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a • Quarterly Groundwater Monitoring Report, December, 1992--February, 1993 April 15, 1993 Manna Pro, 4004 South EI Dorado, Stockton, CA 2 Groundwater Morntorina Data On February 3, 1993, 10 existing groundwater monitoring wells were monitored for depth to water (DTW) The DTW measurements were taken from surveyed points at the top of each well casing, which are referenced to a common datum (designated as 100 feet) The February 1993 groundwater elevations increased an average of 4 30 feet compared with October 1992 elevations The results of groundwater monitoring are included in Attachment 3, Table 1 A potentiometric surface map, as interpreted from the data in Table 1, is presented in Attachment 2, Figure 1 As shown in Figure 1, the average groundwater gradient in the site vicinity is approximately 0 006 ft/ft toward the north Previous potentiometric surface maps, constructed using data from wells MW-1 through MW-6, showed the gradient to the west-southwest The San Joaquin County Flood Control and Water Conservation District potentiometric surface maps (1985 and 1991) show the groundwater gradient generally toward the north, suggesting that the previous water levels measured in wells MW-1 through MW-6 were hydraulically influenced by the excavation With the additional data from wells MW-7 through MW-10, the potentiometric surface map shows an overall gradient consistent with the regional flow and also shows the hydraulic effect (mounding) near the tank excavation Groundwater Duality Data .� The 10 monitoring wells were sampled on February 3, 1993, after groundwater-level monitoring The wells were purged to collect samples representative of aquifer conditions The pH, temperature, and conductivity were measured with a YSI 3650 meter during purging Wells MW-1 and MW-6 bailed dry after purging less than 4 well volumes of groundwater Four well volumes of groundwater were removed from wells MW-2, MW-3, MW-4, MW-5, MW-7, MW-8, MW-9, and MW-10 The wells were allowed to recharge to approximately 80 percent of static water level before samples were collected Samples were collected using a Teflon® bailer and placed into sample containers supplied by the laboratory performing the analyses Equipment was cleaned after each sample was collected, using an Alconoxe' and water solution and rinsed with de-ionized water A field blank and ansate sample were collected in the field for Quality Assurance/Quality Control (QA/QC) measures Samples were labeled and placed in an ice-chilled cooler for transport to GTEL Environmental Laboratories, Inc in Concord, California, and were accompanied at all times by a chain-of-custody record Samples from the February 3, 1993, sampling event were analyzed by the laboratory for ■ Aromatic volatile organics and total petroleum hydrocarbons-as-gasoline (TPH-G) using Environmental Protection Agency (EPA) Methods 5030 and 8020 and modified EPA Method 8015, ■ Total dissolved lead using EPA Method 7421 R2819A3 DH EM GROUNDWATER TECHNOLOGY, INC