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I Mr Ron Rowe Page 2 I per foot A groundwater gradient map generated using the February 25, 1998 monitoring data is shown on Figure 2 Recent groundwater monitoring data are summarized in Table 1 Monitoring logs are included in Attachment A as is a table of well construction details From February 25 through 27, 1998, as directed by San Joaquin County Public Health Services Environmental Health Division (PHS/EHD), Ground Zero collected groundwater samples from vapor wells VW1, VW3, VW4, and VW6 and from groundwater monitoring wells MWI through MW13 Grab samples were collected from vapor wells that contained groundwater At least three well casing volumes were purged from each groundwater monitoring well prior to sampling Physical parameters (pH, conductivity, temperature) were monitored during purging and allowed to stabilize prior to sampling Purged water was contained in 55-gallon drums approved by the Department of Transportation and removed from the site on March 16, 1998 by American Valley Environmental Services Purge logs are presented in Attachment A 4 Samples were placed on ice and transported under chain of custody to Argon Laboratories (DHS #1873) Samples were analyzed for TPHg using modified EPA method 8015 and BTEX using EPA method 8020 Additional sample matrix collected from monitoring and vapor wells was delivered by Argon under chain of custody to American Environmental Network in Pleasant Hill, California (DHS #1172) and analyzed for oxygenates using modified EPA method 8260 Recent analytical results of groundwater samples are summarized in Table 2, and oxygenate results are summarized in Table 3 Attachment B contains laboratory reports and chain of custody documentation Groundwater Analytical Results Petroleum hydrocarbons were detected in groundwater samples collected from all monitoring F wells, except MW7, MWI I, and MW12 Although the TPHg and BTEX concentrations in the sample collected from MW4 showed a decrease since the previous quarter, concentrations were still above historical levels Additionally, a notable decrease in hydrocarbon concentrations was seen in the sample collected from MW2 The highest concentrations of TPHg and BTEX were detected in samples collected from wells MW4, MWS, and MW 13 Petroleum concentrations are summarized in Table 2 Oxygenates were only detected in samples collected from monitoring wells MW4, MW9, MW11 and MW12 MTBE was detected in samples from wells MW4 and MW9 at concentrations of 890 parts per billion (ppb) and 22 ppb, respectively DIPE was detected in the sample from well MW 11 (16 ppb) and TBA was detected in the sample from well MW12 (110 ppb) ETBE and TAME were not detected in any samples The detection limits were raised for some samples r as a result of dilution The oxygenate concentrations are summarized in Table 3 rwklgmrl l qm 1998