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N <br />• <br />:-ffZ..qWArMqK <br />Working To Restore Nature <br />This equation indicates that the distribution coefficient decreases with increasing solubility <br />Assuming a solubility for diesel of 1300 ppb (Guard, Ng & Laughlin, 1983) the soil distribution <br />coefficient for diesel would be approximately 1970 <br />The expected concentration of TPHd in groundwater at the site can now be estimated using the <br />average concentration of TPHd remaining in the soil (mass of contaminant divided by the mass <br />of soil) by the equation <br />Cw = Cs/Kd = 40 ppm/1970 = 0 020 ppm = 20 ppb <br />This value is below the detection limit of 50 ppb for TPHd in water <br />By the same method, soil distribution coefficients were calculated for the residual BTEX <br />constituents and gasoline fuel (TPHg) Potential groundwater concentrations were determined <br />using the average remaining soil concentrations <br />B <br />Kd <br />= 17 <br />Csavg <br />= 0 02 ppm <br />Cwavg <br />= 1 2 ppb <br />T <br />Kd <br />= 38 <br />Csavg <br />= 0 02 ppm <br />Cwavg <br />= 0 5 ppb <br />E <br />Kd = <br />86 <br />Csavg <br />= 0 02 ppm <br />Cwavg <br />= 0 3 ppb <br />X <br />Kd = <br />86 <br />Csavg <br />= 0 12 ppm <br />Cwavg <br />= 1 4 ppb <br />TPHg <br />Kd = <br />127 <br />Csavg <br />= 12 ppm <br />Cwavg <br />= 94 ppb <br />With the exception of benzene and TPHg, all the calculated values are below their respective <br />primary and secondary MCLs The calculated values for benzene and TPHg approach their <br />respective detection limits and are representative of calculated concentrations upon initial contact <br />with the groundwater Natural dilution and diffusion properties would be expected to reduce the <br />dissolved fraction fairly rapidly According to the above, residual soil contamination at the site <br />should not be capable of impacting groundwater to a significant extent <br />Although a formal groundwater pumping test has not been performed on the shallow aquifer <br />beneath the site, the transmissivity of the aquifer was estimated using pumping rates from well <br />purge logs and assuming a saturated thickness of 20 feet Purge logs and time -drawdown graphs <br />for data collected in April 1993 are included in Attachment 3 Estimated transmissivity values <br />ranged from 377 gallons per day per foot (gpolft) 1n well MW 1 to 2456 gpolft in well MW2 <br />The average calculated hydraulic conductivity (K) was determined to be 70 gpolft' Utilizing <br />this value for K, an average groundwater gradient (i) of 0 006 ft/ft, and an effective porosity <br />(,�) of 10%, the groundwater velocity was calculated according to the following equation <br />V -- Ki = 70 X 0 006 - 0 56 ft/day (or 205 ft/yr) <br />7 48 gal 7 48 X 0 10 <br />ft3 <br />1500541R ADA94 F"NI 3 <br />