ARCHIVED REPORTS XR0004628 - Laserfiche WebLink

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CVVZ ATC Associates Inc 1117 Lone Palm Avenue,Suite B Modesto, Calilorma 95351 209-579-2221 n s s U 9 o c r n T e s r N Fax 209-579-2225 GROUNDWATER FLOW DIRECTION Water level measurements collected from monitoring wells MW1, MW2, and MW3 on June 1, 2005, ranged from 27 92 to 28 60 feet below the tops of the well casing elevations representing an average decrease in water table elevation of approximately 0 01 feet since March 2005 The water level data were used to develop the groundwater elevation contour map (Figure 3) Groundwater in the uppermost aquifer beneath the site flowed toward the east, southeast The average hydraulic gradient on June 1, 2005, was calculated to be 0 001 ft/ft or approximately 5 3 ft/mile A summary of groundwater monitoring data is presented in Table 1 ANALYTICAL RESULTS TPHg was detected in the groundwater samples collected from MWl, MW2, and MW3 at concentrations of 290 micrograms per liter (µg/L), 170 µg/L, and 2,100 pg/L, respectively Benzene was detected in the groundwater sample collected from MW3 at a concentration of 4 6 pg/L Toluene was detected in the groundwater sample collected from MW3 at a concentration of 3 2 tig/L Ethyl benzene was detected in the groundwater samples collected from MW 1 and MW3 at concentrations of 3 4 pg/L and 24 pg/L, respectively Xylenes were detected in the groundwater sample collected from MW3 at a concentration of 100 pg/L TBA was detected in fie the groundwater sample collected from MW2 at a concentration of 17 pg/L 1,2-DCA was detected in the groundwater sample collected from MW2 at a concentration of 0 9 pg/L MTBE was not detected in any of the groundwater samples Analytical results of groundwater samples are summarized in Table 2 Laboratory data sheets and chain-of-custody documentation are contained in Attachment 2 CLOSURE ADDENDUM Current/Future Use According to a map provided by the City of Ripon (Figure 4), there are three wells that are planned to be installed in the near future However, these wells lie far enough away that they will not be influenced by the site No well installations are currently planned within 0 5 miles of the site Two groundwater wells that lie directly to the north of the site are screened in a deeper aquifer Groundwater has historically flowed to the northwest, minimizing the chance of contaminant influence on the Stamslaus River approximately 0 5 miles to the southeast Contamination Biodegradation Rates Two contaminant plumes were analyzed for their rates of attenuation Each plume was classified as unstable based on their consistent declining contaminant concentrations over time The equation used for the biodegradation rate was C(t) = Cie ld, where C(t) is equal to the concentration of the plume (in ppb) at a specific point at time t, Ci is the initial contaminant concentration(in ppb)at that point, and k is the first order attenuation rate