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�I Quarterly Groundwater Monitoring Report, March—May 1993 July 15, 1993 Manna Pro, 4004 South EI Dorado, Stockton, CA 2 Groundwater Monitoring Data On April 16, 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 an arbitrary datum (designated as 100 feet) The April 1993 groundwater elevations increased an average of 3 7 feet compared with February 1993 elevations The results of groundwater monitoring are included in Attachment 1, 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 Groundwater Quality Data The 10 monitoring wells were sampled on April 15, 1993 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 Four well-casing volumes of groundwater were purged from wells MW-2, MW-3, MW-4, MW-5, MW-6, MW-7, MW-8, MW-9, and MW-10 Well MW-1 bailed dry after purging less than 4 well-casing volumes of groundwater 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 Alconoe 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 April 15, 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 Analytical results reported detectable concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX) Benzene concentrations ranged from below method detection limits (MDLs) in the samples from wells MW-8 and MW-9 to 29,000 micrograms per liter (µg/1) in the sample from well MW-1 Concentrations of TPH-G ranged from below MDLs in the sample from well MW-8 to 1 10,000u g/I in the sample from well MW-1 Figure 2 illustrates the distribution of TPH-G in the groundwater using the April 15, 1993, analytical results Total dissolved lead was not detected in the samples collected this quarter Concentrations of TRH-G have decreased in samples from wells MW-5, MW-6, and 2819 RO13 070 GROUNDWATER TECHNOLOGY, INC