United States Environmental Protection Agency Risk Reduction Engineering Laboratory Cincinnati, OH 45268 Research and Development EPA/600/SR-92/170 October 1992 EPA Project Summary Soil Vapor Extraction Column Experiments on Gasoline Contaminated Soil Michael E. Miller, Tom A. Pederson, Carole A. Kaslick and George E. Hoag Soil vapor extraction (SVE) is a tech- nique that is used to remove volatile organic compounds from unsaturated soils. Air is pumped through and from the contaminated zone to remove vapor phase constituents. In this work, labo- ratory soil column experiments were conducted using a sandy soil residu- ally saturated with gasoline to evaluate the performance of SVE under con- trolled conditions. Both vapor extrac- tion and aqueous leaching of the soil columns were conducted. The progress of the vapor extraction event was con- tinuously monitored by an in-line total hydrocarbon analyzer (THA). Perfor- mance of vapor extraction was evalu- ated by a series of soil chemical analyses including total petroleum hy- drocarbons, headspace measurements, and extraction techniques with quanti- fication by GC/FID and GC/MS. Con- taminant levels in aqueous percolate were compared before and after SVE. After 60 pore volumes of water flow through a column, the percolate from the contaminated soil still contained at least 100 mg/L of total hydrocarbons. Vapor extraction of contaminated soil reduced total hydrocarbons by 99.96%, and subsequent aqueous leaching re- sulted in percolate concentrations of 3.7 mg/L initially and 0.6 mg/L after 60 pore volumes of water flow. This Project Summary was developed by EPA's Risk Reduction Engineering Laboratory, Cincinnati, OH, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction SVE is an innovative technology used to remove volatile compounds from un- saturated soils. The basic principles of SVE technology are straightforward. By inducing air flow from and through the subsurface vadose zone, vapor phase contaminants are flushed from the soil pores. Extraction wells are the usual route by which contaminants are recovered from the subsurface. The process consists of the following steps. Contaminant laden air is withdrawn through extraction wells under a vacuum created by an above-ground blower or vacuum pump. The air flow is controlled by ball or butterfly valves and is monitored by vacuum gauges. The extracted vapor stream passes through an air/water sepa- rator to remove moisture and protect the blower. After the blower, the air stream passes through a heat exchanger to con- trol the relative humidity and improve the efficiency of the subsequent vapor treat- ment operation. Injection wells are optional as a means to enhance air flow. A cap or surface seal placed over the treatment area to control the vapor flow path is also an optional component of the system. The seal, which could be as simple as plastic sheeting, serves to induce the air to flow in a horizontal manner as opposed to a vertical flow pafhway that may result from air being drawn from the surface only near the extraction well. Vapor extraction is most effective in re- moving compounds that exhibit significant volatility at ambient temperatures in soil (i.e., vapor pressure greater than 0.5 mm of mercury at 20°C and a dimensionless Gjj/0 Printed on Recycled Paper ------- Henry's Law constant greater than 0.01). This includes most gasoline constituents and solvents. Additionally, the mobility of the compound due to its affinity for the soil organic matter and small pore spaces affects the success of the treatment. Compounds strongly sorbed to the soil matrix tend to be more difficult to extract. Although SVE has been used with great success at numerous sites, the behavior of the contaminants remaining in the soil has not been completely assessed. The efficacy of SVE as a remedial technology for extraction of gasoline contaminated soils was investigated using laboratory scale soil column experiments. One di- mensional air flow rates were controlled within a small soil sample with distinct boundaries. Other parameters controlled Included moisture content, chemical con- taminants, and soil characteristics. The experimental program had three main objectives: * Determine the limitations on gasoline removal from soil using vapor extrac- tion under controlled, optimized labo- ratory conditions. • Evaluate the aqueous mobility of the constituents in contaminated soil and those that remain in the soil following SVE. • Assess the appropriateness of avail- able analytical techniques to measure son contaminant levels. Experimental Methods Stainless steel soil columns were 29.8 cm (1 ft) in length with a 10.8 cm (4.25 in.) internal diameter. A rigid, porous Teflon disk sat at the bottom of the column and supported the soil sample; a second po- rous disk rested on top of the soil. Stain- less steel endplates were bolted to either end of the columns, and Won* gaskets were used to create an air- and water-tight seal. The columns terminated at each end in needle valves, which could be closed to isolate the system. The internal volume of each column was 2.5 L The columns were weighed to the near- est gram following each manipulation. Four identical columns were packed with Con- necticut sand; 97% of those particles were between 0.1 mm and 5 mm in diameter, with an average diameter of 0.5 mm. The sand was predominantly quartz (35%) and feldspar (35%) and had a surface area of 1.9 m^g. As the soil was added to the columns in 2.5 cm lifts, the columns were vibrated to achieve maximum compaction. The process of soil addition and shaking were repeated until the columns were full (approximately 4.7 kg of soil). The in-place bulk density was 1.9 g/cm3, and the po- rosity was 30% as determined by mass difference (columns empty, packed with soil, and saturated with water). The packed columns were saturated with water containing 100 mg KCN/L to prevent microbial activity, drained to field capacity moisture content, saturated with gasoline, and allowed to drain to residual saturation. At various stages of the ex- periment, a column was opened, the soil was poured into a stainless steel bowl, transferred to sample vials and jars, and analyzed by the battery of techniques listed in Table 1. One column was sacrificed for soil analysis after residual saturation with gasoline. For headspace analysis, 1 g of soil was spiked with ~ 500 ppm of""a~,a,~d-~ trifluorotoluene in a 10 mL Teflon-lined, septum-sealed vial and was heated in a water bath at 90°C for 20 min. This treat- ment was assumed to displace all volatile compounds in the sample into the headspace. A 200-u.L sample of the headspace was injected directly into a GC/ FID to obtain a value for headspace vola- tiles. Chromatographic peaks eluting between 2-methylpentane and 1,2,4-trimethylben- zene, defined as the gasoline range or- ganic (GRO) compounds, were summed. The area of the internal standard was subtracted from this total area. The sub- sequent sample peak area was directly correlated to a five point calibration curve utilizing the peak area summation of a 10- compound mixture containing 2- methylpentane; 2,2,4-trimethylpentane; heptane; benzene; toluene; ethylbenzene; m-xylene; p-xylene; o-xylene; and 1,2,4- trimethylbenzene. Varying measured amounts of this mixture were added to clean soil, and 1 g portions were analyzed as above to create the calibration curve for soil GRO concentration. The final two columns were vapor ex- tracted simultaneously at a flow rate of 2.1 L/min (4.5 scfh). Air, supplied by com- pressed air cylinders, passed through an activated carbon column to remove hydro- carbons, and then through a flask of water via a diffuser stone. The humidified air split into two equal streams and flowed first through flow meters, then the soil columns. The column exhaust air flowed to a sequential sampler. The vapors from one column, then the other, were sent every minute to a THA for continuous monitoring of total hydrocarbon levels. After 6.5 days of vapor extraction, one column was sacrificed for soil analysis; the other received 60 pore volumes of rainwater flow as above. The soil from this final column was analyzed at the conclu- sion of the experiment. Results and Discussion Measurements are recorded for a soil column residually saturated with gasoline, a column following aqueous leaching, and a vapor extracted column. Multiple values for any measurement are replicate sample analyses. The gasoline retained in each column was measured directly by mass difference. The mass of each column at field capacity moisture content was subtracted from the mass of the same column residually satu- rated with gasoline. Any additional water that was lost during the drainage of the excess gasoline was collected in a gradu- ated cylinder and was also subtracted to yield the final value. Initial gasoline con- centrations ranged from 12,700 to 15,300 Table 1. Soil Analytical Techniques- Measurement USEPA SW-846 Method No. or Ret Procedure • MwMfon of trad* names or commercial products does not constitute endorsement or recommendation for use. Total volatile solids Total petroleum hydrocarbons Gasoline range organics Headspace Semi-volatile organic compounds Volatile organic compounds 160.4 9071/418.1 Enseco-RMAL, 1990 Method described below 8270 8260 Heat to 10&C then 550°C Freon extraction, IP Methanol extraction, GC/FID or GC/MS, integration of peaks GC/FID Methylene chloride extraction, GC/MS Methanol extraction, GC/MS ------- rag/kg. These values were greater than the results of each of the analytical meth- ods employed. The discrepancy can be attributed to volatilization losses during the transfer of soil samples from the columns to vials and jars for storage before analy- sis as well as to losses incurred during the performance of the analytical methods themselves. Aqueous leaching of a soil column re- sidually saturated with gasoline resulted in the mobilization of some of the retained GRO during 60 pore volumes (about 43.2 L) of water flow. In the percolating water during the leaching experiment, the peak concentration of GRO was 3,000 mg/L at the onset of leaching. The concentrations continually dropped, and after about 5 pore volumes, they began to level off; however by the end of the run, they were still as high as 170 mg/L. Integration of the GRO concentrations during the aqueous leaching experiment yielded a total of 9.8 g of GRO removed. The original mass of gasoline in the col- umn was 79 g, so that about 12% of the contaminants were mobilized in the per- colating water. Although benzene dropped below the detection limit of 1.25 mg/L by 60 pore volumes, the levels of toluene, ethylbenzene, and xylene remained steady at 70, 5, and 30 mg/L, respectively. Vapor extraction was conducted con- tinuously for 6.5 days, with only brief breaks in the flow to make adjustments to the system. Vapor extraction of two con- taminated columns brought the soil GRO levels down as low as 0.5 mg/kg, a re- moval of 99.96% of the original GRO. During the initial 7 min of vapor extrac- tion, exhaust gas hydrocarbon concentra- tions exceeded the instrument's THA maximum limit of detection of 100,000 ppm (v/v). After this period, hydrocarbon levels steadily declined with brief periods of in- creased concentration spikes following pauses in the flow. The vapor concentra- tions exiting the two columns were similar at first during the rapid decline and in the final stages as the concentrations leveled off. Hydrocarbon values, however, differed by as much as 50% during the middle stages of the extraction process. These differences can be explained by different hydrocarbon starting concentrations, since the column giving rise to the higher levels had retained 88 g of gasoline before SVE, whereas the other began with only 66 g. Final vapor hydrocarbon levels were be- tween 20 and 40 ppm (v/v). Integration of the vapor extraction data yielded a total hydrocarbon mass removal that was greater than the initial gasoline mass by about 70%. This is understand- able since the THA was operated under circumstances that were best suited for generation of qualitative data only. The THA was calibrated only once and for one concentration range at the start of gas flow, whereas several concentration ranges were used during the course of the ex- periment. Furthermore, the calibration mixture contained only butanes and pen- tane; these were not completely repre- sentative of the complex gasoline mixture. The longer chain hydrocarbons and aro- matic compounds also found in gasoline exhibit greater THA response factors than the calibration gas compounds. Concen- trations fell from a maximum of 3.7 mg/L at 2 pore volumes to a plateau of 0.6 mg/ L after 60 pore volumes. Conclusions The gasoline contaminated sandy soil was susceptible to aqueous leaching of the hydrocarbon components. Alter 60 pore volumes of water flow, however, the majority of product remained in the soil and significant concentrations of BTEX were still found in the percolating water. SVE was effective in removing gasoline from the soil, with a reduction in the GRO of 3.5 orders of magnitude. After this pe- riod, the vapor extracted soil still contained low levels of water-mobile contaminants. GRO was the most reproducible of the analytical techniques tested, and headspace analysis by GC/FID recorded the highest concentrations. Soil column experiments provided a well controlled, effective measurement of the soil processes occurring in the aqueous leaching and vapor extraction of gasoline contaminated soil. The full report was submitted in fulfill- ment of Contract No. 68-03-3409 by COM Federal Programs Corporation under the sponsorship of the U.S. Environmental Protection Agency. •U.S. Government Printing Office: 1992— 648-080/60108 ------- Michael £ Miller, Tom A. Pederson, and Carole A. Kaslick are with Camp Dresser andMcKee, Inc., Cambridge, MA 02141. George E Hoag is with University of Connecticut, Starrs, CT 06268. Chl-Yuan Fan is the EPA Project Officer (see below). The complete report, entitled "Soil Vapor Extraction Column Experiments on Gasoline Contaminated Soil," (Order No. PB92-226430/AS; Cost: $19.00, subject to change) will be available only from: National Technical Information Sen/be 6285 Port Royal Road Springfield,VA22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Risk Reduction Engineering Laboratory U.S. Environmental Protection Agency Edison, NJ 08837 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Official Business Penalty for Private Use $300 EPA/600/SR-92/170 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 ------- |