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�4 Quarterly Groundwater Monitoring Report,Apr[I-June 1995 2 Manna Pro, 40134 South EI Qorado, Stockton, CA July 13, 1995 Groundwater Monitoring Data On April 27, 1995, 10 existing groundwater monitoring wells were monitored for depth to water (DTW) DTW measurements were also taken from future extraction well (EW-1) and the three observation wells (OB-1, OB-2, and OB-3) The DTW measurements were taken from surveyed points at the top of each well casing, which are referenced to an arbitrary datum (designated as 15 feet) This quarter, the groundwater elevations Increased an average of 4 23 feet, since the January 1995 elevations were measured The results of groundwater monitoring are included In attachment 2, table 1 A potentiometric surface map, as interpreted from the data in table 1, Is presented in figure 1 The groundwater elevation data collected from MW-1 was not used in the construction of the potentiometric surface map since it is believed not representative of the water level elevation conditions In the area As shown In figure 1, the average groundwater gradient in the site vicinity is approximately 0 001 foot/foot radiating from the area of the former tank excavation principally in an east-northeast direction Groundwater Quality Data The 10 monitoring wells were sampled on April 27, 1995 Before sampling, the static water in the well was purged to collect samples representative of aquifer conditions The pH, temperature, and conductivity were measured with a YSI 3650 meter during purging Equipment was cleaned after each sample was collected, using an Alconox and water solution, and rinsed with de-ionized water Three well-casing volumes of groundwater were purged from the wells or purged until the well became dry The wells were allowed to recharge to approximately 80 percent of static water level before samples were collected Samples were collected using well-dedicated, polyurethane, disposable bailers, and placed Into sample containers supplied by the laboratory performing the analyses Samples were labeled, placed in an Ice-chilled insulated chest, and transported to GTEL Environmental Laboratories, Inc in Concord, California, accompanied at all times by a chain-of-custody record A trip blank was also collected and accompanied the samples for quality assurance and control Samples from April 27, 1995, sampling event were analyzed by the laboratory for ■ aromatic volatile organics and total petroleum hydrocarbons-as-gasoline (TPH-g) using Environmental Protection Agency (EPA) Method 8020 and gas chromatography/flame-Ionization detector (GC/FID), respectively ■ total dissolved lead using EPA Method 7421 Analytical results reported detectable concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX) Benzene concentrations ranged from below method detection limits (MDLs) in the samples Wa6M35 2M GROUNDWATER ❑❑❑ TECHNOLOGY