaryContamigroundwMonitoring points beyond the containmentperimeter or all monitoring pointsTracer mAquifer restorationAs listed above HydrauliExtraction wells PumpingAll monitoring points HydrauliTracer mGroundwAll monitoring wells in contaminated zone GroundwchemicalFrom soil or rock through borings in contaminated zoneInfluent and effluent of treatment unitMonitoring wells located upgradient and sidegradient ofcontaminant plume if contaminants have migratedbeyond the containment zoneRepresentative locations in the contaminated zone Soil conand orgaPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wfor capture zone analyses. Groundwater contaminant concentra-tion monitoring is performed throughout the whole P&T opera-tion to check whether the temporal and spatial variations ofcontaminant distribution/concentration along or near the down-gradient of the containment boundary are consistent with thedesigns. Failure of the hydraulic containment is suggested if: 1the estimated total contaminant mass in groundwater beyond thecontainment perimeter increases with time; 2 temporal changesof contaminant concentrations in perimeter or downgradientmonitoring wells is inconsistent with the design under effectivecontainment; and/or 3 comparatively retarded contaminants pre-viously restricted to the containment area are detected in perim-eter monitoring wells. In some contaminated sites, tracers wereperiodically released into the containment zone where hydrauliccontrol is considered the least effective. Tracer detection ingroundwater beyond the containment perimeter indicates contain-ment failure and the possible locations of the failure. Moreover,by releasing the tracers in areas of uncertain capture followed bymonitoring of the tracers present in the extracted groundwater, theP&T capture zone can be delineated.Performance on Aquifer Restoration. Attainment of aquiferrestoration by P&T systems is more difficult than the hydrauliccontainment of the plume because of the occurrence of concen-tration tailing and rebound during the P&T operation USEPA1994. This phenomenon is caused by the limited aqueous solu-bility of most contaminants e.g., nonaqueous phase liquidsNAPLs, slow desorption of contaminants from aquifer materi-meters Purposesic gradient Remedial performance and capture zone analysislic gradient Prevention of downward contaminant migrationCreation of hydraulic gradient toward theextraction wells and capture zone analysesFlushing of contaminated groundwater towardthe extraction wellsncentration in Remedial performancent Location of containment failure or delineationof capture zonesainment Prevention of further spread of contaminantplume during restoration effortsPore volume of flushingient Determination and control stagnation zonesntowrateontaminants andncernRemedial performance/progressnt concentrationsbon contentRemedial performance/progresstaminanic carRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 143aste Manage. 2007.11:132-157.als relative to groundwater flow, and slow dissolution of contami- centrations from extraction wells manifolds to a treatment unitand its effluent stacks is required to optimize the contaminantDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.nant precipitates. Therefore, application of the P&T systems foraquifer restoration requires a high degree of performance moni-toring to identify the problem areas and optimize systemoperations.Generally, the hydraulic containment of the contaminantplume is a prerequisite for the aquifer restoration. All the moni-toring parameters for the hydraulic containment are also includedfor the aquifer restoration monitoring, as shown in Table 2. There-fore, further spread of the contaminant plume can be preventedduring the restoration efforts. In addition to the inward hydraulicgradient monitoring and the capture zone analyses, monitoring ofthe pumping rate during the aquifer restoration can also determinethe number of pore volume NPV of groundwater flushingthrough the contaminant zone by the P&T systems. Effectiveaquifer restoration by the P&T systems requires sufficient NPV ofgroundwater flushing to remove both existing dissolved contami-nants, and those that will desorb from aquifer materials and dis-solve from contaminant precipitates. Measuring of the hydraulicgradient, tracer movement, and groundwater flowrate by a down-hole flowmeter can show any stagnant zone in subsurface inducedby low hydraulic gradient created during the P&T operation.Once the stagnant zone is identified, its size and duration can beminimized by changing the extraction and/or injection rates, andthe location of the wells. Groundwater and soil contaminant con-centration in the contaminant zone, and contaminant concentra-tion in the influent and effluent of the treatment unit should beanalyzed periodically to monitor the remedial performance andprogress toward the attainment of the remediation objectives.Concentration of chemicals in groundwater which can affect theperformance of the treatment unit should also be measured. Forexample, iron concentration in groundwater should be monitoredif groundwater is aerated during the treatment process since ironmay precipitate and consequently clog the treatment unit.Soil Vapor Extraction and Air Sparging SystemsSoil Vapor Extraction SystemsSVE, which is also called soil venting or vacuum extraction, is anin situ soil remediation technology. It is effective in removingvolatile contaminants adsorbed onto the soil in vadose zone orcapillary zone with high air permeability to allow sufficient ad-vective air flow ITRC 2004; USEPA 2004a. In this technology,vapor extraction wells are drilled near the contaminant sourceareas in a vadose zone as illustrated in Fig. 9a. Through theextraction wells, a vacuum is applied to the soil matrix to create anegative pressure gradient causing the movement of vapor towardthe extraction wells. As the air advection through the porousmedia travels toward the extraction wells, contaminants in the soilmatrix volatilize. The contaminant vapor then migrates throughthe air-filled pore in vadose zone by diffusion into the advectiveair flow toward the extraction wells and finally is removed fromsubsurface. The extracted vapor is treated prior to discharge to theatmosphere USEPA 2001a; ITRC 2004. SVE is only applicableto the contaminants with sufficient high vapor pressure andHenrys constants. Low air permeability and heterogeneous dis-tribution of air permeability in soil significantly limit the SVEeffectiveness on the soil remediation.Key Monitoring ParametersDuring the operation of the SVE systems, monitoring of the ex-traction flowrates, vacuum pressures, and vapor contaminant con-144 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wmass removal Table 3. Based on these monitoring data, thevacuum pressure can be adjusted to concentrate on the extractionwells, which produce vapors with high contaminant concentra-tions. In addition, periodic monitoring of the extraction flowrateand vapor contaminant concentration from the extraction wellsand the effluent stacks allow the determination of contaminantmass extracted by the SVE systems, as mentioned previously, forthe examination of remedial progress. If the SVE systems aremonitored exhibiting asymptotic behavior with respect to bothvapor contaminant concentration and cumulative mass removali.e., no further decrease in vapor contaminant concentration orincrease in cumulative mass removal with the remediation time,some modification of the SVE systems is needed. These modifi-cations include increasing flow to the extraction wells with highvapor contaminant concentration by ceasing vapor extractionfrom the extraction wells with low vapor contaminant concentra-tion, and installation of additional extraction wells USEPA2004a.Air Sparging SystemsAS is an in situ remediation technology for removal contaminantsin groundwater. Generally, it involves two mechanisms: 1 strip-ping of volatile organic contaminants; and 2 creation of oxygen-ated conditions favorable to aerobic biodegradation. Air strippingof contaminants requires injection of contaminant-free air into theaquifer through the AS wells, which enable a phase transfer ofvolatile organic contaminants from a dissolved phase to a vaporphase. In the case where the air stripping is the primary remedia-tion mechanism, SVE systems are often used with AS systems toremove the vapor vented to vadose zone from subsurface Fig.9b. However, if biodegradation is the main remediation mecha-nism, SVE systems are usually not included. Moreover, air injec-tion flowrate must be minimized to prevent excessive generationof organic vapor in soil gas or nearby receptors ITRC 2004;USEPA 2004a.Key Monitoring ParametersIn the case where the AS systems is worked with the SVE sys-tems, the latter should be started up and optimized in advancebased on the SVE monitoring data. This step is to prevent thepossible accumulation of organic vapor in soil gas or migration oforganic vapor to the nearby receptors during the operation of theAS systems. After starting the AS systems, the air injection flow-rate, extraction flowrate, vacuum pressure, hydraulic gradient, andvapor contaminant concentration are monitored to balance theinjection flowrate and optimize contaminant mass removal rateTable 3. For the sake of determining the contaminant mass ex-tracted and remaining in the aquifers, contaminant levels ingroundwater and vapor in the groundwater monitoring wells,vapor extraction wells, and the effluent stacks of the treatmentunit should be measured periodically. According to these results,the asymptotic behavior of the AS systems with respect to bothdissolved-phase and vapor-phase concentrations can also be iden-tified USEPA 2004a.Surfactant/Cosolvent Flushing SystemsSurfactant/cosolvent flushing involves injection of chemicals intovadose zone or aquifer to sweep the NAPL zone and extraction ofE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.Table 3. Key Monitoring Parameters of SVE Systems and AS Systems Worked with SVE SystemsLocations Parameters PurposeSoil vapor extraction systemsExtraction wells, manifolds to vapor treatmentunit and effluent stacksExtraction flowrate, vacuum pressure, andvapor contaminant concentrationSystem optimization and remedial performance/progressAir sparging systems worked with soil vaporextraction systemsAir sparging wells AS, groundwatermonitoring wells AS, extraction wells SVE,manifolds to vapor treatment unit SVE, andeffluent stacks SVESparging pressure AS, injection flowrateAS, hydraulic gradient AS, extractionflowrate SVE, vacuum pressure SVE,vapor contaminant concentrations SVESystem optimization and remedial performance/progressGroundwater monitoring wells Groundwater contaminant concentrations Remedial performance/progressFig. 9. Typical a SVE systems; b AS systems worked with SVE systems adapted from USEPA 2004aPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 145Pract. Period. Hazard. Toxic Radioact. Waste Manage. 2007.11:132-157.monitoring parameters of the surfactant/cosolvent flushing tech-nologies. Monitoring of the contaminant concentration in the ex-echnolrametent conlevels,, and eion ratvelsDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.an elutricate i.e., a mixture of the injected fluids and swept con-taminants above ground surface for treatment or recovery Fig.10. Surfactants consisting of a hydrophilic head and a hydropho-bic tail and cosolvents including ethanol and isopropanol are ableto lower the interfacial tension between NAPL and the aqueousphase, and increase the solubility of NAPL when they are flushedthrough the contaminated zone ITRC 2004. The criteria of theselection of surfactants and cosolvents for flushing depend oncontaminant characteristics, such as hydrophobicity, density, vis-cosity, and interfacial tension with water and soil matrix. Siteconditions including soil heterogeneity, hydraulic conductivity ofaquifer, groundwater geochemistry, and soil mineralogy are alsothe factors affecting the chemical selection.Key Monitoring ParametersDespite the fact that surfactant/cosolvent are proven to be effec-tive in lowering the interfacial tension and increasing contaminantsolubility, small change in electrolyte concentration in groundwa-ter can potentially exert significant impact on the performance ofthis technology ITRC 2003. Thus periodic monitoring ofsurfactant/cosolvent level, contaminant concentration, andgroundwater characteristics e.g., pH and specific conductanceare key to ensure effective and efficient removal of subsurfacecontaminant by this technology. Table 4 summarizes the keyTable 4. Key Monitoring Parameters of Surfactant/Cosolvent Flushing TLocations PaInjection wells, extraction wells, treatment andrecovery units, and monitoring wells withinand outside of the remediation zoneDissolved contaminasurfactant/cosolventspecific conductanceconcentrationInjection and extraction wells Injection and extractInjection and extraction wells, monitoringpointsGroundwater levelsInjection, extraction, and wastewaterprocessing fluid linesPressureExtraction wells Tracer movementAll monitoring points Free-phase NAPL leFig. 10. Schematic diagram showing the operation of surfactant/cosolvent flushing systems for groundwater remediation146 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wtracted groundwater allows for the determination of contaminantmass extracted. If surfactant/cosolvent is reused, their amountbeing recovered can be calculated by monitoring the surfactant/cosolvent level in the extracted groundwater and recovered fluidsso that the additional amount of fresh surfactant/cosolvent re-quired can be determined. Another important point is that moni-toring of the surfactant/cosolvent concentration in the injectionsolution is needed to ensure that the mixing procedure and theinjection solution meet design specifications. This step becomesmore important if surfactant/cosolvent is reused. For the sake ofmonitoring the quality of the treated groundwater being redis-charged to the aquifers, monitoring of the contaminant concentra-tion in the influent and effluent of the treatment unit is necessary.Besides, to ensure the hydraulic capture of the flushed fluids andswept contaminants, and assess the remedial progress, contami-nant and surfactant/cosolvent concentration should be measuredperiodically from the monitoring points located below, above, andaround the remediation zone. Overall, in pre- and postflushingmonitoring of the contaminant concentration in the injection, ex-traction and monitoring wells are needed to check whether thereis a significant reduction in dissolved contaminant concentrationin subsurface. Postflushing monitoring should be implementedwhen the groundwater systems have been reequlibrated. Long-term monitoring is also needed to assess any rebound in contami-nant concentration. Groundwater sampling from the monitoringpoints outside the remediation zone can be done to verify thatcontaminants and surfactant/cosolvent do not migrate out of thezone during the flushing.Other important parameters that are crucial in affecting theperformance of surfactant/cosolvent flushing are the injection andextraction flowrates. This is because unbalanced injection and ex-traction rates can lead to poor sweep efficiency and mounding ofthe groundwater table. Serious fluctuation of the groundwatertable may create hydraulic control system and thereby may resultin poor hydraulic capture of the flushed fluids. In addition, accu-rate measurement of the injection and extraction rates are requiredfor precise determination of the contaminant mass extracted andamount of surfactant/cosolvent recovered.In Situ Thermal Remediation SystemsThermally enhanced remediation systems are in situ remediationtechnologies for removal of light NAPLs LNAPL or DNAPLsogiesrs Purposecentration,pH, temperature,lectrolyticRemedial performance/progress, and fluidchemistryes Fluid flow propertiesAquifer propertiesFluid flow propertiesImplemented before the flushing to ensureefficient collection of the mobilizedcontaminants by extraction wellsAquifer propertiesE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Electrical Resistance HeatingDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.in soil and/or groundwater. The principles of these technologiesare that heat energy is delivered to subsurface to vaporize and/ormobilize NAPL contaminants. A network of extraction wells invadose zone and aquifers is then used to remove contaminantvapors, mobilized NAPL, and contaminated groundwater forabove ground treatment. During the remediation, continuous boil-ing of groundwater is desirable to create steam stripping andthereby offer sustained treatment of contaminated groundwater.To date, there are three main ways used to deliver thermal energyto vadose zones or to aquifers: 1 direct heating by steam injec-tion steam-enhanced extraction; 2 heating resulting from thenatural resistance of the geologic medium in response to passageof electrical current electrical resistance heating; and 3 heatingby conduction from thermal wells or thermal blankets containingheating elements thermal conduction heating ITRC 2004.Steam-Enhanced ExtractionAs shown in Fig. 11, the thermal remediation system using steam-enhanced extraction involves injection of steam around a pool ofNAPL contaminants in subsurface. Although Fig. 11 only indi-cates steam injection to the vadose zone, it can also be injectedinto aquifers. When steam is injected into the well bores, it heatsthe well bores and the formation around the steam injection zone.The injected steam then condenses as latent heat of vaporizationof water transfers to the well bores and the porous media of theformation where the steam enters. As more steam is injected, thecondensed hot water moves into the formation pushing the origi-nal cold water in the formation further deep into the porousmedia. When the porous media at the point of steam injection hasabsorbed enough thermal energy, steam itself enters the porousmedia and finally pushes the bank of condensed hot water andcold water in front of it toward the contaminant zone. Cold watercan first flush the free NAPLs from the pores. The hot water bankcan reduce the viscosity and residual saturation of the contami-nants. When steam front reaches the contaminant zone, theresidual contaminants are volatilized and transported to steam.Finally, the cold and hot water and the steam containing contami-nants are removed from subsurface by the groundwater and vaporextraction wells Davis 1998.Fig. 11. Diagram of an in situ thermal remediation system usingsteam-enhanced extractionPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. WThis approach of thermal remediation systems applies the thermalenergy generated from the natural resistance of soils or rocks inresponse to the flow of electric current to volatilize NAPL con-taminants and generate steam as a carrier gas to sweep NAPLcontaminants to the extraction wells as described in the previoussection. Electric current is applied to geologic media through anarray of electrodes placed in the subsurface throughout the reme-diation area. In comparison to the steam-enhanced extraction,electrical resistance heating can provide relatively uniform heat-ing throughout the remediation area regardless of lithologies en-countered and whether it is saturated or unsaturated. Furthermore,in situ steam generation can occur in fractured or porous rock,and in all soil types irrespective of permeability Beyke andFleming 2005.Thermal Conduction HeatingThermal conduction heating is applied to the thermal remediationsystems for soil remediation only. Heat energy is applied to thesoils through arrays of vertical or horizontal heaters i.e., thermalwells containing heating elements. Heat flows through the soilsfrom the heating elements primarily by thermal conduction. Asthe soils are heated, NAPL contaminants are vaporized and/ordestroyed by a number of mechanisms including steam distilla-tion, boiling, oxidation, and pyrolysis. Then the vaporized waterand contaminants are drawn from the subsurface via vacuum ex-traction wells. Since the energy input into the soils by the thermalwells are uniform over each heaters length and the soil thermalconductivity does not vary substantially over a wide range of soiltypes, the rate of heat front propagating from each heater is highlypredictable ITRC 2004.Key Monitoring ParametersMonitoring of the in situ thermal remediation systems should beimplemented in subsurface and the key locations throughout theremediation train, such as steam injection/extraction wells, elec-trodes, unit process inlet and outlet piping, effluent stack, andgroundwater monitoring wells. Table 5 shows the key parametersneeded to be monitored during the thermal remediation usingsteam-enhanced extraction, electrical resistance heating, and ther-mal conduction heating.To monitor the heating progress and completeness, tempera-ture in subsurface should be monitored through thermocouplestrings placed between injection and extraction wells and atextraction wells in steam injection systems, or at various locationsbetween electrodes in electrical resistance heating systems orbetween heating elements in thermal conduction heating systems.Temperature data obtained from the thermocouple strings colo-cated with peripheral monitoring wells and the monitoring wellssituated inside and outside the remediation area can show if thereis loss of hydraulic control of contaminated groundwater andmobilized contaminants during the thermal remediation. For in-stance, temperature increases at a certain depth of the monitoringwell outside the remediation area can evince flow of heated waterfrom the remediation area. In fact, understanding the directionwhere water is leaving the remediation zone allows adjustment tobe made to improve the hydraulic control. Besides, during thethermal remediation, the area beneath the contaminated zone isusually heated up first to establish a hot barrier. In situationswhere contaminants move downward, the hot barrier can quicklyvaporize the contaminants, which are then removed by the vaporextraction wells. To ensure the establishment of the hot floor,thermocouples should be installed below the contaminant zoneRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 147aste Manage. 2007.11:132-157.Table 5. Key Monitoring Parameters for In Situ Thermal Remediation Systemser levesurfaceperatuperatutaminaperatutaminaperatuperatuer, curer flower, curDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.for the temperature monitoring. In addition to temperature, sub-surface pressure is also an important monitoring parameter of thethermal remediation systems for the control of vapor generated. Itcan be measured using the piezometers located within and aroundthe perimeter of the remediation area, and colocated with moni-toring wells ITRC 2004.Monitoring of the vapor contaminant concentration in soil gasor extracted vapor can provide valuable information related to theremediation progress toward attainment of remediation objec-tives. They can also indicate whether the concentrations in theextracted vapor reach asymptote, as described in SVE and AS.Contaminant concentration in soil should be monitored during theinstallation of the remediation system and immediately after thesystem shutdown. Soil concentration data obtained during the sys-tem installation can provide information on the extent of contami-nation and provide baseline information of the soil contamination.Soil concentration data obtained after completion of remediationcan indicate whether the remediation objectives are met or addi-tional remediation is required. Moreover, soil concentration datamust be obtained from some locations where complete heating isdifficult to achieve. Interim soil sampling can also be imple-mented to evaluate the remedial progress. If there is an extractionof groundwater during the thermal remediation, monitoring of thecontaminant concentration in the extracted fluids can provide datato calculate the mass of contaminant extracted from subsurface.In Situ Chemical Oxidation SystemsUnlike P&T, SVE/AS, and surfactant/cosolvent flushing systemsextracting the contaminants from subsurface, ISCO systems di-rectly destroy the contaminants in soil and groundwater. ISCOinvolves an injection of oxidants and potential coamendments di-Locations/mediaGeneralPeripheral monitoring wells Groundwater WatVapor SubThermocouples TemRemediation zone before, during,and after the remediationThermocouples TemSoil ConGroundwaterVaporExtraction and treatment systems Extracted fluids water, vapor,and NAPL contaminantsTemConSteam-enhanced extraction onlyInjection wells Steam, liquid, and vapor lines TemSteam systems Steam headers TemElectrical resistance heating onlyElectrical heating systems Electrodes PowWatThermal conduction heating onlyElectrical heating systems Thermal well circuits Pow148 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wrectly into the contaminant source zone and the downgradientcontaminant plume. When the oxidants contact with the contami-nants, the latter are oxidized into benign products, such as carbondioxide, water, and/or inorganic chloride for chlorinated sol-vents ITRC 2005. Nowadays, there are four types of oxidantsmost frequently used in ISCO, which are permanganate MnO4,sodium persulfate Na2S2O8, hydrogen peroxide H2O2, andozone gas O3. Permanganate oxidizes contaminants through theelectron transfer, whereas the others create free radicals to breakthe chemical bonds of the contaminants. The oxidative power ofthese oxidants is in the following order: ozonesodiumpersulfatehydrogen peroxidepermanganate ITRC 2004,2005.Permanganate is a stable oxidant and has a unique affinity foroxidizing organic compounds containing carbon-carbon doublebonds, aldehyde groups, or hydroxyl groups. Under normalgroundwater pH and temperature, the carbon-carbon double bondof alkene is broken spontaneously and converted to carbondioxide through either hydrolysis or further oxidation by thepermanganate4KMnO4 + 3C2Cl4 + 4H2O 6CO2 + 4MnO2s + 4K+ + 12Cl + 8H+ 6Hydrogen peroxide alone is an oxidant but it is not kineticallyfast enough to destruct many hazardous organic contaminants atlow concentration. Therefore, ferrous salt is usually added to dra-matically increase the oxidative strength of hydrogen peroxide.Such enhancement of the strength is ascribed to the production ofhydroxyl radicals OH Eq. 7. The reaction of iron catalyzedperoxide oxidation at pH 2.5 to 3.5 is called Fentons reaction andthe iron/peroxide mixture is known as Fentons reagent, whichParameters Purposesls and contaminant concentrations Migration controlpressure Migration controlre Migration controlre Heating progress and completenessnt concentration Remedial performance/progressre, pressure, and flowrate Mass and energy balance, cooling,and vacuum confirmationnt concentrations Remedial performancere, pressure, and flowrate Confirmation of steam injection,injection safety and process control,energy balance, and site balancingre, pressure, and flowrate Safety and process control, massand energy balancerent, and voltage Energy balance and site balancingrate Power delivery maintenancerent, and voltage Confirmation of heat deliveryand energy balanceE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Table 6. Key Monitoring Parameters of In Situ Chemical Oxidation TechnologiesParamcentratationscentratcentratoncentand ote andDO, pnce, anganesechromameter, naturnityDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.has the strongest oxidation power among MnO4, Na2S2O8, H2O2,and O3. In the case where hydrogen peroxide is in excess, moreradicals can be produced from H2O2 itself in comparison to Fen-tons reaction because of the radical-propagation reactionsFe2+ + H2O2 Fe3+ + OH + OH 7Sodium persulfate or persulfate anion S2O82 is kineticallyslow in destroying many organic contaminants even though it is astrong oxidant. Under dilute acid conditions, hydrolysis of theS2O82 yields bisulfate anion and H2O2. However, activation byheat or catalysis by chelated metals e.g., iron, copper, and silvercan substantially increase the oxidative strength of S2O82 becauseof generation of sulfate free radical SO4, as shown in Eq. 8Fe2+ + S2O82 Fe3+ + SO4 + SO42 8Ozone is the strongest oxidant available for ISCO. The ozone-based process is different from the other ISCO processes becauseof the involvement of the ozone gas injection leading to verydifferent design and operational issues. In practice, ozone gas iseither injected into the vadose zone or sparged below the watertable. When the contaminants are in contact with the injectedozone, they are either oxidized directly by ozone itself Eq. 9 orindirectly by the generated hydroxyl radicalsO3 + RC = RC RCOCR + O2 9Key Monitoring ParametersSince ISCO is a destructive remediation technology, assessmentof the contaminant mass treatment is more difficult than the tech-nologies involving extraction of contaminants from subsurface sothat pre- and posttreatment subsurface evaluation must be applied.Generally, precise estimation of the amount of contaminantsbeing destroyed may be impractical so that ISCO performancemonitoring customarily focuses on the evaluation of source treat-ment progress ITRC 2004. As listed in Table 6, groundwatercontaminant concentration in the monitoring wells downgradientof the injection wells and soil contaminant concentration are theLocationsGroundwater in monitoring wells downgradientof injection wells pre- and postinjectionContaminant conOxidant concentrSoil within contaminant zone pre- and postinjection Contaminant conSoil vapor within the contaminant zone Contaminant conSoil vapor within the contaminant zone Residual ozone cInjection wells during injection Injection flowrateExtraction wells associated with recirculation system,if installedExtraction flowraGroundwater in monitoring wells, downgradientof injection wells pre-, during, and postinjectionField parametersspecific conductaTracer movementMetals e.g., manarsenic, lead, andWater quality parchloride, calciumnitrate, and alkaliPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wparameters that should be measured before and after the injectionoperation for assessing the remedial progress and/or the effective-ness of the ISCO technologies. These monitoring data are alsouseful to evaluate the seriousness of the displacement of contami-nated groundwater away from the injection wells during theoxidant injection. If the contaminant concentration in all themonitoring wells shows a decline after the injection, the displace-ment is expected to be minimal. On the other hand, the increase inthe concentration at one or more monitoring locations may showa displacement of contaminated groundwater from the contami-nant zone. In addition, measurement of the oxidant concentrationin groundwater can indicate the oxidant persistence in subsurfaceand the distribution of oxidant concentration across the contami-nant zone. In some situations, improper application of Fentonsreagent or ozone can lead to a significant generation of heatand/or oxygen gas in the subsurface, which may be volatile and/orstrip contaminants in aquifers to vapor phase and consequently toatmosphere. Transfer of contaminants to atmosphere can alsooccur in the vadose zone when using ozone as an oxidant. Toensure that contaminants are destroyed in subsurface rather thantransferred to atmosphere, monitoring of the contaminant concen-tration in soil gas is needed.During the injection, injection flowrate and oxidant concentra-tion should be measured from the injection wells so as to checkwhether the oxidant concentration injected and radius of influencecreated in subsurface is consistent with the design specifications.Monitoring of certain field parameters helps evaluate the systemefficiency because oxidant injection can lead to changes ingroundwater DO, pH, temperature, and/or redox potential. More-over, these monitored field parameters can be used to decide if thesite condition, after the injection, has returned to equilibrium con-dition for effectiveness monitoring. Analysis of the dissolvedmetal levels in groundwater during and after the injection is alsoessential. This is because certain redox-sensitive metals, such asarsenic, barium, cadmium, chromium, copper, lead, and selenium,can be oxidized to a more soluble state by the injected oxidant.Originally, these metals are in reduced forms and insoluble stateeters Purposesions Remedial progress and effectivenessEstimation of oxidant persistence and radialinfluenceions Remedial effectivenessions Prevention of transfer of contaminants to vaporphaserations Prevention of ozone emission into atmospherexidant concentrations Confirmation of the oxidant concentrationand volumes injected, and radial influenceoxidant concentrations Oxidant recoveryH, temperature,d redox potentialEvaluation of system efficiencyObservation of travel times and distributionof oxidants, aluminium,iumMobilization of insoluble metals by injectedoxidants and evaluation of potential for theformation cloggings sulfate,al organic matter,Progressing of the treatment, and influenceof nontarget oxidizable mattersRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 149aste Manage. 2007.11:132-157.systemDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.in natural aquifers so that they are naturally not detected ingroundwater. However, ISCO can mobilize these metals andthereby may harmful to downgradient receptors. Furthermore,metal monitoring data can be used to evaluate the potential ofmetal precipitation in subsurface during ISCO, which may clogthe porous media, thereby reducing the aquifers hydraulic con-ductivity. Monitoring of certain specific indicating parameters,such as chloride for chlorinated solvents, can provide evidence ofongoing treatment by injected oxidant. Water quality parameterscan also help to evaluate the influence of nontarget oxidizablematters e.g., natural organic matter in subsurface on the injectedoxidant consumption ITRC 2004; ITRC 2005.In Situ BioremediationIn situ groundwater bioremediation is a technology that stimulatesgrowth and reproduction of indigenous microorganisms toenhance biodegradation of organic contaminants in aquifersBedient et al. 1999. This remediation approach can effectivelydegrade organic contaminants both dissolved in groundwater andadsorbed onto aquifer matrix. To stimulate and maintain themicrobial activity in subsurface, a delivery system providingelectron acceptors e.g., oxygen, nitrate, nutrients nitrogen,phosphorous, and/or energy sources carbon is required forbioremediation. Typically, during the bioremediation, groundwa-ter is extracted from extraction wells and treated to remove dis-solved contaminants if necessary. The treated groundwater is thenmixed with the electron acceptors, nutrients, and/or other con-stituents and reinjected upgradient of or within the contaminantsource, as illustrated in Fig. 12. The major biological processesinvolved in degrading organic contaminants include aerobic,anaerobic, and cometabolic degradation processes. Aerobic deg-radation is the most effective process in treating aliphatic organicse.g., hexane and aromatic petroleum hydrocarbons e.g., ben-zene, naphthalene in subsurface USEPA 2004a, while biodeg-radation of chlorinated hydrocarbons involves all three processesITRC 1998. Under anaerobic condition, reductive dechlorina-tion occurs, which involves replacement of chlorine atoms in thechlorinated organics by hydrogen. In this process, an electrondonor, either hydrogen gas or a precursor carbon compound, isrequired. Aerobic cometabolism involves a fortuitous degradationFig. 12. Schematic diagram of typical in situ bioremediation150 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wof chlorinated compounds by nonspecific enzymes intended tometabolize compounds, such as toluene and phenol. Direct deg-radation or oxidation of less chlorinated organics can occur inanaerobic or aerobic environments ITRC 1998.Key Monitoring ParametersIn addition to assessing the progress and effectiveness of biore-mediation technologies, the monitoring parameters selectedshould also be able to examine the nutrients and/or oxygen deliv-ery and distribution in subsurface, occurrence of contaminant bio-degradation, and whether the subsurface condition is suitable forbioremediation ITRC 2004; USEPA 2004a. Table 7 summarizesthe key monitoring parameters of in situ bioremediation technolo-gies. To ensure that stable and balanced extraction and injectionflows are established for oxygen and/or nutrient delivery, volumeof water extracted and injected and water level should berecorded periodically. Monitoring of the general water qualityparameters, such as pH, temperature, and specific conductance isalso required to ensure a suitable subsurface conditions for biore-mediation. Since oxygen is a key element for aerobic biodegra-dation of petroleum hydrocarbons, dissolved oxygen, H2O2, orozone levels and distribution in aquifers should be measured.These monitoring data can also provide the information indicatingthe performance of the delivery system and the transmitted dis-tance of H2O2 or ozone in aquifers. Furthermore, measurement ofthe carbon dioxide level in the soil gas can show the occurrenceof biodegradation. If in situ bioremediation is applied for the deg-radation of chlorinated hydrocarbons, monitoring of the subsur-face redox sensitive parameters, such as redox potential, levels ofdissolved hydrogen, ferrous iron, sulfate, and methane is requiredto ensure the achievement of specific conditions for reductivedechlorination. Microbial community in aquifers should also beanalyzed to check if there are specific types of microbes andenough microbial population for reductive dechlorination. Moni-toring of the dechlorination indicating parameter i.e., chloride,and carbon isotope of the chlorinated contaminants and by-products can evince the occurrence of reductive dechlorination.Certainly, periodic monitoring of the concentration of petroleumand chlorinated hydrocarbons, and their by-products in ground-s for groundwater remediation adapted from USEPA 2004aE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Table 7. Summary of Key Monitoring Parameters of In Situ Bioremediation Technologies for Petroleum and Chlorinated Hydrocarbons Degradationand tuosphatns andns andns andants, chDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.water and soil is an obligatory step to assess the remedial progressand effectiveness of the in situ bioremediation technologiesITRC 2004; USEPA 2004a.Monitored Natural AttenuationMonitored natural attenuation MNA is a remediation approachrelying on natural attenuation processes for soil and groundwaterremediation in which the remediation performances are carefullymonitored. It can achieve site-specific remediation objectiveswithin a time frame that is reasonable compared to that requiredby other more active remediation technologies. Generally, sourcecontrol systems and long-term performance monitoring are thekey elements of any MNA remedy USEPA 1999. The naturalattenuation processes usually involved in the MNA include a va-riety of in situ physical, chemical, or biological processes, such asdispersion, dilution, volatilization, radioactive decay, and sorp-tion, as well as chemical, or biological stabilization, transforma-tion, or destruction of contaminants. These processes are capableof reducing the mass, toxicity, mobility, bioavailability, volume,or concentration of contaminants in soil or groundwater withouthuman intervention. The in situ processes, which only result inLocations/media ParametersGeneralExtraction and injection wells Extraction and injection volumesMonitoring wells Water levelsGroundwater pH, temperature, specific conductance,Groundwater Bio-nutrients, such as ammonia and phAerobic bioremediation of petroleum hydrocarbonsGroundwater Dissolved oxygen and redox potentialGroundwater H2O2 or ozoneGroundwater Concentration of petroleum hydrocarboby-productsSoil vapor Carbon dioxideSoil vapor Oxygen, H2O2, or ozoneSoil vapor Concentration of petroleum hydrocarboby-productsSoil Concentration of petroleum hydrocarboby-productsBioremediation of chlorinated hydrocarbonsGroundwater Concentration of chlorinated contaminby-products and final benign productsGroundwater Dechlorination indicating parameter eGroundwater Redox sensitive parameters, such as redissolved hydrogen and oxygen, ironImanaganeseII, nitrate, sulfate, and mGroundwater Electron donor parameters, such as chedemand, total organic carbon, volatilespeciated electron donorsGroundwater Respiration indicator, such as carbon dalkalinityGroundwater Microbial parameters, such as microbiand molecular parametersGroundwater Stable carbon isotopes of chlorinated cthe by-productsPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wreducing contaminant concentration, are called nondestructiveprocesses, whereas the processes result in actual reduction in con-taminant mass are named as destructive processes. Nowadays, itwas found that biodegradation is the major process to degradepetroleum-related contaminants, such as benzene, ethylbenzene,toluene, and xylene BTEX, and chlorinated solvents in subsur-face into innocuous products during the MNA. Degradation ofBTEX can occur under a variety of geochemical conditions butdegradation of chlorinated solvents can only happen at a specificcondition. Attenuation of the concentration of inorganics metalsor nonmetals is usually achieved by sorption reactions, such asprecipitation, adsorption on soil surface, absorption into soil ma-trix, or partitioning into soil organic matter. Oxidationreductionreactions can also transform the valence states of some inorganicsto less soluble and/or less toxic forms. Typically, MNA is used forlow contaminant concentration areas, whereas active remediationtechnologies are applied in high concentration areas, or MNA isused as a follow-up to the active remediation technologies.Key Monitoring ParametersIn comparison to other remediation technologies, performancemonitoring is more important for MNA because of the potentiallyPurposesBalanced and stabilized extraction and injection flowsDetermination of hydraulic conditionsrbidity Confirmation of suitable subsurface conditions forbioremediatione Performance of delivery systems and remedial progressPerformance of delivery systems on establishing aerobicenvironments and optimization of system operationsTheir transmitted distance in subsurface by the deliverysystemsthe Remedial progress/effectivenessEvidence showing occurrence of biodegradationPotential loss of injected oxygen, H2O2, or ozonethe Possible escape of petroleum contaminants to vadosezonethe Remedial progress/effectivenesslorinated Remedial progress/effectiveness, and extent ofdechlorinationoride Remedial progress and extent of dechlorinationtential, Suitability of subsurface conditions for reductivedechlorinationoxygencid, andMeasurement of availability and distribution of electrondonors in subsurfaceand Evaluation of areas of increased biological activitymunities Presence of sufficient or suitable microbes forbioremediationinants and Remedial progress and extent of dechlorination.g., chldox poI,ethanemicalfatty aioxideal comontamRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 151aste Manage. 2007.11:132-157.Table 8. Key Monitoring Parameters of Monitored Natural AttenuationonitoriDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.longer remediation timeframes, potential for ongoing contaminantmigration, and the absence of human intervention for the opera-tion. Typically, parameters to be selected for monitoring MNAremedial performance should be able to: Evince that natural attention is happening as expected; Detect any change in hydrologic, geochemical, and microbio-logical conditions, as well as other changes that may curtailthe efficacy of the natural attenuation processes; Identify any potentially toxic and/or other transformationby-products; Testify that contaminant plumes are not expanding, eitherdowngradient or laterally and vertically; Verify no acceptable impact to downgradient receptors; Detect the new release of contaminants to subsurface that canimpact the effectiveness of MNA; Demonstrate the efficiency of institutional controls e.g., state,LocationsPaInitial samplesUpgradient wells Contaminants, by-products, andfull suite of geochemicalparametersSidegradient wellsSource impact wells or monitoringwells within contaminant sourcezoneContaminants, by-products, andfull suite of geochemicalparametersDowngradient source impact wellsor monitoring wells within theplumeContaminants, by-products, andfull suite of geochemicalparametersMonitoring wells just and furtherdowngradient of the plumeContaminants, by-products, andfull suite of geochemicalparametersFarthest downgradient monitoringwellsContaminants, by-products, andfull suite of geochemicalparametersTable 9. Suite of Geochemical Parameters Required for Performance MGeochemical Parameters WaterDissolved oxygen Determination of metabolic pathway andIron IIITotal organic compounds Determination of extent of groundwater cCarbon dioxide Determination of bioactivity in aquifersNitrate Indication of respiration in the absence ofIron II Indication of anaerobic degradationChloride By-product from chlorinated solvent reduSulfate Indication of anaerobic microbial respiratiOxidation-reduction potential Indication of nature of degradationAlkalinity Indication of buffering capacityMethane MethanogenesispH Condition for some metabolic process toTemperature Indication of particular microbial speciesthe approximate degradation rateConductivity Water quality parameter152 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wfederal that were put in place to protect potential receptors;and Verify the attainment of the remediation objectives.Table 8 summarizes the key monitoring parameters of theMNA, and the suite of geochemical parameters needed to be mea-sured is listed in Table 9. During the performance monitoring,both contaminants originally present in the contaminated sites andtheir transformation by-products are needed to measure. This isbecause contaminant transformation processes may produce moretoxic, mobile, or recalcitrant by-products. Monitoring of thegroundwater and soil quality in the upgradient and sidegradientmonitoring wells can provide the background water and soil qual-ity data of the contaminated site for subsequent indication ofoccurrence of the attenuation processes. Monitoring of the con-centration of contaminants and the by-products in the upgradientand sidegradient wells can indicate if there is a plume expansion.rsPurposesSubsequent samplesertinent geochemical parameters Background water and soil qualityontaminants, by-products, andertinent geochemical parametersChanging source strengthontaminants and by-products Contaminant plume behavior overtimeontaminants and by-products Detection of plume migrationontaminants and by-products Compliance monitoringng of Monitored Natural AttenuationPurposesSoilvity Determination of bioactivity in vadose zonePrediction iron reductionnation Determination of extent of soil contaminationDetermination of bioactivity in vadose zonesn Electron acceptor for organic compounds oxidation undersome conditionsElectron donorBy-product from chlorinated solvent reductionElectron acceptor for organic compounds oxidation undersome conditionsIndication of nature of degradationIndication of buffering capacityCondition for some metabolic process to occururface and Indication of particular microbial species in subsurface andthe approximate degradation ratebioactiontamioxygectiononoccurin subsrametePCpCCCE WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.rmeabDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.Besides, periodic analysis of the soil and groundwater contami-nant concentration inside the contaminant source zone and thedowngradient plume can illustrate the change of source andplume strength over time during the MNA. These data can beused to compute field-scale attenuation rates and contaminantmass reduction so that the results can be compared with the de-sign specification to see whether MNA is performing as expected.An increase in the contaminant concentration or detection of newcontaminants at the monitoring points may indicate new releasesof contaminants to subsurface. Furthermore, periodic monitoringof the contaminant distribution can ensure that appropriateprogress is made toward contaminant reduction objectiveUSEPA 2004b. Contaminant concentration data from the moni-toring wells located just and further downgradient from the plumecan examine the possible migration of the plume to downgradientreceptors.In analogy to contaminants of concern, geochemical param-eters, as summarized in Table 9, are also needed to measurethroughout the plume to monitor the MNA performance. This isbecause changes in geochemical conditions may affect the micro-bial populations and consequently the transformation processesresulting in contaminant destruction. Another reason is that bio-logical degradation of contaminants can cause specific geochemi-cal changes in subsurface. Thus, monitoring of the geochemicalparameters during the MNA can establish a correlation betweencontaminant degradation and microbial activity. Besides,geochemical monitoring should also be implemented in the up-gradient and sidegradient monitoring wells to determine the rangeand variation in ambient conditions so as to differentiate thechanges in geochemistry related to natural attenuation or micro-bial processes from unrelated processes USEPA 2004b.Permeable Reactive Barrier TechnologiesPermeable reactive barriers PRBs or chemical reactive barriersare in situ and passive groundwater remediation technology. Un-like other physical barriers constraining plume migration, PRBsare designed as conduits for contaminated groundwater flow. Bydefinition, PRB involves emplacement of reactive materials insubsurface designed to intercept a contaminant plume, provide aflow path through the reactive media, and transform the contami-nants into environmentally acceptable forms to attain remediationFig. 13. Schematic diagram showing groundwater remediation by pePRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wconcentration goals downgradient of the barriers Fig. 13USEPA 1998. Currently, zero-valent metals, such as zero-valentiron Gillham and OHannesin 1992; Lai et al. 2006a, zero-valent tin Su and Puls 1999, and zero-valent zinc Arnold andRoberts 1998, bimetallic reactive materials Gavaskar et al.2000, nanoscale zero-valent iron Lien and Zhang 2001, andCercona iron foam Bostick et al. 1996 are the reactive materialsavailable for PRBs. When the contaminants are in contact withthe reactive materials, they are transformed into either benignproducts or immobile forms through the process of abiotic reduc-tion, reductive precipitation, or adsorption USEPA 2002; Laiet al. 2006a; Lo et al. 2006a.Key Monitoring ParametersPerformance monitoring of PRBs includes the evaluation ofchemical, geochemical, and mineralogical parameters in ground-water over time. Geochemical parameters to be measured gener-ally include pH, specific conductance, redox potential, dissolvedoxygen, hardness, alkalinity, total dissolved sulfide, ferrous iron,and dissolved hydrogen. These parameters can indicate the pro-ceeding of the degradation or removal processes exerted by thereactive media, and the extent of precipitate formation within thePRBs that eventually deteriorates the PRB performance overtime. For instance, application of zero-valent iron in the PRBs forthe removal of chlorinated solvent correspondingly results innegative redox potential, increase in pH and dissolved hydrogengas, and substantial drop in groundwater hardness and alkalinityLai et al. 2006a. Unlike performance monitoring of other reme-diation technologies sampling large water volume, relativelysmall volume of water is usually sampled within or around thePRBs during the monitoring because of a comparatively smallzone of the PRB within the aquifer USEPA 1998; Gavaskar et al.2000.As shown in Table 10, concentration of groundwater contami-nants and geochemical parameters are needed to be measuredfrom the monitoring wells located within and immediately down-gradient to the PRB reactive zone to confirm the proceeding ofthe degradation or removal processes and attainment of downgra-dient concentration goals. In analogy to MNA, monitoring of theconcentration of the transformation by-products generated insidethe reactive zone of the PRBs during the degradation processes isle reactive barriers PRBs From Lo et al. 2006b, with permissionRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 153aste Manage. 2007.11:132-157.Table 10. Key Monitoring Parameters of Permeable Reactive Barrier Technologiesrameteic by-trationing paityyationount aDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.also required because they may be toxic and harmful to downgra-dient receptors. The contaminant concentration profile obtainedalong the PRBs can subsequently be used to determine the fielddegradation rates, which can be used to check if the PRBs areperforming as designed and to assess the temporal change in thePRB reactivity. Contaminant concentration monitored from theupgradient monitoring wells can indicate if there is a sudden in-crease in the influent contaminant concentration, which may re-sult in the contaminant breakthrough inside the PRB. Hydraulicgradient, tracer movement, and groundwater velocity measuredover the PRB can be used to determine the actual extent of thehydraulic capture zone of the PRB and the actual residence timeinside the reactive barriers for contaminant removal, which sub-sequently can be compared to the designed specifications.Hydraulic conductivity within the PRB should be monitored pe-riodically to observe if there is a decrease in PRB permeabilityover time or even clogging of the reactive media. Sampling ofreactive medium cores from the emplaced PRBs for mineralogicaland microbiological analyses can provide useful information forassessing the effect from groundwater geochemistry and micro-bial activity on the PRB reactivity and longevity.PhytoremediationPhytoremediation is an emerging technology using various plantsto degrade, extract, contain, or immobile contaminants in soil andgroundwater. Contaminant degradation by phytoremediation in-volves the mechanism of rhizodegradation and/or phytodegrada-tion. Rhizodegradation is the breakdown of organic contaminantsin soil by the enhanced microbial activity in rhizosphere due tothe presence of root exudates and better aeration conditions. Phy-todegradation is the breakdown of contaminants absorbed by theplants through the metabolic processes, or outside the plantsthrough the compounds produced by the plants. Extraction ofcontaminants from soil and groundwater by plants involves themechanisms of phytoextraction, rhizofiltration, and/or phytovola-tilization. Phytoextraction is the process of uptaking contaminantsby plant roots and subsequent accumulation in the shoots andleaves of plants, whereas rhizofiltration is an extraction of con-taminants in solution surrounding the root zone by biotic or abi-otic processes in which contaminants are adsorbed, precipitatedonto plant roots, or absorbed into the roots. Harvesting the cropand the roots can ultimately remove the contaminants from con-Locations PaMonitoring wells located within orimmediately downgradient to the PRB reactivezoneContaminant and toxconcentrationsUpgradient monitoring wells Contaminant concenMonitoring wells located within orimmediately downgradient to the PRB reactivezoneGeochemical indicatMonitoring wells inside the PRBs Hydraulic conductivMonitoring wells inside and across the PRBs Hydraulic gradientMonitoring wells inside and across the PRBs Tracer movementMonitoring wells inside and across the PRB Groundwater velocitReactive medium cores from PRB Mineralogical informmediumReactive medium cores from PRB Heterotrophic plate cfatty acid profiles154 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVPract. Period. Hazard. Toxic Radioact. Wtaminated sites. Phytovolatilization is the uptake and transpirationof contaminants by plants, with release of the contaminants ormodified forms of the contaminants to the atmosphere. Contami-nant containment using plants either binds the contaminants tothe soil and renders them nonbioavailable, or immobilizes thecontaminants by removing the means of transport. Physical con-tainment of contaminants by plants involves binding of the con-taminants within a humic molecule, physical sequestration ofmetals or root accumulation in nonharvestable plants. Hydrauliccontrol of contaminant plume can be achieved by using plants toremove groundwater through uptake and consumption. Vegetativers Purposesproduct Remedial effectiveness, contaminantdegradation rates, and temporal changes in thePRB reactivitys Potential breakthrough of contaminantsrameters Remedial progress and potential impact ofprecipitate formation on the PRB effectivenessPotential clogging or decrease in the PRBpermeabilityHydraulic capture zone and residence timeHydraulic capture zone and residence timeHydraulic capture zone and residence timeof the reactive Potential impact of precipitate formation onthe PRB effectivenessnd phospholipid Microbial activityFig. 14. Degradation, extraction, and containment mechanismsgenerally involved in phytoremediation adapted from USEPA 2000E WASTE MANAGEMENT ASCE / JULY 2007aste Manage. 2007.11:132-157.Table 11. Summary of Key Monitoring Parameters of Phytoremediation TechnologiesDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.cover systems can minimize the water infiltrating into the sub-surface, thereby minimizing the plume migration driven by infil-tration. Fig. 14 summarizes the degradation, extraction, andcontainment mechanisms generally involved in phytoremediationUSEPA 2000.Key Monitoring ParametersClimatic factors are the monitoring parameters of phytoremedia-tion technologies see Table 11 since plant growth and some ofthe remediation processes are highly dependent on climatic con-ditions. For example, phytovolatilization processes are prone tobe affected by the temperature, precipitation, humidity, insolation,and wind velocity because these factors can affect the transpira-tion rate. The obtained climatic data can also indicate the neces-sity of appropriate maintenance actions, such as irrigation.Besides, visual monitoring of the plant characteristics, such asleaf mass, damages from insects and animals, can indicate if plantreplacement, fertilizer, and pesticide additions are needed. Mea-suring the plant tissue composition, transpiration gas composition,transpiration rate, and root density can provide the data for thedetermination of the amount of contaminants and by-products ex-tracted. Soil geochemical condition should be measured to deter-mine if it provides suitable condition for the plant growth andmicrobial growth in rhizosphere. Measuring the microbial popu-lation in soil within the rhizosphere can ensure that there aresufficient microbes for rhizodegradation of contaminants. Moni-toring data on the concentration of contaminants and the relevantby-products in soil and groundwater can be used to evaluate thecontaminants mass extracted, reduced, and/or remaining in sub-ParametersClimatic dataTemperaturePrecipitationRelative humiditySolar radiationWind speed and directionPlantsVisual characteristics viability, damage from insects oranimals, growth, leaf mass, etc.Tissue compositions roots, shoots, stems, leaves, etc.Transpiration gasesTranspiration ratesRoot densitiesSoilGeochemical parameters pH, nutrient concentrations, watercontent, oxygen content, etc.Microbial populationContaminant and breakdown product levelsGroundwaterAquifer information direction and rate of groundwater flowand depth to groundwater, etc.Contaminant and breakdown product levelsPRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Wsurface and thereby be capable of assessing the remediationprogress toward the remediation objectives. Groundwater flow-rate, and direction, and water level are the crucial factors affectingthe success of the hydraulic control of contaminant plume by thewater consumption, thereby requiring periodic monitoring. In ad-dition, in cases where contaminants are extracted to the edibleportions of the plants, such as leaves and seeds, monitoring of thefood chain for bioaccumulation of the contaminants is neededUSEPA 2000.SummaryPerformance monitoring is an obligatory component of site reme-diation projects since it provides the data for assessing whetherthe remediation technologies are proceeding as expected towardthe attainment of remediation objectives and if a contingency planis required to implement for further site treatment. Site managersor engineers also decide the termination of the remediationprojects based on the long-term monitoring data. Generally, per-formance monitoring involves assessment of the effectiveness andefficiency of the applied remediation technologies. Effectivenessrefers to the capability of the remediation technologies to meetremediation objectives at contaminated sites. On the other hand,the efficiency refers to the optimization of the time, energy, andcosts expended toward the attainment of remediation effective-ness in which it is typically assessed by comparing system oper-ating parameters to the relevant design specifications. To assessPurposesMaintenance requirements e.g., irrigationDetermination of water balance and evapotranspiration rateMaintenance plant replacement, fertilizer, pesticide application, etc.Quantification of contaminants and by-products extractedQuantification and/or prediction of system operationOptimization of vegetative, root, or microbial growthDetermination of water balance and evapotranspiration ratesRemedial progress/effectivenessQuantification of contaminants and by-products removed andremainingQuantification and/or prediction of system operationRemedial progress/effectivenessQuantification of contaminants and by-products removed andremainingQuantification and/or prediction of system operationRADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 155aste Manage. 2007.11:132-157.the remediation effectiveness and efficiency, each remediationtechnology has its own set of monitoring parameters. Generally,isotopic fractionation during reductive dehalogenation of chlorinatedethenes by metallic iron. Org. Geochem., 308, 755763.Downloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.concentration of contaminants and/or the relevant transformationby-products in groundwater, soil, and/or soil gas are the overrid-ing monitoring parameters being measured periodically sincethese monitoring data can be directly used to assess the remedia-tion progress, contaminant mass reduction, and/or the decrease intoxicity, mobility, and/or mass flux of contaminant sources.Specifically, P&T systems require monitoring of the inwardand upward hydraulic gradient toward the extraction wells to en-sure the successful control and capture of the contaminant plumefor aboveground treatment. SVE and AS systems require moni-toring of the vacuum pressure and the sparging pressure appliedto the extraction wells and sparging wells, respectively. Periodicrecording of the injection and extraction flow rates is needed forthe surfactant/cosolvent flushing systems to ensure the propersweep of contaminant zones and extraction of the elutricate. Re-garding in situ thermal remediation systems, temperature distribu-tion in the subsurface is a unique parameter needed to measure tocheck the heating progress and completeness. When ISCO is ap-plied for soil and groundwater remediation, monitoring of theconcentration of dissolved metals in aquifers, such as chromiumand arsenic, is necessary because of the possible mobilization ofinsoluble metals precipitated onto soil by the injected oxidants.During the in situ bioremediation, monitoring of the water qualityand redox sensitive parameters in groundwater is required to as-sure that the subsurface condition is favorable for biodegradationof contaminants. Besides, MNA requires monitoring of anychange in hydrologic, geochemical, and microbiological condi-tions in subsurface, which can affect the attenuation processes.Periodic monitoring of the residence time inside the reactivemedia for the contaminant degradation or removal and the reac-tive media permeability is necessary when PRBs are applied forgroundwater remediation. In addition, climatic factors are the spe-cific monitoring parameters for phytoremediation technologies.ReferencesArnold, W. 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Demonstration and validation of a water and solute flux mea-suring device. ESTCP CU-0114.Garabedian, S. P., LeBlanc, D. R., Gelhar, L. W., and Celia, M. A. 1991.Large-scale natural gradient tracer test in sand and gravel, Cape Cod,Massachusetts. 2: Analysis of spatial moments for a nonreactivetracer. Water Resour. Res., 275, 911924.Gavaskar, A., Gupta, N., Sass, B., Janosy, R., and Hicks, J. 2000. De-sign guidance for application of permeable reactive barriers forgroundwater remediation, Strategic Environmental Research and De-velopment Program, Battelle Columbus, Ohio, http://www.itrcweb.org/Documents/PRB-2a.pdf, http://www.itrcweb.org/Documents/PRB-2b.pdf, http://www.itrcweb.org/Documents/PRB-2c.pdf.Gillham, R. W., and OHannesin, S. F. 1992. Metal-catalyzed abioticdegradation of halogenated organic compounds. Proc., IAH Conf.Modern Trends in Hydrogeology.Heraty, L. J., Fuller, M. E., Huang, L., Abrajano, Jr, T., and Sturchio, N.C. 1999. 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M. 2006. Practical handbook of environmental site charac-terization and ground-water monitoring, 2nd Ed., CRC/Taylor &Franics, Boca Raton, Fla.Rao, P. C. S., Jawitz, J. W., Enfield, C. G., Falta, R. W., Annable, M. D.,and Wood, A. L. 2001. Technology integration for contaminatedsite remediation: Cleanup goals and performance criteria. Ground-water quality: Natural and enhanced restoration of groundwater pol-lution, IAHS, 275, 571578.Song, D. L., Conrad, M. E., Sorenson, K. S., and Alvarez-Cohen, L.2002. Stable carbon isotope fractionation during enhanced in situbioremediation of trichloroethene. Environ. Sci. Technol., 3610,22622268.Sturchio, N. C., Clausen, J. L., Heraty, L. J., Huang, L., Holt, B. D., andAbrajano, Jr., T. A. 1998. Chlorine isotope investigation of naturalattenuation of trichloroethene in an aerobic aquifer. Environ. Sci.Technol., 3220, 30373042.Su, C., and Puls, R. W. 1999. 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EPA/510/R-04/002Solid waste andemergency response 5401G, Washington, D.C.USEPA. 2004b. Performance monitoring of MNA remedies for VOCsDownloaded from ascelibrary.org by Iowa State University on 09/22/13. Copyright ASCE. For personal use only; all rights reserved.USEPA. 1989. Methods for evaluating the attainment of cleanup stan-dards. Volume 1 soils and solid media. EPA/230/02-89/042, Office ofPolicy, Planning and Evaluation, Washington, D.C.USEPA. 1992. General methods for remedial operation performanceevaluations. EPA/600/R-92/002, Robert S. Kerr Environmental Re-search Laboratory, Ada, Okla.USEPA. 1994. Methods for monitoring pump-and-treat performance.PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, ANDPract. Period. Hazard. Toxic Radioact. Win ground water. EPA/600/R-04/027, Ground Water and EcosystemsRestoration Division, National Risk Management Research Labora-tory, Office of Research and Development, Ada, Okla.VanStone, N. A., Fotch, R. M., Mabury, S. A., and Lollar, B. S. 2004.Effect of iron type on kinetics and carbon isotopic enrichment ofchlorinated ethylenes during abiotic reduction on Fe0. GroundWater, 422, 268276.RADIOACTIVE WASTE MANAGEMENT ASCE / JULY 2007 / 157aste Manage. 2007.11:132-157.

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