Detecting Organic Contaminants in the Unsaturated Zone Using Soil and Soil-Pore Water Samples
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HAZARDOUS WASTE & HAZARDOUS MATERIALSVolume 7, Number 2, 1990Mary Ann Liebert, Inc., Publishers
Detecting Organic Contaminants in theUnsaturated Zone Using Soil and Soil-Pore
Water SamplesK. W. BROWN, G. C. BARBEE, and J. C. THOMAS
Texas A&M UniversitySoil and Crop Sciences Department
College Station, TX 77843
H. E. MURRAYDepartment ofBiological and Environmental Sciences
McNeese State UniversityLake Charles, LA 70609
A lysimeter study was conducted to compare the effectiveness of soil coreand soil-pore water samples in detecting the movement of organic constituentsfrom land-treated industrial wastes. Lysimeters collected from the Bastrop (UdicPaleustalf), Padina (Grossarenic Paleustalf), and Weswood (Fluventic Ustochrept)soils were amended with a refinery separator sludge, a wood-preserving bottomsediment sludge, or a nonhalogenated solvent recovery sludge at rates of 50, 15,and 50 g kg , respectively. Soil-pore water samples from porous ceramic cupsand soil cores were collected monthly at three depths in the lysimeters tomonitor n-alkanes and polynuclear aromatic hydrocarbons from the petroleumwaste, phenols and cresols from the wood-preserving waste, and aromatic solventsfrom the solvent recovery waste.
The organic carbon normalized soil sorption coefficient (K ) may be usefulfor determining when soil-pore water or soil core samples will be most effectivein detecting organic chemicals in the unsaturated zone of soils. N-alkanes withlog K values < 4.4; PNAs < 3; chlorophenols (mono-, di-, tri-) < 4.0; nitro
-phenols (mono-, di-) < 2.3; and aromatics < 3.3, are best detected using soil-pore water sampling methods. N-alkanes with log K values between 4.8 and 6.2are equally detected by either sampling method. Otherwise, all these classesof chemicals with log K values greater than those mentioned are best detectedby obtaining soil cores.
Unsaturated zone monitoring systems are required at hazardous waste land-treatment facilities to determine if pollutants are moving beneath the treatmentzone. Either soil-pore water samples or soil cores are collected to monitorcontaminant migration into the unsaturated zone. A considerable amount ofinformation is available concerning the proper procedures to follow forinstalling soil-pore water samplers or for obtaining soil core samples (1,2),but information is lacking on the effectiveness of these two techniques indetecting organic chemicals in the unsaturated zone.
Often, porous ceramic cup samplers are the soil-pore water sampler ofchoice, primarily due to their ease of installation and operation, low cost, andability to repeatedly sample at the same location. However, little is knownabout how representative these samples are of the porous medium. There are
indications that the ceramic cup has a significant capacity to adsorb or excludeorganic chemicals, but the extent of sample alteration has not been determined.Another limitation to using porous cup samplers is that if a vacuum is appliedto obtain a sample, an unknown fraction of the organic chemicals of interest maybe lost due to volatilization (3)
An organic vapor trap similar to thatsuggested by Wood et al. (4) may then be required.
The physical and chemical properties of an organic chemical (ie., density,miscibility, viscosity, surface tension) may significantly differ from the soil-pore water. The pollutant could be unevenly distributed in the unsaturated zoneas: a separate immiscible phase, droplets, or thin films floating on the soil-pore water (5). In addition, macropores can act as isolated conduits to alloworganic chemicals to rapidly move to significant depths, and the porous cupsamplers may be completely bypassed in the process (3). Heterogeneitycharacterizes the soil-pore water environment and the porous cup samplerscapability to effectively obtain a representative sample under such nonuniformconditions is unknown. Additional difficulties are encountered when soil-porewater samplers are used during dry periods or in arid climates. Inadequatesample volume caused Law Engineering (6) to conclude that porous cup samplerswere not adequate for monitoring the unsaturated zone of a refinery land-treatment unit.
The distribution of an organic chemical in soil is often difficult todetermine due to the spatial variability of the soil. Thus, the size of a soilcore sample is an important consideration when designing a sampling plan. Theinterpretation of soil core sample data is also complicated by the presence ofboth soil and soil-pore water in the soil core because a high concentration ofchemical in a small quantity of soil-pore water may be significantly diluted bya large volume of soil which has a very low chemical concentration. Soil samplestherefore, are thought to be best suited for sampling compounds such as heavymetals and organic chemicals that are strongly adsorbed to the soil.
From the foregoing discussion, one realizes that there are limitations tousing soil core and soil-pore water sampling systems for monitoring thedistribution of organic chemicals in the unsaturated zone of soil. Thisfield study was undertaken to determine if soil-pore water samples, collectedusing porous ceramic cup samplers, and soil cores are effective in detectingorganic pollutants that are migrating in the unsaturated zone beneath industrialwaste amended soil.
MATERIALS AND METHODS
Thirty six undisturbed soil monoliths (200 L barrel lysimeters), 0.85 mtall by 0.57 m in diameter, were collected using the procedure of Brown et al.(7). Twelve lysimeters were collected from each of these three soils: Bastrop(Udic Paleustalf), Padina (Grossarenic Paleustalf), and Weswood (FluventicUstochrept). The physical and chemical properties of the soils are given inTable 1. The Bastrop soil is composed of 0.15 m of sandy clay loam overlying aclay subsoil with an abrupt boundary separating the two layers. The Padina soilhas a uniform sandy loam texture to the 0.9 m depth. The top 0.15 m of theWeswood soil is sandy clay, below which the silt and clay content increase,causing it to be classified a clay loam soil.
Each lysimeter was equipped with three porous cup suction drains in thebottom (Figure 1) to maintain a vertical hydraulic gradient in the monolith.Before waste application, 0.15 m of soil was removed from the top of eachlysimeter to allow installation of side-wall flow barriers. The hydraulicintegrity of each lysimeter was tested by measuring the breakthrough of a 200 mgL" bromide solution to assure the absence of any sidewall or other bypassflow (7).
Three wastes were selected for study and included an API separator sludge(API) from a petroleum refinery, a wood-preserving bottom sediment sludge (WPW)from a wood treatment facility using pentachlorophenol and creosote, and asolvent recovery sludge (SRS) produced by reclaiming nonhalogenated solvent-containing wastes. The major chemical constituents in each waste are given inTable 2.
TABLE 1Physical and Chemical Properties of the Three Soils Used In
Sand Silt Clay ---
scl - sandy clay loam, c - clay, si = sandy loam,cl - clay loam.
sc = sandy clay,
Nine lysimeters of each soil were amended with the three wastes, with theremaining three lysimeters of each soil serving as controls. Waste applicationrates in the zone of incorporation (ZOI, 0.10 m) were 50 g kg for the API andSRS wastes and 15 g kg" for the WPW waste. A lower application rate was usedfor the WPW waste due to its higher toxicity. The loss of volatiles during wasteapplication were not measured, thus precluding a mass balance determination forthe chemicals.
ZONE OF WASTEINCORPORATION
Figure 1. Barrel lysimeter equipped with porous ceramic cup soil-pore watersamplers and suction drains.
TABLE 2The Primary Chemical Constituents in the API, SRS,
and WPW Wastes Used in this Study
WasteChemical(ug g-1) Concentration
WPW Waste Phenol 132-chlorophenol 12-nitrophenol 52,4-dime thylpheno12,4-dichlorophenolP-chloro-m-cresol2,4,6-trichlorophenol2,4-dinitrophenol4-nitrophenol4,6-dinitro-o-cresol 8Pentachlorophenol 44
SRS Waste Ethylbenzene 209.8Xylene 785.52-butoxyethanol 131.61,2,4-trimethylbenzene 47.3Acetophenone 245.62-phenyl-2-propanol 1,386.6Isophorone 112.3Naphthalene 17.6
* ND not detected.
Natural rainfall was supplemented with applications of stored rainwater toprovide sufficient moisture to cause the contaminants to move through themonoliths. The total depth of water reaching the lysimeters during the 8 monthstudy was 0.99, 1.19, and 1.13 m for the Bastrop, Padina, and Weswood soils,respectively. These rates were the maximum amount possible without incurringcontinuous saturated conditions.
The porous cup samplers were constructed using 0.062 by 0.048 m diameterceramic cups (Soil Moisture Corp., model 653X1-B2M2) epoxied to the appropriatelength of 0.038 m diameter PVC pipe and endcap. Nylon tubing, sufficiently longMention of trade name does not constitute endorsement.
TABLE 3GC/MS and GC Operating Conditions
Parameter GC/MS GC
Fused silica25 m x 0.2 mm ID5% phenylmethyl silicone0.33 urn film thickness(Hewlett Packard)
Helium, 1 cc/min
Fused silica30 m x 0.53 mm ID100% methyl polysiloxar.31.5 urn film thickness(DB-1, J&W Sei. , Inc.)
Helium, 13.2 cc/min
to reach the inside tip of the ceramic cup, was passed through a hole in the PVCend cap and epoxied into place. The samplers were installed vertically in thelysimeters with the ceramic cups 0.25, 0.41, and 0.61 m below the soil surface(Figure 1). Once installed, a soil slurry followed by finely sieved soil wasused to backfill between the sampler and the surrounding undisturbed soil. Apolyethylene collar, taped around the sampler tube at the 0.15 m depth,prevented side-wall flow and thus sample contamination.
Soil core and soibpore water samples were taken at one month intervals.Soil-pore water samples were collected by drawing a small vacuum on the porouscup samplers for 20 min. The water samples were sealed in 0.06 L bottles with aminimum of headspace and frozen until extraction. Soil samples were taken usinga 0.018 m diameter hollow tube hand auger (SOILTEST, Inc., Evanston, IL). Loosesoil was prevented from falling down the sample hole and contaminating deepersamples. Between samples, the auger was washed with high pressure spray,scrubbed with acetone and rinsed with distilled water to prevent crosscontamination. Each sample hole was plugged with a stoppered PVC pipe toprevent bypass flow and resampling at the same location. Soil samples weretaken from the 0.2-0.3, 0.36-0.46 and 0.58-0.64 m depths which correspond to the0.25, 0.41 and 0.61 m deep soil-pore water samples. The soil cores were placedin sealed, solvent washed soil moisture cans and kept frozen until extraction.
Soil-pore water samples were extracted using the procedure of Giam (8).Soxhlet extraction of soil samples was accomplished using USEPA Method 3540 (9)with a 2.33:1 dichloromethanermethanol (v:v) solvent mixture. Identification ofthe major chemicals in the waste, soil, and soil-pore water was accomplishedusing a Hewlett-Packard model 5880 gas Chromatograph (GC) interfaced with a massspectrophotometer (MS) (model 5970). The chemicals were quantified with a Tracormodel 560 GC. The GC-MS and GC operating conditions are given in Table 3.
RESULTS AND DISCUSSION
Corresponding soil core and soil-pore water samples often did not havedetectable concentrations of a particular chemical. Therefore, to meaningfullycompare the sampling methods, data interpretation was limited to paired data.Paired data consists of a chemicals concentration in parallel soil core andsoil-pore water samples on a given date and from the same depth in a lysimeter.Paired data allows a direct comparison to determine if a particular organicchemical is equally detected by both methods, or if, due to such factors as thechemicals sorption characteristics, the chemical is more effectively sampled byone sampling method over the other.
The organic carbon normalized soil sorption coefficient (K ) has beenemployed as a means to translate the soil or soil-pore water sampling resultsinto a practical procedure for selecting which sampling method would be mosteffective for detecting an organic chemical or class of organic chemicals in theunsaturated zone beneath a land-treatment facility. The sorption of organicchemicals by soil is highly correlated with the soil organic carbon content,upon which K is based, and much less on other soil properties such as the typeand amount of clay, soil pH, hydrous oxide content, etc. (10). Therefore, thesoil organic carbon content is usually the major factor influencing the sorptionof an organic chemicals, which directly affects whether or not the chemical willbe effectively sampled by the soil core or soil-pore water sampling method.API Amended Soils
Paired data from the Bastrop and Padina soils showed that nonane and decanewere detected 80 and 88 percent of the time, respectively, in the soil-porewater (Table 4). The next three longer n-alkanes (undecane, dodecane, andtridecane) were detected about equally in either the soil cores or soil-porewater. For those n-alkanes longer than tridecane, 77 to 100 percent of thedetections were in the soil core samples only. These results indicate that n-alkanes with ten or fewer carbons (log K < 4.4) will be best detected bycollecting a soil-pore water sample. N-alkanes with 11 to 14 carbons (log K4.8 to 6.2) are about equally detected by either soil core or soil-pore watersamples and n-alkanes with greater than 14 carbons (log K > 6.2) are bestdetected by taking soil cores.
In the Weswood soil, measurable amounts of n-alkanes were found only in thesoil core samples (Table 4)
This may be due to their bypassing the porous cupsamplers by moving through macropores in this structured soil, or theirincreased adsorption due to the Weswood soils higher organic carbon content.
The mean concentrations of n-alkanes in the soil core and soil-pore watersamples in the three soils follows the same trend as for the frequency ofchemical detection in the samples (Table 5). In the Bastrop and Padina soils,the mean concentrations of the shorter n-alkanes, which were detected most oftenin the soil-pore water samples, were higher in the soil-pore water than the soilcores. As the carbon chain length increased, the mean concentrations of the n-alkanes in the soil core samples increased while those in the soil-pore watersamples decreased. In the Weswood soil, no n-alkanes were observed in the soil-pore water samples and the concentrations measured in the soil cores showed noapparent pattern (Table 5) . The paired samples from the two sampling methodswere infrequently statistically different due to the large variability in thesample concentrations, but when they were significantly different (p = 0.05) thedata support the previous observations. No soil-pore water or soil core samplesfrom the controls contained measurable amounts (i.e. > 1 ug L" or ug kg" ) ofany of the n-alkanes. Nonane, decane, undecane, tricosane and tetracosane werenot detected in the original waste and none was found in the control soils;therefore, concentrations of these n-alkanes measured in the soil-pore and soilcore samples are thought to have resulted from the degradation of longer n-alkanes present in the waste.
In all three soils, with the exception of naphthalene, PNAs from the APIwaste were detected almost exclusively, 89 to 100 percent of the time, in thesoil cores. Naphthalene, a two ring PNA with a considerably higher watersolubility, was detected 86% of the time in the soil-pore water samples (Table6). PNAs with a log KQC value > 3.3 have a greater affinity for the soil thanthe soil-pore water, and thus soil core samples are most effective formonitoring their movement in soils.
Dragun and Helling (11) observed that the strength of chemical binding tosoil may be influenced by the inherent aliphatic or aromatic character of thechemical. With water solubilities (Sw) less than about 3.6 mg L , whichincludes all the n-alkanes and PNAs (except naphthalene) monitored in thisstudy, the aliphatics (i.e., n-alkanes) are considerably more mobile than thearomatic chemicals (i.e., PNAs) at the same water solubility. Since K is
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In the few soil-pore water samples from the Padina and Weswood soils inwhich PNAs were detected, the mean concentrations were less than 1.0 ug L"(Table 7). In the Bastrop soil, the mean concentration of fluorene,phenanthrene, fluoranthene and pyrene in the soil-pore water samples ranged from1.8 to 14.9 ug L" and did not differ significantly (p - 0.5) from thecorresponding soil core samples (Table 7)
The mean concentrations of PNAs inthe soil cores from the three soils were typically significantly greater (p -0.05) than the mean concentrations in the soil-pore water samples. PNAs weresometimes detected only in the soil core samples but the mean concentrationswere not significantly different (p 0.05) from those in the soil-pore watersamples due to concentration variability.WPW Amended Soils
Due to the lower application rate for the WPW waste, few of the soil coreand soil-pore water samples taken in the WPW amended soils contained measurableamounts of WPW constituents, which made it difficult to obtain enough paireddata to adequately compare them. Phenol, 2-nitrophenol, 2,4,-dichlorophenol, p-chloro-m-cresol and 2,4,6-trichlorophenol were best detected by taking soil-porewater samples (Table 8). The chemicals 2,4-dimethylphenol, 2,4-dinitrophenol,4,6-dinitro-o-cresol, and pentachlorophenol (PCP) were best detected by takingsoil cores.
In addition to the aromatic ring, the phenols and cresols have functionalgroups on the aromatic ring which also affects their mobility in soils. For thisreason, it is difficult to determine when a soil-pore water or soil core samplewould be most appropriate based upon K alone without also knowing whichfunctional groups are present on the chemical.
Harter (13), and others, observed that the major portion of the surfaceinteraction of organic molecules with clay minerals can be attributed tofunctional groups containing nitrogen and oxygen. In this study, thosechemicals containing two NO2 groups appeared to be preferentially adsorbed tothe soil and thus better detected using soil core samples (Table 8). Thus,nitrophenols and cresols with log K values < 2.3 are best detected by takingsoil-pore water samples and those with log K > 2.3 by taking soil cores.
With the chlorophenols, additional chlorine substitution on the ringdestroys the retardation effects of the aromatic ring (11). Thus, a chlorophenolwould tend to be partitioned into the soil water and best detected using soil-pore water samples. This was true for the mono-, di-, and trichlorophenols(Table 8).
In the case of the highly chlorine substituted PCP, Dragun and Helling (11)observed that chlorine substitution in the meta position of the ring can causesignificant retardation of PCP movement in soil. However, increasing chlorinesubstitution around the aromatic ring results in decreasing the pK of PCP whichwould cause it to exist as an anion, and thus have some degree of mobilityenhancement caused by the dissociated hydroxyl group. PCP was mostly adsorbed tothe soils in this study and thus was best detected using soil core samples.Therefore, those chlorophenols with log K values < 4.0 are best detected bytaking soil-pore water samples and those with log K > 4.0 by soil coresamples.
The mean concentrations of chemicals in the soil-pore water and soil coresamples from the three WPW amended soils did not significantly differ (p =0.05), even when no chemicals were detected in one of the pairs, due toconcentration variability (Table 9).
SRS Amended Soils
Acetophenone, xylene, and 1,2,4-trimethylbenzene were detected from 69 to83 percent of the time exclusively in the soil-pore water samples (Table 10)
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The remaining SRS chemicals: Isophorone, 2-butoxy-ethanol, ethylbenzene, and 2-phenyl-2-propanol were about equally detected in the soil-pore water or soilcore samples.
Several of the aromatic solvents in the SRS waste (ketones and alcohols)have relatively high water solubility or low soil adsorption which causes themto be partitioned into the soil-pore water and therefore have a higherprobability of being detected by soil-pore water sampling methods. Those SRSwaste constituents with higher K values (i.e., ethylbenzene, trimethylbenzene,and xylene), which would tend to be adsorbed to the soil (12), were probably atleast partially dissolved in the other solvents present which caused all the SRSchemicals to be ubiquitous in the soil profiles. Thus, when soil cores weretaken, due to the chemicals presence in the soil-pore water within the soil, alarge percentage of the paired samples had chemicals in both the soil core andthe soil-pore water (Table 10)
Had the SRS waste been applied at a lowerloading rate or the experiment been conducted under drier soil moistureconditions, it is probable that a higher percentage of several of the SRS wastechemicals would have been found in the soil core samples.
The mean concentrations of paired soil-pore water and soil core sampleswere only significantly different (p = 0.05) when the SRS chemicals weredetected in the soil-pore water sample only and not in the soil core sample(Table 11).
Sample Concentration Variability
For all waste/soil combinations, the standard deviation of the chemicalconcentrations in either the soil-pore water or soil core samples was oftengreater than the mean value (Tables 5, 7, 9, and 11). Since analytical variabil-ity was accounted for, this indicates considerable heterogeneity of the chemicaldistribution within the soil profile. This heterogeneity may be attributable tononuniform chemical movement due either to the physical and chemical propertiesof the waste constituents, or to the soils physical properties. Such variabilitymust be incorporated into the design of and interpretation of results from soilsampling programs for organic chemicals.
The results from this study indicate that to effectively detect themovement of unknown organic chemicals or a wide spectrum of organic chemicals in
TABLE 12Selection of Optimum Sampling Method For Detecting the
Movement of Different Classes of Organic Chemicals in SoilsBased Upon Their Soil Sorption Coefficient (log K )*
PNAs < 3.3 > 3.3
Phenols/cresolschloro- (mono,di,tri) < 4.0 > 4.0
Methyl & nitro-(mono, di) < 2.3 > 2.3
Aromatics < 3.1 > 3.3
Log Koc = 3.95-
0.62 log Sw; Sw= water solubility (mg L"1);from Griffin and Roy (12).
the unsaturated zone of diverse soils, it is best, when possible, to use bothsoil core and soil-pore water sampling systems. If this is not feasible, or ifthe sampling is for a limited number of known chemicals, the organic carbonnormalized soil sorption coefficient (K ) may be used as a basis for choosingwhether soil-pore water or soil core samples will be needed (Table 12).
Contribution of the Texas Agricultural Experiment Station, Texas A&MUniversity, College Station, TX 77843. This work was funded in part byCooperative Agreement CR-810945-01 from the U.S Environmental Protection Agency.
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9. USEPA., Test Methods for Evaluating Solid Waste: Physical/Chemical Methods,2nd Ed. Office of Water and Waste Management, Washington, D.C. USEPA SW-846.(1982).
10. Hassett, J.J. and Banwart, W.L., The sorption of nonpolar organics by soilsand sediments. In: Reactions and Movement of Organic Chemicals in Soils.B.L. Sawhney and K.W. Brown, eds. Soil Science Society of America, Inc.Madison, WI. (1989).
11. Dragun, J. and Helling, C.S., Evaluation of molecular modeling techniquesto estimate the mobility of organic chemicals in soils: II. Water solubilityand the molecular fragment mobility coefficient. In: Land Disposal:hazardous waste. Seventh Annual Research Symposium. USEPA MunicipalEnvironmental Research Lab., Cincinnati, OH. EPA 600/9-91-002b. (1981).
12. Griffin, R.A. and Roy, W.R., Interaction of Organic Solvents with SaturatedSoil-water Systems. Environmental Institute for Waste Management Studies,
University of Alabama. Open File Report No. 3, The University of Alabama,P.O. Box 2486, University, Alabama 35486. (1985).
13. Harter, R.D., Reactions of minerals with organic compounds in the soil. In:Minerals in Soil Environments. J. B. Dixon and S. B. Weed, eds. Soil ScienceSociety of America, Inc. Madison, WI. (1977).
Mdress reprint requests to:
K.W. BrownTexas A&M UniversitySoil and Crop Sciences DepartmentCollege Station, TX 77843