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h vertical extent of impacted soil in the vicinity of the former underground storage tanks (USTs) and possible lateral migration of impacted groundwater between wells MW4 and MW5. The results were submitted on a Summary Report on Additional Subsurface Investigation dated August 11, 2004. GROUNDWATER SAMPLING AND GRADIENT EVALUATION ATC personnel performed the latest quarterly groundwater monitoring of six monitoring wells on August 24, 2004. Field work consisted of measuring depth-to-water levels in all wells, collecting groundwater samples from all wells, and submitting groundwater samples for laboratory analyses. Water level measurements were collected from groundwater monitoring wells MW-1 through MW-6. The depth to water ranged from 7.08 to 8.76 feet below top of well casing elevation. Groundwater elevations have increased an average of 0.49 feet since the previous quarterly monitoring event conducted on January 12, 2004. The water level data were used to develop the groundwater elevation contour map (Figure 2). Groundwater in the uppermost aquifer flows beneath the site in a northerly direction. The average hydraulic gradient on August 24, 2004, was calculated to be approximately 0.002 ft/ft or approximately 11 feet/mile. The data indicate a groundwater gradient that is generally consistent with previous groundwater gradients observed at the site. A summary of groundwater monitoring data is presented in Table 1. Groundwater elevations are referenced to feet above mean sea level. Groundwater samples were collected from monitoring wells MW-1 through MW-6. Prior to collection of groundwater samples a minimum of three well casing volumes were purged. The pH, electrical conductivity, and temperature were measured and allowed to stabilize before groundwater samples were collected. Samples were then collected from each well using disposable bailers. Purge water was contained in 55-gallon DOT approved drums. Groundwater well purge and sample logs are provided in Appendix A. The groundwater samples collected from each well were submitted with Chain-of-Custody Record to State-certified Argon Laboratories Inc. (ELAP Cert. No. 2359). The samples were analyzed for TPHg by EPA Method 801513; BTEX by EPA Method 802113; and MTBE, tert- amyl methyl ether (TAME), TBA, di-isopropyl ether (DIPE), ethyl-tert-butyl ether (ETBE), 1,2- dichloroethane(1,2-DCA), and 1,2-dibromoethane (EDB) by EPA Method 8260B. RESULTS OF LABORATORY ANALYSES The laboratory analytical results indicate constituents of concern (COCs) were detected in the groundwater samples collected from monitoring wells MW-1, MW-2, MW-3, MW-4 and MW-5. TPHg and/or BTEX constituents were detected in wells MW-2, MW-3, and MW-4. Only the _ groundwater samples collected from wells MW-2 and MW-3 contained benzene at concentrations that exceeded the Maximum Contaminant Level (MCL) of 1.0 micrograms per liter(µg/1). All of s:\Environmental\17861\reports\1gmr2004.doc 3 ATC Associates Inc.