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Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results biologically-mediated breakdown products of PCE under anaerobic aquifer conditions. The lateral extent of TCE is similar to that of PCE and its concentrations increase relative to PCE in the downgradient(northeast)direction,suggesting it is a breakdown product of PCE. cis DCE has been detected consistently in well MW-11 and only sporadically in wells MW-4 and MW-14. This compound is commonly the endpoint breakdown product in biologically-enhanced dechlorination reactions of PCE and TCE in settings where organic waste is present. The effect of the biological breakdown process is to reduce the concentration of parent compounds in groundwater as the breakdown compounds are produced. Based on the existing data,compound adsorption onto aquifer soils does not appear to be _ significant in affecting compound migration rates or concentrations in groundwater north of ARL. This finding is based on results of TOC analyses performed on soils obtained during the field drilling and sampling program (Table 3-3)and on mechanisms leading to sorption of VOCs. _ Sorption of VOCs onto aquifer soils is dependent primarily on a soil and total organic carbon content. Organic carbon present in aquifer soils provides sorption sites for the dissolved organic compounds of concern at the ARL site. The soil organic carbon results shown in Table 3-3 indicate that there is very little organic material in the aquifer soils sampled,so potential for VOC sorption sites is low. VOC compounds can also sorb onto fine grained aquifer soils such as silts and clays, but usually at a much lower rate than if organic carbon is present. The prevalence of interbedded _.. silts and clays in the aquifer deposits(as shown by ML,CL and SM symbols on Figures 3.1 through 3.3)suggests that there is at least some VOC sorption occurring in the aquifer within the fine grained deposits. For the purposes of the evaluating groundwater plume migration,however, it is conservative to assume that the process of compound sorption is not significant in the aquifer system north of the ARL site. Compound migration rates were evaluated using the existing quarterly groundwater sampling data. Although compound concentrations generally decrease over time,there is a distinct increase in PCE concentration in results from wells MW-11 and MW-14 that can be used to assess VOC - migration rates. At well MW-11 PCE concentration shows a sudden increase,to 227 pg/l,at the September-1996 sampling event. Well MW-14 shows a similar notable increase,to 100 pg/l,during the May 1997 sampling event eight months later. The contoured groundwater elevation maps over the fall 1996 to spring 1997 time period indicates that well MW-14 is generally downgradient of well MW-11 (CH2M Hill,1997). Assuming that the notable increase in PCE in these wells is related,then the PCE migration rate can be calculated. Based on Figure 1-2,wells MW-11 and MW-14 are 1020 feet apart. Assuming the PCE detections in these wells represent the same part of the PCE plume as it migrates north,then the PCE migration rate is 1020 feet/8 months travel,or approximately 4.2 feet per day. This number agrees very closely with the calculated groundwater flow rate. Using aquifer parameters presented in Appendix D(K=255 feet per day,hydraulic gradient(I) =0.0024), assuming an aquifer effective porosity(n)of 0.15 and using the equation for groundwater flow velocity(v)given by v=KI/n,then the groundwater flow rate is 4.1 feet per day. The calculated PCE migration and groundwater flow rates are within about 3 percent of each other. Given the assumptions involved in each calculation,the similarity in estimated PCE and groundwater flow velocities suggests that the numbers obtained from the aquifer pumping test are reasonable,that groundwater does flow toward the north in the vicinity of wells MW-11 and MW-14,and that PCE migration rates are not affected significantly by compound sorption. CDM Camp Dresser&McKee 3-9 W:\REPORTS�STOCKTONMLPLUME.MEC3.WPD CS ®1®779 City of Stockton 190855.000b