ARCHIVED REPORTS XR0008612 - Laserfiche WebLink

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1• INTRODUCTION 1 Exxon Retail Site (RS) 7-0137 is located at 1605 South El Dorado Street, Stockton, California, on the southwest corner of Second and South El Dorado streets The site is an active service station that has three double-walled fiberglass gasoline underground storage tanks (USTs) The site currently contains seven groundwater monitoring wells (MW 1—MW7) and two vapor extraction wells (VEW 1 and VEW2) The seven groundwater monitoring wells (Figure 1) are sampled quarterly ' Groundwater sampling at Exxon RS 7-0137 has been conducted regularly since 1991 A soil vapor extraction (SVE) system operated at the site from November 1995 to March 1996 and resumed in February 1997 The SVE system in operation until March 1996 extracted soil vapor through five wells (VEW1, VEW2, MW2, MW3, and MW5) and abated the petroleum hydrocarbons with a catalytic oxidizer unit The catalytic oxidizer unit was removed in March ' 1996, following a decline in the concentrations of extracted hydrocarbon vapors The SVE system installed in February 1997 extracts from the same five wells, and vapors are treated through carbon adsorption On 21 July 1997, the seven groundwater monitoring wells were purged and sampled Samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPH-g), for benzene, toluene, ethylbenzene, and total xylenes (BTEX), and for methyl t-butyl ether (MTBE) r2. SUMMARY OF RESULTS The depth to water in each well was measured prior to purging and sampling Liquid-phase hydrocarbons (LPH) were not detected in any of the wells Groundwater elevations at the site ranged from 19 44 to 18 91 feet below mean sea level Groundwater elevations have decreased an average of 0 80 feet since the previous gauging on 1 April 1997 The monitoring wells at the site were constructed at a time when the water table was more than 10 feet lower than it is currently Monitoring wells MW 1, MW2, MW3, and MW6 are screened from 35 to 55 feet below the surface, and MW4, MW5, and MW7 are screened from 40 to 60 feet below the surface Table 1 presents monitoring well construction details Historical and current depths to water are presented in Table 2 The depth to water on 21 July 1997 ranged from 29 76 to 32 34 feet below the top of the well casing, therefore, none of the wells are currently screened across the water table In order to estimate the groundwater gradient and flow direction, a water table contour map was prepared (Figure 1) Groundwater flow was bidirectional Groundwater on the site flows to the southeast at an approximate gradient of 0 01 Groundwater at the eastern edge of the site flows to the southwest at an approximate gradient of 0 02 Gauging data and calculated groundwater elevations from 21 July 1997 are presented in boldface type in Table 2, along with previous gauging data Field documents are included as Appendix A After the depths to water were measured, each groundwater monitoring well was purged with a 2-inch PVC pipe attached to a vacuum pump until at least 3 casing volumes of groundwater were removed Samples were collected using factory-cleaned disposable polyethylene bailers The W Pb 1 70137 Q797 TX 2 1