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1 QLWJnEFkY GROUNOMlATER MOWFMMO RMU 6131 Pacific Avenue Stockton,Cw1tomia ' All water-level measurements were made from referenced wellhead elevations (i e , top Of casing) to the nearest 0 07 foot Wellhead elevations were surveyed and referenced to a temporary bench mark with an assumed elevation of 100 feet above mean sea level ' by a licensed surveyor Depth-to-liquid measurements were collected on April 9, 1991, and indicated groundwater at 55 96 feet below surface in MW-1, 55 61 feet in MW-2, and 56 97 feet in MW-3 A thin layer (0 02 foot) of floating liquid hydrocarbons was observed in MW-3 Depth-to-liquid measurements were again collected on July 17, 1991, and indicated groundwater at ' 56 90 feet below surface in MW-1, 56 57 feet in MW-2, and 58 02 feet in MW-3 Floating hydrocarbons were observed in MW-3 at 0 08 foot thickness The depth to groundwater in MW-3 was corrected for hydrocarbon suppression according to the formula noted in ' Table 1 Liquid levels rose 0 63 foot in wells MW-2 and MW-3 and 0.68 foot in MW-1 from January to April Liquid levels decreased 0 94 foot, 0 96 foot and 1 05-foot in wells MW-1, MW-2, and MW-3, respectively, from April to July A groundwater elevation hydrograph ' is included as Figure 3 On the basis of the April 9, 1991, measurements, the estimated direction of groundwater flow was to the east-southeast at an average gradient of 0 001 ft/ft (0 1 percent) However, on the basis of the July 17, 1991, measurements, the ' estimated direction of groundwater flow is to the west at an average gradient of 0 01 ft/ft (1 0 percent) Figures 4 and 5 are potentiometric surface maps of the shallow water- bearing zone for April 9 and July 17, 1991, respectively 4 2 Groundwater SamDlina and Analysis Groundwater samples were collected from monitoring wells MW-1 and MW-2 on April 9 and July 17, 1991, according to the SOP included in Appendix A, and delivered under chain-of-custody to NET Pacific, Inc , of Santa Rosa, California, a state-certified analytical laboratory Well MW-3 was not sampled on either date due to the presence of floating hydrocarbons The samples were analyzed for a) TPH, as gasoline, by GC/FID Method 5030, b) benzene, toluene, ethylbenzene, and total xylenes (BTEX) by EPA Method 602, and c) total and fecal coliform by Colilert Methods The groundwater analytical results are summarized in Table 2 The analytical reports and chain-of-custody forms are included in Appendix B Concentrations of TPH and benzene reported in Table 2 are also shown on Figures 6 and 7 f Analysis of samples collected in April from well MW-2 indicated a concentration of 4 5 ppm TPH and BTEX concentrations ranged from 0 012 ppm (ethylbenzene) to 1 2 ppm (benzene) The analysis did not indicate concentrations of TPH or BTEX above the respective detection limits in the sample collected from MW-1 For MW-2, analysis indicated a bacteria count of 1 1 MPN (most probable number) per 100 milliliters for both total and fecal coliform Both total and fecal coliform were below the 1 1 MPN reporting lima in MW-1 f 90-0208 RPT 2- I