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to -36.76, representing an average decrease in the groundwater table elevation of approximately
<br /> 8.39 feet since April 2005. The water level data were used to develop the groundwater elevation
<br /> contour map (Figure 4). Shallow groundwater beneath the site generally flowed towards the
<br /> northeast. The average hydraulic gradient on June 5, 2005, was calculated to be approximately
<br /> 0.0006 ft/ft or approximately 3.2 ft/mile. A summary of groundwater monitoring data is
<br /> presented in Table 1. Groundwater elevation data is presented in Table 1; historic groundwater
<br /> analytical data is summarized in Table 2.
<br /> 3.0 RESULTS
<br /> 3_1 Petroleum Hydrocarbons in Groundwater
<br /> The groundwater samples collected on June 5, 2005 from MW 1, MW2, MW3, MWS, MW6, and
<br /> MW7 contained detectable concentrations of TPHg at 380 ug/l, 1,900 ug/l, 250 ug/l, 1,300 ug/1,
<br /> 520 ug/l, and 180 ug/1, respectively. The groundwater sample collected from MW2 and MW6
<br /> contained detectable concentrations of benzene of 9.7 ug/l and 1.0 ug/l, respectively. The
<br /> groundwater samples collected from MW1, MW2, MW3, MW5, MW6, and MW7 contained
<br /> detectable concentrations of toluene at 0.8 ug/l, 6.5 ug/l, 1.1 ug/l, 6.0 ug/l, 2.0 ug/l, and 1.3 ug/1,
<br /> respectively; ethyl benzene at concentrations of 1.7 ug/l, 110 ug/l, 1.7 ug/l, 33 ug/l, 13 ug/1, and
<br /> 0.7 ug/l, respectively; and xylenes at concentrations of 5.0 ug/1, 85 ug/I, 3.8 ug/l, 130 ug/1, 25
<br /> ug/1, and 2.7 ug/l, respectively. The groundwater sample collected from MW4 contained no
<br /> detectable concentrations of TPHg or BTEX constituents.
<br /> MTBE was detected in the samples collected from MW1, MW2, MW3, MW4, and MW7 at
<br /> concentrations of 21 ug/l, 32 ug/l, 51 ug/1, 31 ug/1, and 30 ug/I, respectively. TBA was detected
<br /> in the samples collected from MW 1, MW2, MW3, and MW7 at concentrations of 43 ug/1, 41
<br /> ug/1, 40 ug/l, and 49 ug/l, respectively. 1,2-DCA was detected in the samples collected from
<br /> MW1, MW2, MW3, MW4, MW5, MW6, and MW7 at concentrations of 38 ug/l, 32 ug/l, 43
<br /> ug/l, 34 ug/l, 2.9 ug/l, 2.9 ug/1, and 42 ug/l, respectively. Copies of lab data are contained in
<br /> Attachment 2.
<br /> 4.0 RECEPTOR SURVEY
<br /> ATC reviewed PHS/EHD groundwater well construction logs, Department of Water Resources
<br /> (DWR) groundwater well construction logs, and conducted a site walk in the vicinity of the site
<br /> to identify any groundwater wells located within 2,000-foot radius of the site. The search
<br /> revealed fourteen wells Iocated within approximately 2,000 feet of the site. None of the fourteen
<br /> wells are located within 500 feet of the site (Figure 3).
<br /> The wells were located on the map according to "well location" description information provided
<br /> on the well construction logs. Available well construction logs are contained in Attachment 3.
<br /> Based on ATC's current work at a project located approximately one mile southwest of the site,
<br /> the inferred regional groundwater gradient is to the northeast.
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