remediation of a former dry cleaner using nanoscale zero valent iron

REMEDIATION Winter 2013

Remediation of a Former Dry CleanerUsing Nanoscale Zero Valent Iron

Michael Jordan

Nanjun Shetty

Matthew J. Zenker

Christopher Brownfield

Nanoscale zero valent iron (nZVI) was evaluated in a laboratory treatability study and subsequently

injected as an interim measure to treat source area groundwater impacts beneath a former dry

cleaner located in Chapel Hill, North Carolina (the site). Dry cleaning operations resulted in re-

leases of tetrachloroethene (PCE) that impacted site soil at concentrations up to 2,700 mg/kg and

shallow groundwater at concentrations up to 41 mg/L. To achieve a design loading rate of 0.001

kg of iron per kilogram of aquifer material, approximately 725 kg of NanoFeTM (PARS Environmen-

tal) was injected over a two-week period into a saprolite and partially weather rock aquifer. Strong

reducing conditions were established with oxidation–reduction potential (ORP) values below –728

mV. pH levels remained greater than 8 standard units for a period of 12 months. Injections resulted

in near elimination of PCE within one month. cis-1,2-Dichloroethene accumulated at high concen-

trations (greater than 65 mg/L) for 12 months. MAROS software (Version 2.2; AFCEE, 2006) was used

to calculate mass reduction of PCE and total ethenes at 96 percent and 58 percent, respectively,

compared to baseline conditions. Detections of acetylene confirmed the presence of the beta-

elimination pathway. Detections of ethene confirmed complete dechlorination of PCE. Based on

hydrogen gas generation, iron reactivity lasted 15 months. c ⃝ 2013 Wiley Periodicals, Inc.

INTRODUCTION

Site Setting

The site is located in the piedmont region of central North Carolina. The topography ischaracterized as relatively flat, sloping from the southeast to the northwest withapproximately 2 ft of relief. The site is at an elevation of approximately 455 ft above meansea level. No surface water features are on-site. The nearest surface water is a stream,Tanbark Creek, located approximately 0.5 mile north–northwest.

The site is underlain by saprolite extending to 15 ft below land surface (ft bls). Theupper 5 ft of saprolite is comprised of yellowish-orange and brown sandy silt (ML) thatgrades into silty fine and medium sands (SM). Saprolite transitions into partiallyweathered rock with grain size increasing with depth. The site-specific porosity insaprolite was measured as 35 percent with a specific retention of 14 percent. Specific yieldwas calculated as 21 percent. Bedrock, which is encountered at depths of 18 to 30 ft bls, iscomposed of granite (Bradley et al., 2004).

c ⃝ 2013 Wiley Periodicals, Inc.Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/rem.21376 31

Remediation of a Former Dry Cleaner Using Nanoscale Zero Valent Iron

Chlorinated ethenesdetected include PCE, tri-chloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride.

Shallow groundwater is at a depth of approximately 5 ft bls and flows to thenorthwest. The average hydraulic conductivity for the surficial aquifer based on slug testswas 0.13 ft/day. The average hydraulic conductivity for shallow bedrock was estimated tobe 4.7 ft/day. The average hydraulic gradient for shallow groundwater and bedrockgroundwater is approximately 0.025 and 0.026 ft/ft, respectively. Groundwater velocityin saprolite was calculated as less than 5 ft/yr.

Site History and Previous Work

The site was utilized as a dry cleaning facility from 1970 to 1974. Dry cleaning operationsused tetrachloroethene (PCE). The building was subsequently occupied by numerousbusinesses since 1974, and was most recently utilized as a fast food restaurant.

Previous site investigations identified subsurface soil and groundwater impactsbeneath the floor slab of the building and in the parking areas on an adjacent property.Chlorinated ethenes detected include PCE, trichloroethene (TCE),cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride. Shallow soil samples collectedbeneath the floor adjacent to the suspected dry cleaning machine locations hadconcentrations of PCE up to 2,700 mg/kg. The water table is approximately 7 ft belowfloor grade and groundwater was impacted with PCE at concentrations up to 41 mg/L.

Limited remediation was previously performed to address soil impacts and mitigatevapor intrusion. Approximately 101 tons of impacted soil including 90 tons ofcharacteristically hazardous soil was removed from beneath the floor slab of the building.Following excavation, a rubberized asphalt vapor barrier (Liquid Boot R⃝; manufactured byCETCO Liquid Boot Company, Hoffman Estates, Illinois) was applied above the existingsoil grade to limit migration of volatile organic compound (VOC) vapors from thesubsurface to indoor air. In addition, a subslab depressurization system was installedbeneath the newly constructed floor slab. The depressurization was initially operated inconjunction with a radon-type fan (i.e., low-vacuum) and subsequently upgraded to a soilvapor extraction system powered by a 10-horsepower blower. Groundwater impactsbeneath the building served as a continuing source of vapors. Additionally, impacted soiladjacent to the footing, which could not be excavated, also served as a source of vapors.

Technology Description

ZVI (Fe0) is a strong reducing agent which reacts with dissolved oxygen in water toproduce ferrous iron, hydrogen gas, and hydroxide ions. Ferrous iron and hydrogen arepossible reducing agents for chlorinated solvents (Zhang, 2003). nZVI is an engineeredremediation product that utilizes nanoscale (less than 10−6 μm) sized particles of ZVI.Because of its high reactivity and extremely small particle size, nZVI represents anextremely versatile remediation tool. It can be used to treat a wide range of recalcitrantcontaminants in both in situ and ex situ applications. For in situ applications, thenZVI-water slurry can be injected under pressure or by gravity into the treatment area.The particles are then transported by groundwater to establish a treatment zone. Thetechnology can effectively treat certain dissolved chlorinated solvents, such as PCE and itsdaughter products. Chlorinated ethenes, such as PCE or TCE, can be directly reduced toethene without forming common degradation products (e.g., cis-1,2-DCE and vinylchloride). The specific reductive elimination process relevant for ZVI and chlorinated

32 Remediation DOI: 10.1002/rem c ⃝ 2013 Wiley Periodicals, Inc.


ethene reactions is termed beta-elimination (Roberts et al., 1996). It works throughoxidation–reduction processes where the contaminant serves as the electron acceptor.Reductive elimination is the preferred pathway, as it can result in the most directdegradation of parent-chlorinated compounds (Liu et al., 2005). Additionally, theformation of hydrogen gas from ZVI can also facilitate abiotic reduction of chlorinatedethenes via hydrogenolysis (i.e., step-wise dechlorination: Tobiszewski & Namiesnik,2012).

The technology will also reduce other oxidized compounds (e.g., metals) in additionto the target-chlorinated ethenes. There are three pathways for reduction of chlorinatedsolvents using ZVI: (1) direct electron transfer to adsorbed halocarbon at the metal–waterinterface; (2) ferrous iron from ZVI reduction may dechlorinate chlorinated solvents; and(3) hydrogen from iron corrosion may react with chlorinated solvents if an effectivecatalyst is present (Matheson & Tratnyek, 1994). An added benefit of the nZVI is thepotential for stimulating the growth of anaerobic microbial consortia as a result of thestrongly reducing conditions and generation of hydrogen from iron corrosion.

Remediation Objectives

Remedial activities were initiated in response to significant concentrations of chlorinatedethenes beneath and adjacent to the former dry cleaner. This mass in the saturated zoneserved as a source of vapors that resulted in indoor air concentrations above applicablestandards. Additionally, the mass acts as a continuing source to the dissolved phasegroundwater plume that extends for approximately one half mile in length and results insurface water impacts over an additional 600 linear feet.

The nZVI injections aimed to: desorb and deplete residual sorbed mass in thesaturated zone; decrease dissolved phase concentrations of source area groundwater;establish and maintain reducing conditions in the aquifer throughout the targetedtreatment area; and implement the interim measures using a technology that producesrapid results. Additionally, the long-term goals of the nZVI injections are to stabilize theplume to control further migration and reduce the mass of CVOCs, such thatconcentrations below applicable groundwater standards can be achieved via naturalattenuation for the remaining dissolved CVOCs.

MATERIALS AND METHODS

nZVI Treatability Study

To evaluate the performance of nZVI in treating chlorinated solvents in groundwater, abench-scale treatability study was performed by New Jersey Analytical Laboratories-AFTConsulting (Pennington, NJ). A groundwater sample was obtained from monitoring wellMW-2 with a PCE concentration of 41 mg/L. The study involved setting up separatesacrificial treatment reactors with two nZVI doses, 5 g/L and 20 g/L, and a groundwatersample from the site. The noncatalyzed nZVI material was procured from LehighNanotech (Bethlehem, PA). The samples were incubated on a rotating table over thecourse of the specified time. At each sampling time, a set of reactors were sacrificed foranalysis.

The nZVI injections aimedto: desorb and depleteresidual sorbed mass inthe saturated zone; de-crease dissolved phaseconcentrations of sourcearea groundwater; estab-lish and maintain reducingconditions in the aquiferthroughout the targetedtreatment area; and imple-ment the interim measuresusing a technology thatproduces rapid results.

c ⃝ 2013 Wiley Periodicals, Inc. Remediation DOI: 10.1002/rem 33


nZVI was successful inrapidly reducing the chl-orinated solvents, PCE,TCE, cis-1,2-DCE, andvinyl chloride, to ethene.

Strong reducing conditions (oxidation–reduction potential of –387 to –519 mV)were developed after the addition of nZVI. The study results indicated that PCE and TCEwere reduced by over 84 percent and 98 percent, respectively, in 5 g/L nZVI dose at 28days. In the 20 g/L nZVI dose, PCE and TCE were reduced by over 99 and 100 percent,respectively, at 28 days. cis-1,2-DCE was reduced over 98 percent by 28 days in both nZVIdoses. Vinyl chloride was completely reduced after seven days. Ethene and ethaneconcentrations were 2,054 mg/L and 3,443 mg/L, respectively, seven days aftertreatment.

nZVI was successful in rapidly reducing the chlorinated solvents, PCE, TCE,cis-1,2-DCE, and vinyl chloride, to ethene. The use of nZVI will take advantage of thealready slightly reducing conditions and ongoing natural processes responsible forreducing PCE to TCE, cis-1,2-DCE, and vinyl chloride. The results further demonstratedthat beta-elimination was the dominant reaction pathway, as the formation of daughterproducts was not observed. The ability to minimize creation of PCE daughter productsand the rapid reaction rate make the use nZVI particularly suited for source areagroundwater treatment. Additionally, the use of uncatalyzed nZVI, which was successfulduring the bench-scale study, offers a significant cost savings over palladium catalyzednZVI.

Remedial Design

The nZVI injections focused on source area groundwater impacts in the saprolite (5–12 ftbls) and partially weathered rock (PWR) (12–22 ft bls) portions of the surficial aquifer.Loading results were designed to be consistent with literature recommendations andbased on the results of a laboratory treatability study. Based on previous experienceinjecting into similar lithology, each point was anticipated to accept up to 1,000 to 2,000gallons of injected solution (or the equivalent of an effective pore fraction of 20 percentfor screen lengths of 10–20 ft). Based on a design loading rate of 0.001 kg of iron perkilogram of aquifer material and an 8.5-ft radius of influence, a design injectantconcentration of 15 g/L to 25 g/L was calculated. An approximate injectantconcentration of 21.5 g/L was selected for injections to minimize precipitation and toallow for completion of injections within a reasonable time frame.

To maximize nZVI travel and dispersion and reduce agglomeration and subsequentprecipitation in the subsurface, the nZVI was in a suspension with a food grade inorganicmaterial. Additionally, injections were designed to be under low pressure (less than 5pounds per square inch) to facilitate optimal distribution while minimizing the likelihoodof daylightiing or fracturing. The Sidewinder tool (Wavefront Technology Solutions, Inc.,Edmonton, Alberta, Canada) was used at the wellheads of the vertical wells to facilitatemore rapid injection of the nZVI solution and to minimize daylighting. A potassiumbromide tracer was added to two injection wells to evaluate the solution distribution.

Well Network

To deliver sufficient iron to impacted areas, an injection network of permanent verticaland angled wells was used. Exhibit 1 depicts the site location on a topographic map andExhibit 2 presents the location of the injection monitoring well network at the site. Aseries of four, angle wells (MW-5 through MW-8) were installed in a north–south line



Exhibit 1. Site location diagram

adjacent to the western property boundary. The wells were installed at a 45◦ angle withthe well screens located beneath the former dry cleaner building. The wells weremanifolded to a subgrade piping network that extends to a single access point that permitsinjections to be conducted in a central, off-site location. Each well was isolated by a ballvalve. Two additional vertical injection wells (MW-27 and MW-28) were installed totarget mass outside of the building footprint. Injection wells were installed with amini-sonic drill rig. Prior to well installation, the boreholes were drilled to refusal, whichwas encountered at depths of 12 to 22 ft bls.

The monitoring well network used to evaluate the nZVI injections consisted of 7wells (MW-1, MW-2, MW-3, MW-20, MW-24, MW-25, and MW-26) spaced atdistances of 4 to 40 ft from the injection points (Exhibits 2 and 3).

Injection Procedures

A dilute nZVI suspension was delivered through a serious of vertical and angled injectionwells. The nZVI suspension was obtained from PARS Environmental (NanoFeTM,



Exhibit 2. Well location diagram

Robbinsville, NJ), which consisted of noncatalyzed ZVI nanoparticles. The nZVI materialwas shipped as a suspension in a food grade inorganic material.

A single nZVI injection event was implemented over a two-week period. The nZVIwas delivered to the site as concentrated slurry contained in 30-gallon plastic drums. Eachdrum contained 75 pounds of iron which equates to an nZVI concentration (weight ofiron per volume of solution) of approximately 499 g/L.

The solution was diluted on-site in 250-gallon polyethylene totes to an injectionconcentration of approximately 21.5 g/L using water collected from a nearby firehydrant. Once diluted, the injection solution was mixed via recirculation using apneumatic double diaphragm pump. After mixing, the injection solution was conveyed toinjection wells using a double diaphragm pump. Injection solutions were continuallyrecirculated during injection to allow the nZVI to remain suspended in solution. Pressuregauges were installed on injection manifold equipment, and were continually monitoredduring the injection activities. Groundwater elevations were gauged twice daily, beforeand after injections, from wells MW-2, MW-24, MW-25, MW-26, MW-27, andMW-28 during the injection period. Geochemical parameters (temperature, pH, ORP,specific conductivity, and dissolved oxygen [DO]) were also measured down well each daybefore/after injections and periodically during the day in wells MW-2 and MW-27.

The Sidewinder tool (Wavefront Technology Solutions, Inc.) was installed at verticalinjection wellheads (i.e., MW-27 and MW-28) to facilitate more rapid injection of thenZVI solution and to minimize daylighting. The Sidewinder was utilized for all subsequent



Exhibit 3. Well summary table

Well IDScreen(ft bls)

Diameter(inches) Orientation Status

Distance toInjection Well (ft)

nZVI Injected(lbs)

Monitoringwells

MW-1 12–17 2 Vertical Active 12 –

MW-2 6–11 2 Vertical Active 4 –MW-3 3–8 2 Vertical Active 30 –MW-20 36–41 2 Vertical Active 40 –MW-24 13–18 1 Vertical Active 6 –MW-25 7–12 1 Vertical Partially

Fouled8 –

MW-26 10–20 1 Vertical Active 7 –Injection

wellsMW-5 7–21 2 45◦ Active – 52

MW-6 7–21 2 45◦ Active – 248MW-7 7–21 2 45◦ Fouled – 614MW-8 7–18 2 45◦ Fouled – 100MW-27 12–22 2 Vertical Fouled – 97MW-28 12–22 2 Vertical Active – 489

ft bls: feet below land surface; ft: feet; lbs: pounds.

nZVI injections at the site. To aid in evaluation of injectant distribution, a bromide tracerwas added. Approximately 231 g of potassium bromide was injected in angled well MW-7and MW-67 g into MW-28.

RESULTS

Postinjection groundwater monitoring was performed at 1 week, 2 weeks, 1 month, 3months, 12 months, 21 months, and 2.5 years. A field analysis for total and dissolved ironwas also performed on samples from select wells during these sampling events.

Zone of Influence

The design injection volume and nZVI mass for each injection well are shown in Exhibit 4.The total planned nZVI injection mass was 1,600 pounds of nZVI. During injection,however, several difficulties were encountered that altered the actual volume/massapplied to each well. Specifically, injection into wells MW-5 and MW-6 resulted ininfiltration of the nZVI solutions into the building (daylighting). Well MW-27experienced daylighting in the vicinity of its well vault, and injection into well MW-8created a failure of the well seal, which caused injection solution to enter the well vault.As a result, the majority (69 percent) of the nZVI was injected into angled injection wellMW-7 and vertical injection well MW-28.



Exhibit 4. Summary of nZVI injection data

Total Volume (gallons) Mass of nZVI (lbs)

Well ID Design Actual Design Actual

MW-5 973 83 170 52MW-6 973 1,118 170 248MW-7 973 2,723 170 614MW-8 729 375 127 100MW-27 2,431 475 424 97MW-28 2,431 1,825 424 489

lbs: pounds.

Exhibit 5. pH values in source area monitoring wells

Even with the changes to the injection program, subsequent groundwater monitoringdata indicate that the injection solution was able to create desired geochemical conditionsthroughout the groundwater monitoring network. The zone of influence is most clearlyindicated by an analysis of pH and ORP parameters collected during postinjectionmonitoring. When present in an aqueous solution, corrosion of ZVI will create high pH(approximately 10) and very low (less than –500 mV) ORP values.

Exhibits 5 and 6 illustrate the pH measured in source area monitoring and injectionwells, respectively, before and after nZVI injection. ORP measurements in source areamonitoring and injection wells are displayed on Exhibits 7 and 8, respectively. ORP valuesdecreased over 700 mV in injection wells and remained depressed over two years afterinjection. Similar changes in pH were observed in injection well samples with increases of



Exhibit 6. pH values in source area injection wells

Exhibit 7. Oxidation–reduction potential (ORP) in source area monitoring wells

3 to 5 standard units (SUs). pH levels in injection wells were still elevated two yearssubsequent to injection.

The pH values in monitoring wells located within the injection area increased by anaverage of 2.5 units after injection, and remained elevated for approximately one year.Similarly, the ORP decreased in these same wells by an average of –400 mV, and remaineddepressed for over one year postinjection. These results confirm the impact of nZVIinjections within the target injection zone.



Exhibit 8. Oxidation–reduction potential (ORP) in source area injection wells

Groundwater migration was also evaluated through detections of the bromide tracersolution. Bromide was not detected in any wells during the baseline sampling event, butwas detected three months after injection in wells MW-2 and MW-24 through MW-28.Bromide was analyzed for and not detected in upgradient well MW-21 and side-gradientwell MW-3. Bromide was detected as far as 250 ft downgradient of the injections 15months postinjection. The bromide distribution confirms the results of the fieldparameters and verifies groundwater migration within the target injection zone.

Select monitoring wells were analyzed for the presence of hydrogren gas at 15 and 21months postinjection. Elevated levels of hydrogen gas were observed during the 15-monthevent in monitoring wells MW-2 (190,000 nM) and MW-26 (230,000 nM) whileMW-24 had little hydrogen gas (2 nM) (Exhibit 9). Wells MW-2 and MW-26 are locatedin close proximity to the injection wells that received the majority of the nZVI (i.e., wellsMW-7 and MW-28). The ORP and pH changes in well MW-24 were muted incomparison to other source area monitoring wells, which agrees with the depressedhydrogen gas concentrations that were observed. Additionally, reductions in chlorinatedethene concentrations in well MW-24 were less than those observed in adjacent wells.

Mass Reduction

Changes in molar chlorinated ethene concentrations in four source area monitoring wells,MW-2, MW-24, MW-25, and MW-26, are illustrated in Exhibits 10–13. Initially, themolar mass of total dissolved chlorinated ethenes increased following nZVI injection (e.g.,wells MW-2, MW-24, and MW-25). The increase in mass is indicative of desorption ofresidual dense nonaqueous phase liquid (DNAPL) from the aquifer matrix in response tochanges in equilibrium. Short-term increases in mass were quickly followed by significantreduction during the first one to three months following the injection.



Exhibit 9. Concentrations of dissolved gases

Well Sample Date Acetylene (μg/L) Ethane (μg/L) Ethene (μg/L) Methane (μg/L) Hydrogen (nM)

MW-2 15-month 41 1,200 1,400 1,000 190,00021-month NS 26 500 1,600 39

MW-6 15-month 1.9 300 310 1,500 75,00021-month 5.3 37 84 680 44,000

MW-24 15-month <0.500 1,300 1,800 1,700 221-month NS 320 850 660 2.8

MW-26 15-month 1.9 2,500 1,100 1,600 230,00021-month <1 1,400 2,300 1,300 110,000

MW-28 15-month 5.5 340 220 330 450,00021-month 38 360 560 920 250,000

𝜇g/L: micrograms per liter; nM: nanomolar; NS: not sampled. Sample date is months since injection.

Exhibit 10. Stacked molar concentration bar chart of chlorinated ethenes in well MW-2

Changes in dissolved phase mass were evaluated using MAROS 2.2 software and aresummarized in Exhibit 14. Mass was calculated based on concentration data from 14 wellsscreened in the saprolite and PWR zones portions of the source area aquifer.

Compared to baseline conditions, PCE and TCE mass were reduced by 96 percentand 89 percent, respectively, after 21 months. Conversely, cis-1,2-DCE and vinyl chlorideincreased 181 percent and 717 percent, respectively, during the same time period.




Exhibit 14. Mass of chlorinated ethenes pre- and postinjection

Constituent Baseline (kg) 21 months (kg) % Removal

PCE 1.1 0.046 96%TCE 0.21 0.023 89%cis-1,2-DCE 16 45 −181%trans-1,2-DCE 0.0022 0.0024 −9%Vinyl chloride 0.012 0.098 −717%Total 1.4842 0.6194 58%

kg: kilograms. Mass calculated using MAROS version 2.2 (AFCEE).

Despite increases in cis-1,2-DCE and vinyl chloride, nZVI injections still resulted in theelimination of 58 percent of the chlorinated ethene mass within the first 21 months.

Evidence for complete dechlorination was found with the generation of ethene.Elevated ethene concentrations (greater than 1,000 μg/L) were sustained in multiplemonitoring wells (MW-2, MW-24, and MW-26) at 15 and 21 months postinjection.During the injections, a subslab depressurization system was operated to mitigate vaporintrusion. Ethene was detected in the depressurization system vapor effluent at aconcentration of 23 μg/m3 at three months after injection.



Exhibit 15. Changes in chlorine number during treatability test

Degradation Pathways

The changes in CVOC concentrations indicate that the nZVI is stimulating degradationprocesses. Based on the increase in daughter product concentrations, it is evident thathydrogenolysis is the dominant degradation mechanism over beta-elimination. Thisobservance corroborates other practitioners (Henn & Waddill, 2006), who have statedthat beta elimination has not been observed under field conditions. This is inconsistentwith the results of the bench-scale treatability study, which demonstrated degradation viathe beta-elimination pathway (i.e., limited/no formation of daughter products). Exhibits15 and 16 illustrate the change in chlorine number versus time during the bench-scale andfield studies, respectively. During beta-elimination, the chlorine number does not changeappreciably during degradation, as all chlorinated compounds react simultaneously, albeitat different rates. During hydrogenolysis, the chlorine number will decrease, as thereaction pathway occurs in a stepwise fashion, with the parent molecules beingsequentially dechlorinated as the reaction progresses. As illustrated in Exhibit 16,degradation observed in the field is that of a step-wise dechlorination consistent withhydrogenolysis.

While degradation via hydrogenolysis was likely a combination of biotic and abioticprocesses, several lines of evidence support that initial reactions were likely abiotic. pHvalues in source area monitoring wells remained at elevated levels (above 8 SUs) that arenot conducive for dechlorinating bacteria for over three months to nearly a year.Degradation during this time period was responsible for near elimination of PCE and TCEwhile accumulating significant concentrations of cis-1,2-DCE and vinyl chloride.



Exhibit 16. Changes in chlorine number in monitoring wells

Subsequent degradation of cis-1,2-DCE and vinyl chloride was likely dominated bybiotic hydrogenolysis. nZVI reaction with water created substantial concentrations ofdissolved hydrogen gas (up to 450,000 nM) that are believed to have stimulated themicrobial populations. Dehalococcoides populations were 3.8E+05 and 2.8E+04 cells/mLat 15 and 21 months postinjection, respectively. Robust populations of Dehalobacter werealso observed at the 15-month sampling event (1.2E+04 cells/mL) and 21-monthsampling event (8.4+E03 cells/mL).

While beta elimination was not the dominant pathway, its presence was confirmedthrough the detections of acetylene (Exhibit 9). Acetylene was detected in monitoringwells MW-2 and MW-26, but not detected in MW-24 at 15 months postinjection. It wasalso detected in injection wells MW-6 and MW-28 at 15 and 21 months postinjection.

Aquifer Fouling and Distribution of nZVI

Injections of nZVI have the potential to agglomerate within the aquifer matrix as well as inthe well filter pack and inner casing annulus. Difficulty injecting nZVI into several wellsresulted in a disproportionate quantity of iron being concentrated in two wells (MW-7and MW-28). Well fouling was first observed at three months after injection in injectionwells MW-7 and MW-8. They remain inoperable three years postinjection. Injection wellMW-27 was fouled from three months through 15 months, was temporarily rehabilitatedthrough additional development, and subsequently became fouled again. Monitoring wellMW-25 was fouled beginning at one year and the well screen remained partiallyobstructed after three years.



Although nZVI injectionsachieved many of thestudy objectives, imple-mentation of the injectionand monitoring programwas not without chal-lenges. Effective deliveryof substrate is of thegreatest significance toany injection program.

Well fouling was generally limited to the injection wells and only occurred in oneadjacent monitoring well, MW-25. While MW-7 and MW-28 received the greatestamount of nZVI, only MW-7 fouled. Fouling appears to be limited to individual wellsrather than aquifer-wide fouling. This is supported by no change in groundwater flowpatterns as well as the results of slug tests performed prior to and after injections.Preinjection slug tests indicated an average baseline hydraulic conductivity of 3E-05cm/sec. Slug tests were repeated on source area monitoring wells one year after injection.Postinjection slug tests had an average hydraulic conductivity (7E-04 cm/sec) that wasconsistent with baseline conditions.

The fouling of wells provides a means for confirming the extent of nZVI distribution.Iron fouling in well MW-25 confirms the distribution of nZVI at a radius of at least 8 ftfrom the injection point.

Considerations for Future Use

Although nZVI injections achieved many of the study objectives, implementation of theinjection and monitoring program was not without challenges. Effective delivery ofsubstrate is of the greatest significance to any injection program. Given the nZVI potentialfor nZVI to agglomerate (as evidenced in the observed well fouling), injections should beperformed at the lowest possible concentrations that will still achieve the desired loadingrate. Our iron loading rate of 0.001 kg nZVI/kg soil confirmed literaturerecommendations needed to achieve mass elimination (e.g., 0.004 kg/kg; Gavasakar et al.2005). While iron concentrations of less than 25 g/L are necessary to avoid significantaquifer fouling, even lower concentrations of less than 10 g/L to 15 g/L will offerenhanced distribution with less fouling risk. The injection of chase water subsequent toinjections will further benefit iron distribution and minimize the potential for well fouling.

nZVI injections resulted in a strongly negative ORP that provides flexibility for futureremedial efforts, if needed. For example, aquifer conditions subsequent to the nZVIinjections would be optimal for “polishing” efforts via injection of a carbohydrate solutionto stimulate enhanced reductive dechlorination through biotic processes.

SUMMARY AND CONCLUSIONS

Injections resulted in nZVI distribution at least 8 ft from the injection points and reducingconditions throughout the target treatment zone, with no noticeable effect ongroundwater flow patterns. nZVI injections were responsible for noticeable changes ingeochemical parameters (pH and ORP) and the relative concentrations of chlorinatedethenes. The in situ degradation pattern of chlorinated ethenes did not progress asobserved during the treatability study. The degradation observed during the treatabilitystudy progressed predominantly via beta-elimination, as daughter product formation(TCE, cis-1,2-DCE, and VC) were not observed to accumulate. For the interim measures,the degradation pattern was consistent with hydrogenolysis, as significant amounts ofdaughter products were observed. Complete elimination of PCE and TCE was nearlyachieved by three months. Complete dechlorination was confirmed through elevatedconcentrations of ethene, which were sustained for over 1.5 years. Concentrations of1,2-DCE and vinyl chloride display a declining trend at 2.5 years postinjection.



It is possible that further abiotic degradation of the remaining chlorinated ethenes maycontinue. Ferrous iron that formed from the ZVI reduction may continue to dechlorinatethe remaining chlorinated ethenes. In addition, biotic degradation processes maycontribute to degradation, as geochemical conditions within the source area are favorablefor biological reductive dechlorination (i.e., low DO, low ORP, and a pH level near 7 SU).

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Michael Jordan, PG, is a senior geologist with Terracon Consultants, Inc. He holds bachelor’s degrees in

geology and science education and a master’s degree in hydrogeology from North Carolina State University. He

currently serves on an advisory board for the Water Resources Research Institute, which is funded by the United

States Geological Survey to support research related to water resources. He has experience with implementation

of various technologies including enhanced reductive dechlorination, soil vapor extraction, pump and treat, as

well as more innovative remedies such as nZVI and phytoremediation.

Nanjun Shetty, P.E., is a senior engineer at AECOM. He specializes in designing and implementing innovative

and cost-effective in situ remediation strategies. Shetty has authored/coauthored numerous publications on

application of various innovative technologies to remediate impacted soil and groundwater at industrial sites and

chaired technical sessions at international conferences organized by Battelle. He received his bachelor’s degree



in civil engineering from Bangalore University in India and his master’s degree in environmental engineering

from the University of Oklahoma in Norman, Oklahoma.

Matthew J. Zenker, PhD, P.E., is an environmental engineer at AECOM. He received his PhD in civil engi-

neering from North Carolina State University and is a board-certified environmental engineer in the American

Academy of Environmental Engineers. His professional experience includes site investigation, natural attenu-

ation, enhanced reductive dechlorination, in situ chemical oxidation, and engineering design of groundwater

remediation systems.

Christopher Brownfield, P.E., is an engineer at AECOM. He specializes in designing and implementing re-

mediation strategies for environmental media impacted with chlorinated solvents and petroleum hydrocarbons.

He earned bachelor’s degrees in chemical engineering and chemistry from North Carolina State University and

a master of science degree in soil science also from North Carolina State University.


remediation of a former dry cleaner using nanoscale zero valent iron

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