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ECanepa's Car Wash(Pacific Avenue) Problem Assessment Report and Corrective Action Plan Page 7 iThe historical groundwater quality data are presented in Appendix C. Volatile organic compounds (VOCs) were detected in samples from five of the eight monitor wells in March 1994. VOCs were detected in samples from monitor wells installed close to the dispenser island and the distribution suggests radial migration away from the source area. The suggested migration pattern is consistent with the apparent groundwater mound depicted by the 1994 groundwater contours. Subsequently, following survey of collar elevations of all of the monitor wells in 2001, the groundwater mound was found to be the result of an error in the original 1994 survey. Based on the 2001 collar-elevation survey, the March 1994 groundwater gradient was to the northeast. The gradient in April 2002 was approximately 0.003, also to the northeast(Figure 8). The groundwater plume in 2002 is illustrated by the plan view distribution of benzene in Figure 9 and in cross section in Figure 10. The distribution of MTBE in groundwater is shown in plan view in Figure 9 ' and in cross section in Figure 11. The concentration of TPH-G ranged from 40 mg/1 in MW-2 to less than 1 mg/I in monitor wells MW-1, MW-11, MW-3, and MW-4. TPH-G was not detected in groundwater samples from the other monitor wells. The groundwater sample collected from MW-12d in April 2002, ' the deep monitor well installed near the southwest corner of the dispenser island, contained benzene at 16 �ig/1 and THP-G at 0.53 mg/l. The second quarter(April) 2002 groundwater quality data from all monitor wells is plotted in Figure 12. A conservative order-of-magnitude estimate of the volume of benzene in groundwater is based on the plan view distribution of benzene shown in Figure 9, the data plotted in Figure 12, and water quality data from monitor wells MW-1 and MW-2. The data used are as follows: • Estimated plume volume 372,000 ft3 + Geometric mean porosity 0.35 ' a Volume of water in the saturated zone (130,250 ft )3,690,000 liters • Geometric mean benzene concentration 6.53 mg/l a Mass of benzene in the given volume of groundwater 24.1 kg or 6.7 gallons. The evolution of water quality in monitor wells MW-1 and MW-2 is illustrated by the plots of benzene concentrations in Figure 13 and the plot of MTBE concentrations in Figure 14. The hydrograph and ' screen position also are plotted for each well. The concentration of benzene in samples from MW-1 decline sharply as the water level rises above the well screen; however, the distribution of benzene in MW-2 indicates an upward trend with time similar to the trend of the hydrograph. The trends for MTBE in the samples from both wells generally are upward. Both lateral migration of the VOC plume toward MW-2 (Table 2) and down gradient migration from the source area toward MW-1 are indicated by the water quality data. Benzene has been near or below the analytical detection limit in samples from MW-3, ' MW-4, MW-5, and MW-8. Benzene concentrations in MW-6 and MW-7 declined rapidly with the water table rise. To date, no direct actions have been taken to remove the petroleum hydrocarbons from the groundwater bene.,+3, the r;+A The re Alnl of the ynlat;lP.0 exsnlvarl frnm oroundwa.ter by the soil vapor extraction VV��HCLLh he site � J system has had negligible effect on groundwater quality. 1 6.0 SENSITIVE RECEPTOR SURVEY AND RISK BASED CORRECTIVE ACTION EVALUATION Condor conducted a sensitive receptor survey within a 2,000-foot radius of the site to identify potential areas of groundwater recharge or discharge (surface water bodies), schools, and potential water well users �� CONDOR