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A S S O C I A T E S I N C RESULTS Water levels have increased an average of 0 35 feet since the last monitoring event on September 25, 1998 (Table 1) A groundwater gradient map was created using the data collected on December 14, 1998 (Figure 2) Groundwater flows northeast, away from the slough, with a gradient of approximately 0 05 feet/foot or 264 feet per mile No petroleum hydrocarbon constituents were detected in the groundwater samples collected from wells MW 1 and MW2 TPHg and BTEX were not detected in any of the groundwater samples Wells MW3, MW4R, MW5, and MW6 contained TPHd at concentrations of 55 parts per billion (ppb), 57 ppb, 250 ppb, and 230 ppb, respectively The samples from wells MW5 and MW6 contained MTBE at concentrations of 12,000 ppb and 48 ppb, respectively Analytical results are summarized in Table 2 Laboratory reports with chain-of-custody documents are included in Attachment 2 ADDITIONAL SITE ACTIVITIES On October 28, 1998 ATC personnel replaced three damaged well boxes and three locking well plugs from wells MW1, MW2, and MW4R In addition, one 4-mch steel bollard was installed adjacent to well MW-4R and one 4-inch steel bollard was installed adjacent to well MW1 to protect the wells from heavy traffic and future damage • Following the repair of the well boxes, all the wells, MWi through MW6, were resurveyed on December 14, 1998 Upon calculating the elevations of the top of well casings, it was discovered that the elevation of well MW2 differed by 0 83 feet from the original survey even though the casing had not been modified ATC reviewed the field notes from the original survey performed by RESNA Industries Inc m April 1991 and determined that there were no mathematical errors made when calculating the elevations ATC resurveyed the wells on January 25, 1999 The results matched the December 1998 results indicating that an error was likely made in the field by RESNA in 1991 ATC briefly reevaluated water level measurements for all four quarters in 1998 and determined that because of the location of well MW2 and steep gradient at the site, the difference in casing elevation had very little impact on the groundwater gradient direction and magnitude RECOMMENDATIONS Due to the detectable concentrations of MTBE in the groundwater sample from wells MW5 and MW6, ATC recommends continued morutonng and sampling of all site wells (MW 1 through MW6) for TPHg, TPHd, BTEX, and MTBE by EPA Methods 8015/8020 w 1625 MquardyWW-1998 doc 2