Soil washing treatability tests for pesticide-contaminated soil

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Soil Washing Treatzibility Tests for Pesticide-Contaminated Soil Raymond M. Frederick S. Krishnamurthy R a y d M. Frederick is a pbysical scientist at EPA's Risk Reduction Engineering Laboratory, Releases Control Brancb in Edison, New Jersey. S. Krisbnamurtby is a senior cbemistJhm tbe Anrerlcan Association of Retired Persons wbo i s working at tbe Releases Control Brancb under tbe Senior Environmental Employment Program Tbey are members of a researcb group investigating new and innovative extraction tecbnologies for tbe remediation of contaminated soiL llje autbors wisb to acknowledge tbe contributions of tbe late Patrick Augustin, wbo was a cbemical engineer witb tbe Soil & Material Engineering Section and was responsible for tbe pilot-scale activities conducted witb tbe VRU Mr. Augustin served as EPA'S on-site project "anager for the VRU during tbe Sand Creek treatabiuty tests. or commercial products does not constitute endorsement or recommendation for use by tbe U.S. Environmental Protection Agemy. Mention of trade names m e 1987 Sand Creek Operable Unit 5 record of decision (ROD) identified soil washing as the selected technology to remediate soils con- taminated with high levels of organochlorine pesticides, herbicides, and metals. Initial treatability tests conducted to mess the applicability of soil washing technology did not effectively evaluate the removal of the elevated contaminant concentrations that were found. To further evaluate the applicability of soil washing at this industrialsite, a second more compre- hensive pilot-scale treatability test was conducted. Twenty-three test runs were conducted over a two-week period in late September 1992, using a pilot-scale soil washing device called the volume reduction unit ( W U ) . The experimental design evaluated the effects of two wash temperatures, two p H levels, three su$actants, four su factant con- centrations, and two liquid-to-soil ratios on the contaminant removal efficiency of the soil washing process. Site soils from layers at three different depths were used in the study. Results from thepilot-scale treatability test indicated that the WUcould achieve contaminant reduction giciencies of 97percent for heptachlor and greater than 91 percent for dieldrin in the uppermost contaminated soils (surface to l-ft. depth). Residual concentrations of heptachlor and dieldrin in the treated soil ranged from 50ppm to less than 1 .6ppm, and 6.8ppm to less than 1 .Gppm, respectively. However, the analytical method detection limit of 1.6 ppm was not low enough to provide residual concentration data at the risk-based action levels of 0.55ppm for hep- tachlor and 0.15ppm for dieldrin. The Sand Creek Superfund Site is the former site of the Colorado Organic Chemical Company, which manufactured pesticides during the 1960s. The 300-acre site is located in an industrial park five miles northeast of downtown Denver and was listed on the National Priorities List (NPL) in 1982. The Sand Creek site is slated for cleanup by the end of 1994. According to the 1990 record of decision (ROD), soil washing was the selected technology for the site. However, bench-scale studies did not effectively evaluate the removal of elevated contaminant concentrations of pesticides found at the site. Also, the remedial design (RD) for this site is complicated by three factors: 1. The action levels identified in the ROD are very low. 2. The volume of contaminated soil reported in the ROD is based on CCC 1051 -565W94IO40443-12 Q 1094 John Wiley & Sons, Inc 443 RAYMOND M. FREDERICK S. KRISHNAMURTHY sUCCe88fil application of soil washing technology to aparticular site depenrie on the phy8ical characteristic8 of the mil and the chemical nature of the contaminant. only 17 samples. 3. Cost figures provided in the ROD were not accurate because of uncertainty about the actual volume of soil to be remediated and the limited knowledge of soil washing costs at the time the ROD was prepared. To obtain additional information on the potential effectiveness of soil washing and to better predict the cost of remediation, EPA Region VIII personnel conducted a more comprehensive pilot-scale treatability test at the Sand Creek Superfund site. EPAs Office of Research and Development provided assistance to Region VIII by making available its pilot-scale soil washing unit from the Risk Reduction Engineering Laboratorys Edison, New Jersey, facility. TECHNOLOGY DESCRIPTION Soil washing is an aqueous process for removing contaminants by either solubilizing and suspending them in the wash solution or by concentrating them into a smaller volume of soil through particle size classification. Soil washers generally employ a combination of these techniques and auxiliary systems to perform wastewater treatment. The concept of soil washing is based on the finding that most organic and some inorganic contaminants tend to bind or adsorb, either physically or chemically, to the clay, silt, and humic components (fines) of the soil matrix. Washing processes scrub and separate the fines from the larger sand and gravel-size particles, effectively concentrating the contaminants into a smaller volume of soil. The fines are suspended in the wash slurry along with dissolved or solubilized contaminants. The highly contami- nated soil fines are recovered by gravity separation and flocculation units and are usually processed further by other technologies such as incinera- tion or solidification. The cleaned soil fraction (sand and gravel-size particles) are returned to the site as backfill material. In some cases, topsoil may be added to the backfill to promote revegetation of the site. Successful application of soil washing technology to a particular site depends on the physical characteristics of the soil and the chemical nature of the contaminant. Soil washing is most economical on sandy loam soil containing less than about 30 percent fines-i.e., particles less than 200 mesh (75 microns). The technology is applicable to a wide range of contaminant classes inchding volatile and semivolatile organics and metals. Complex mixtures of organics and metals usually require sequen- tial washing steps with surfactants and chelating agents or acid leaching. The Sand Creek soil characteristics provided a challenge to soil washing technology. Particle size distribution analysis determined the soil to be a clayey silt matrix with 33 percent less than 200 mesh (see Exhibit 1). Contaminants in the soils contained a complex mixture of organochlo- rine pesticides, herbicides, and metals. VOLUME REDUCTION UNIT PROCESS DESCRIPTION The volume reduction unit (VRU) is a mobile pilot-scale system for 444 REMEDIATION/AUTUMN 1994 Son. WASHING TREATABILITY TESTS FOR P E ~ T I C I D E - C ~ N T ~ A T E D SOIL ~ Exhibit 1. Particle Size Distribution by Soil Depth. Weight 010 Particle Size Mesh Size (-1 0 - 1' 1 - 3' 3 - 5' 0 - 3/8" > 9.5 0.00 0.00 0.00 3/8 - 4" 9.5 - 4.75 0.74 0.66 0.55 4 - 10" 4.75 - 1.70 1.94 1.88 1.74 10 - 40" 1.70 - 0.425 15.96 17.39 16.19 40 - 200" 0.425 - 0.075 46.58 48.87 46.39 200 - PAN < 0.075 34.78 31.20 35.13 conducting soil washing research and treatability studies. The system uses commercial washing equipment and support utilities mounted on two trailers. Nominal design capacity of the system is 100 pounds of dry soil feed per hour. The VRU is small enough to economically conduct a wide range of experiments, yet large enough to obtain cost and performance data of sufficient quality for scale-up. The VRU has a flexible design that allows the addition of new extraction units with minimal modifications and permits individual process units to be isolated or bypassed if necessary. The main or "process" trailer contains equipment for soil feeding, volatiles recovery, soil washing and screening, gravity separation, flocculation, and water clarification. An oil-fired boiler on the main process trailer supplies steam for the thermal desorption or steam stripping of VOCs. The utility trailer contains water storage tanks, pumps, a water heater, water filters, and carbon adsorption drums for recycling the process water. An air compressor, small electric generator, and equipment storage compart- ments are also on this trailer (see Exhibit 2). (The VRU is described in greater detail by Masters et al. [19911.) "REATABILMY STUDY VARIABLES The contamination levels at the site are highest in the surface soils. During remedial action (RA), the surface soils will be excavated along with some of the deeper soils to depths of 5 feet in some areas. Therefore, surface soils mixed with deeper soils will be somewhat diluted and will have lower contaminant concentrations than the soils that were ultimately used in this treatability study. The surface to 5-ft. depth soils are more representative of concentrations to be encountered during the actual remediation effort. However, only two soil samples from this depth range were used during the treatability test. Most of the soils selected for this study are representative of the uppermost contaminated areas (surface to 1-ft. depth) and were selected to provide information on a worst-case scenario. The treatability test was designed to evaluate the effects of the ~~ REMEDIATION/AUTUMN 1994 445 RAYMOND M. FREDERICK S. KIUSHNAMURTHY Exhibit 2. Typical VRU Field Setup. following set of process variables in remediating the Sand Creek soils: 1. Surfactant type: (a) Witcolate WAC-LA (this is a sodium dodecylsulfate compound (b) A mixture of 50/50 Adsee 799 and Witconol NP-100. (c) Tergitol NP-10. SDS is an anionic surfactant. Adsee 799, Witconol NP-100, and Tergitol NP-10 are nonionic. Tergitol is a product of the Union Carbide Company. The other surfactants are products of Witco Chemical Company. 2. Surfacmt concentration: Surfactant performance was evaluated at 0, 0.4, 1.0, and 1.5 percent by volume in water. referred to during the test program as SDS). 446 REMEDIATION/AUTUMN 1994 SOIL WASHING TREATABILITY TESTS FOR PE~TICIDE-CONTAMINATED Son, ~ Test r u m lasted two hours and were started once the system manager determined that the proper comistency and mixing action ezisted in the washer unit. 3. Wash water temperature: The effect of wash water temperature was evaluated at ambient 70F and at 130F. The water heater on the utility trailer was adjusted to heat the process feed water and monitored throughout the test period. 4. Wash water pH: The wash water pH was adjusted with sodium hydroxide to achieve a value of either 7 or 10 units as necessary. SOIL PROCESSING Contaminated soils were excavated from two 50 ft. x 50 ft. sampling grids. Three depth ranges (surface to 1 ft., 1 to 3 ft., and surface to 5 ft.) were excavated by a backhoe, mixed for four hours in a cement mixer, then sampled and stockpiled on polyethylene sheets in a staging area. Soils were manually sieved through 1/4-inch-mesh wire screens and stored in 8-gallon pails. The pails were weighed, recorded, and transferred to the bucket hoist on the VRU. As anticipated, chemical analyses of samples from the three soil depths revealed differences in contaminant concentrations. An average concentration of 25 ppm of dieldrin and heptachlor was targeted for the surface soils, but the actual concentrations varied considerably. At the start of each run, buckets were hoisted and dumped into the soil feed hopper. The screw feeder was set to convey soil to the washer unit at a rate of 100 pounds per hour. Water, surfactant, and caustic flows were set for the proper conditions for each run and mixed with the soil in the washer unit. The two vibrating screens contained 60-mesh and 200-mesh screen inserts. The washed soil slurry was divided into three streams consisting of coarse soil particles in the ranges of -1/4 inch to +10 mesh, -10 mesh to +60 mesh, and -60 mesh to +200 mesh, and a fourth stream consisting of fines at -200 mesh and wash water (see Exhibit 3). Most of the runs with 1 percent or more surfactant experienced foaming problems. After preliminary evaluations of several approaches to control the foaming, two silicone-based anti-foaming agents were selected to address this problem. SDS anti-foam from Witco Chemical and SAG Mark X from Union Carbide were selected. The SAG Mark X proved effective for all the surfactants and was used for the majority of the program. Dosage rate of anti-foam to surfactant varied from 20:l to 1OO:l. Test runs lasted two hours and were started once the system manager determined by visual observation that the proper consistency and mixing action existed in the washer unit, and that steady-state operation was achieved. Process readings were taken every 15 minutes. At 30-minute intervals, a feed soil sample, washed soil from the overflows of each screen, and the slurry underflow from the 200-mesh screen were collected, weighed, and recorded. The coarse soil sample was a composite of soils from the three coarse streams mixed in proportion equal to the mass ratio from each screen. An off-site laboratory measured the weight of the fines (soil material less than 200 mesh) by filtering the fines slurry from the screen underflow. The fines settling system and the water treatment system of the VRU were not evaluated as part of this project. Waste- water containing the soil fines from the process trailer was redeposited REMEDIATION/AUTUMN 1994 447 RAYMOND M. PREDERICK S. KRISHNAMURTHY Exhibit 3. Pilot-Scale Process Diagram. SCREENED FEED SOIL (0.2Sh/&~mm) T R O W E L SCREEN 2x1000 go1 FEED WATER TANK U.S. Environmental Protection Agency Risk Reduction Engineering Laboratory Edison, New Jersey on the Sand Creek site. In addition to the scheduled tests, several other runs were conducted. Two runs were duplicated to evaluate the repeatability of the process. A repeat wash was performed on the washed coarse soil from two runs to determine if double washing would substantially reduce contaminant residuals. Two runs were performed with a mixture of 66 percent SDS and 34 percent Tergitol, because initial results indicated SDS to be more effective in removing dieldrin, while Tergitol seemed more effective in removing heptachlor. And finally, a water rinse was performed on several washed coarse soil samples at the laboratory to determine any effect rinsing might have on decreasing residuals. 448 REMEDIATION/AUTUMN 1994 Son. WASHING TREATABILITY TESTS FOR PESTICIDE-CONTAMINATED Son. Exhibit 4. Summary of Results from the Sand Creek Site Treatability Test. Test Conditions Surfactant Temp. SOilDepth vs Run I O h OP PH Feet Ratio 1 2 11 12 6 7 23 3 8 9 10 17 4 5 18 18A 19 19A 20 20A 21 22 13 14 15 16 none none none none 0.4A 1.OA 1.OA 0.4s 1 .os 1.0s 0.4s 0.4s 0.4T 0.5T 1 .OT 1 .OT 1.5T 1.5T 1 .OT 1 .OT 1 .OM 1.OM 1 .os 1 .OT 1 .OT 1 .os ambient 130 130 130 130 130 130 130 130 130 130 130 1 30 130 130 130 130 130 1 30 130 130 130 130 130 130 130 7 10 10 7 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 7 10 10 10 10 10 0- 1 0- 1 0- 1 0-1 0-1 0-1 0-1 0- 1 0- 1 0- 1 0- 1 0- 1 0-1 0- 1 0- 1 0- 1 0- 1 0- 1 0- 1 0- 1 0- 1 0-1 1-3 1-3 0-5 0-5 6: 1 (9:U 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 (9:l) (9:1> 6: 1 6: 1 6: 1 6: 1 6: 1 6: 1 A = Adsee; S - SDS; T = Tergitol; M = SDS/Tergitol mix Heptachlor (mg/kg) Diefdrin (m- Feed Residual Rinse Feed Residual Rinse 220 87 150 170 180 230 230 100 200 270 260 22 220 220 120 1 20 250 250 64 64 460 210 63 120 8 12 50 29 37 34 26 25 15 24 36 28 34 12 27 30 18 16 20 12 14 16 22 19 16 1.8 2.0 4 . 6 19 4.6 16 2.4 24 4.5 23 4.5 17 6.8 7.9 18 2.9 4 . 6 18 5.6 19 2.0 7.2 17 4.4 4 . 6 27 3.9 25 4.9 2.8 1.5 18 5.0 20 5.0 17 13 1.7 5.0 16 1.8 18 3.2 18 2.0 19 1.9 19 1.6 20 20 2.6 2.5 13 17 3.8 1.8 9.7 1.9 6.2 17 2.2 4 . 6 2.7 4 . 6 3.4 4 . 6 TEST RESULTS The lowest residual concentrations of heptachlor and dieldrin obtain- able for the surface-to-1-ft. soils were 7.2 ppm and less than 1.6 ppm, respectively. Residual concentrations on the two samples representing surface-to-5-ft. soils were 2.0 ppm for heptachlor and less than 1.6 ppm for dieldrin for sample number 15, and less than 1.6 ppm for both contami- nants for sample number 16. Exhibit 4 provides a summary of the test results for 23 runs included in the treatability study. REMEDIATION/AUTUMN 1994 449 RAYMOND M. FREDERICK S. KRISHNAMURTHY The lower the contaminant concentration, the more resistance there is to be overcome by the extractive process. Temperature Because of unexpected program changes encountered during the start of the treatability test program, only one run was conducted at ambient temperature and was used as one of the control runs. Based on the contaminant residual levels from runs 1 and 12, the effect of elevated temperature alone (13OOF) on contaminant removal was insignificant. Heptachlor residuals dropped from 50 ppm to 34 ppm while dieldrin residuals remained unchanged at 4.5 ppm. PH The effect of pH on removal efficiency was inconclusive. Although a comparison of runs 2 and 12 indicates that increasing the wash water pH may improve removal of both heptachlor and dieldrin, this effect is difficult to interpret because the feed concentrations in run 2 were much lower than the feed concentrations in run 12. Run 11 was expected to give moderately better results than run 2 since it was performed at the same conditions except for a higher liquid-to-soil ratio, but it did not. Also, data from run 11 are identical to run 12, which had a lower pH and liquid-to-soil ratio. The data from runs 21 and 22 indicate that increasing the wash water pH may in fact encumber the removal of contaminants from the soils. However, it is easier to obtain higher removal percentage figures using samples with higher contaminant concentrations. The lower the contami- nant concentration, the more resistance (soil-chemical bonding) there is to be overcome by the extractive process. Liquid-to-Soil Ratio Increasing the liquid-to-soil (L/S) ratio from 6:1 to 9:l did not significantly enhance contaminant removal. Run 11 is not an improvement over the control runs and may be worse than run 2. A comparison of run 20 with the remaining Tergitol runs indicates that the results from run 20 are very similar to those for runs 18 and 19, particularly for dieldrin. The heptachlor results are difficult to interpret because of the variability in feed soil contaminant concentrations. Therefore, any improvement gained from increasing the L/S ratio could not be demonstrated by this test. Surfactant v p e and Concentration The addition of surfactant-improved removal efficiencies of both heptachlor and dieldrin. Increases in surfactant concentration from 0.4 percent to 1 percent seemed to enhance this effect. However, the overall effectiveness of the surfactant solutions in removing dieldrin from the soils was less pronounced than in the removal of heptachlor. The dieldrin results were less variable with residuals ranging between below detection limit (BDL) and 6.8 ppm for the runs with less than 1 percent surfactant, and between BDL and 5.6 ppm for runs with 1 percent surfactant. Removal of dieldrin was greatest using SDS surfactant. Percent removal figures for heptachlor were best with Tergitol surfactant, with residual concentrations in the range of 12 to 30 ppm. At elevated pH, results for the 34 percent Tergitol/66 percent SDS mixture at a surfactant concentration of 1 percent 450 REMEDIATION/AUTUMN 1994 Son. WASHING TRFATAEILITY TESTS FOR PESTICIDE-CONTAMINATED SOIL The results indicate that soil waahing could be a viable technology for the remediation of the industrial purposes. ~ site for future by volume were very similar to the results obtained with Tergitol and SDS surfactants independently at 1 percent. Double Wash To simulate a two-stage wash system, the washed coarse soils from runs 3 and 7 were retained and reprocessed through the VRU under the same conditions. Results from the two reprocessed samples 17 and 23 indicate that a 50 percent reduction in residual levels was obtained for heptachlor, 24 ppm to 12 ppm, using either SDS or Adsee surfactants. Dieldrin residuals were lowered from 2.0 ppm to BDL (1.6 ppm) with SDS, but increased from 2.9 ppm to 5.6 ppm with Adsee. Water h e A laboratory tap water rinse was performed on samples from runs 7, 8, 18, 21, and 22 to determine if residual concentrations could be lowered further. The water rinse would flush the surfactant, which contains solubilized contaminants, from the soil particles and was done to simulate a rinse cycle proposed for future installation on the VRU. Three of the five samples rinsed with laboratory tap water resulted in substantially lowered residual concentrations for both heptachlor and dieldrin. Reductions of 50 percent or greater over washed-only samples are typical with final contaminant residual concentrations for dieldrin at or below detection limit. Final heptachlor concentrations were 7.9 ppm, 7.2 ppm, and 13 ppm for samples 7 , 8 , and 22, respectively. Two samples displayed no difference in concentrations and are suspect for laboratory error. We assume that the rinse step was unintentionally omitted during processing. Mass Balance The contaminant mass balance results for both pesticides of interest was poor. Heptachlor balances ranged from 4 percent to 36 percent, while dieldrin balances ranged from 14 percent to 48 percent. Analyses of the runs using surfactants yielded widely varying mass balance results, mostly from 10 percent to 40 percent. The results for the control runs were better, ranging from 33 percent to 68 percent for heptachlor and 39 percent to 51 percent for dieldrin. The best mass balance results for the surface-to-1-ft. depth soils were obtained on run number 3 for heptachlor using SDS (36 percent) and run number 18 for dieldrin using Tergitol(48 percent). These values may indicate that surfactants in the wash water hindered analytical procedures for the soils. Low feed concentrations for the 0-5-ft. and 1-3- ft. soils prevented accurate contaminant mass balance calculations. CONCLUSION The results from this pilot-scale treatability test indicate that soil washing could be a viable technology for the remediation of the Sand Creek site for future industrial purposes. Test soils from surface-to-54. depth gave results below detection limits for both contaminants and are representative of what could be expected during full-scale remediation efforts. The study also indicated that for surface soils, a single washhinse REMEDIATION/AUTUMN 1994 45 1 RAYMOND M. FREDERICK S. KRISHNAMURTHY ~~ Exhibit 5. Results of Other Contaminants: Run No. 18. Parameter Feed Residual [mg/kgJ O/o Removal [w4xl a-BHC b-BHC d-BHC g-BHC (Lindane) Heptachlor Aldrin Heptachlor Epoxide Dieldrin 4,4'-DDE 4,4'-DDD 4,4'-DDT Methoxychlor Endrin Endrin Ketone Chlordane (Total) Toxaphene BDL = Below Detection Limit 14 BDL 37 120 200 BDL 13 2.6 8.1 9.6 42 22 BDL 23 41 BDL BDL BDL - 3.9 16.0 3.7 1.7 BDL BDL 6.5 5.5 BDL 4.3 - >88.0 >38.5 67.6 86.7 98.2 - 86.9 >80.2 >83.3 84.5 75 .O >93.0 89.7 - system can reduce residual levels of heptachlor and dieldrin to the range of 7 to 10 ppm and 1 to 2 ppm, respectively. These figures represent contaminant reduction efficiencies of 97 percent and 93 percent. A full- scale soil washing system of proper design should provide results exceeding those obtained in this pilot-scale treatability study. However, because the risk-based action levels for this site are an order of magnitude lower than the analytical method detection limit of 1.6 ppm, no definitive statement on the ultimate success of the technology at Sand Creek can be made. Although the focus of this work was primarily on heptachlor and dieldrin, satisfactory removal of other organic contaminants was also obtained. Some of the pertinent results are summarized in Exhibit 5. The development and/or enhancement of analytical methods to effectively isolate and quantify the contaminants in the presence of surfactant is necessary. Soils with a high percentage of silt and clay represent a challenge to current methods, as do surfactant-laden water samples. Percent recovery figures for spiked samples fell sharply when surfactants were introduced in the wash water. Sample preparation techniques such as solid phase extraction (SPE) and supercritical fluid extraction (SFE) should be investigated for use in this area (Farley et al., 1994). Son. WASHING TREATABILITY TESTS FOR PESTICIDE-COND Son. The wide variation in feed concentrations makes the interpretation of results difficult. Although soils were mixed for four hours, the effectiveness of the mixer proved to be insufficient. Improved mixing methods are needed to provide more consistent contaminant concentrations. The influence of some of the variables studied on the contaminant removal efficiency was not clear. The following theoretical considerations from the literature (Shinoda and Friberg, 1986; Somasundaran et al., 1992) should be taken into account in future work to determine optimum conditions for efficient removal of contaminants: 1. Temperature rise in a nonionic surfactant solution causes a shortening of the hydrophilic chain length of the surfactant, while the effect of temperature on an ionic surfactant solution is nominal and the size of the hydrophilic group is fixed. 2. In order to obtain substantial contaminant solubility, the selection of optimum hydrophilic chain length or optimum temperature is the most important factor for nonionic surfactant solutions. The hydrophilic-lipophilic balance (HLB) changes with the hydrophilic chain length and/or temperature. To change the HLB of ionic surfactants, a change in counter ions, the addition of salts, and the mixing of hydrophilic and oleophilic surfactants are effective. 3. Nonionic surfactants with long ethoxyl chains impart hydropho- bicity to silica surfaces, while shorter ethoxyl chains do not show this change in hydrophobicity (Somasundaran et al., 1992). BH( REFERENCES Acheson, E. and P. Augustin. 1993. Will Soil Washing Work for the Sand Creek Superfund Site? Proceedings, National Association of Remedial Project Managers Conference, Seattle, Washington, March 15-18, 1993. Ellis, W., J. Payne and D. McNabb. 1985. Treatment of Contaminated Soils with Aqueous Surfactants, ElJA/60O/S2-85/129. Environmental Protection Agency. 1990. Engineering Bulletin-Soil Washing, EPA/540/ Environmental Protection Agency. 1989. Guide for Conducting Treatahflfty Studfes Under CERCLA, Interim Final, EPA/540/2-89/058. Farley, J.A., G.B. Hunter and M.C. Crim. 1994. Technical Feasibility and Conceptual Design for Using Supercritical Fluid to Extract Pesticides from Aged Soil. Remedfatfon 4(3):301- 18. Masters, H., B. Rubin, R. Gaire, and P. Cardenas. 1991. EPAs Mobile Volume Reduction Unit for Soil Washing. Proceedings, 17th Annual Conference on Remedial Action, Treatment and Disposal of Hazardous Wastes. Cincinnati, April 9-11, 1991, pp. 98-104. Shinoda, K. and S. Friberg. 1986. Emukfons and Solubflization. John Wiley, New York. Somasundaran, P., E.D. Snell, E. Fu, and Qun Xu. 1992. The Effects of Adsorption of Nonionic Surfactant and Nonionic-Anionic Surfactant Mixtures on Silica-Liquid Interfacial Properties. CoIIotds and Surfaces, 6349-54.. URS Consultants. 1993. Iilot Scale Soil Washing Study-Sand Creek Superfund Site. Contract No. 68-W9-0053. Prepared for USEPA VIII, Hazardous Waste Management Division, Denver, Colorado. 2-90/017. _______ ~~ ~~ RJMEDIATION/AUTUMN 1994 453

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