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1.0 INTRODUCTION <br /> This report summarizes the results of the Third Quarter 1998 groundwater monitoring <br /> activities conducted on August 24 and 25, 1998 at the Shop 10 Maintenance Station (site, <br /> Figure 1) The work presented herein was conducted in accordance with Task Order <br /> Number 10-911175-MA and Contract 43YO97 The purpose of this project is to perform <br /> quarterly sampling requirements <br /> 2 0 SITE HISTORY <br /> The following information was obtained from the Fourth Quarter 1996 groundwater <br /> sampling report prepared by Geocon Geotechnical and Environmental Consultants <br /> (Geocon) for the Shop 10 Maintenance Station "it is understood that four underground <br /> storage tanks (USTs) were removed from the site in August 1986 Fourteen on-site and <br /> four off-site borings were subsequently advanced between 1987 and 1991 to determine <br /> the extent of subsurface soil impacts beneath, and adjacent to, the site Total Petroleum <br /> Hydrocarbons as Gasoline and Diesel (TPH-G and TPH-D) were detected in soil samples <br /> collected from the soil borings at maximum concentrations of 3,800 and 4,500 milligrams <br /> per kilogram (mg/kg), respectively <br /> Between 1988 and 1991, five on-site and three off-site groundwater monitoring wells (MW- <br /> 1 through MW-8) were constructed to evaluate the lateral limits of petroleum hydrocarbon <br /> impacted groundwater These wells were ultimately abandoned and replaced by fourteen <br /> additional monitoring and/or vapor extraction wells These wells have been monitored <br /> since 1990 with the results indicating elevated concentrations of TPH-G and Benzene, <br /> Toluene, Ethylbenzene, and total Xylenes (BTEX) in the groundwater samples adjacent to <br /> the former wells " The approximate locations of the monitoring wells are depicted in Figure <br /> 2 <br /> On August 3, 1998 Wells VW-1 and MW-26 were abandoned because they were located <br /> within the footprint of a future maintenance building at the site Under supervision of PSI <br /> personnel, V & W Drilling of Rio Vista, California overdnlled the wells using a hollow-ste <br /> auger The wells were backfilled to grade with a portland cement/bentonite mixture <br /> e.m E U CA-C.0— Z rV o 6,-,:QcA e4,a_4-' A,v�c.Q�,�o� <br /> 3 0 GROUNDWATER MONITORING ACTIVITIES <br /> 1 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT <br /> On August 24, 1998, static groundwater elevations were measured in 12 monitoring and/or <br /> vapor extraction wells (Figure 2) Groundwater depths were measured using a <br /> groundwater interface probe A summary of the depth-to-groundwater data collected <br /> during this monitoring event and previous monitoring events is presented in Table 1 <br />� 1 <br /> F Transfer\Report\CALTRANS\Ca11yO97\71017\gm3986n <br />