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Human Health Risk Assessment Farmer Mobil Oil Bulk Plant 04-343 500 East Grant Line Road 1 ; Tracy, California Retardation factors may range from 1 to over 10,000, depending :pon the solute and the geologic media (Miller and Benson, 1983). ' The second set of chemical and biological mechanisms that affect no.icon servative solutes in ground water are irreversible decay processes. Solute decay is a resilt of processes such as oxidation and reduction, microbial conversions, and formation of complex ions. A term that represents the rate at which the dissolved species is removed from solution is usually used to quantify the effects of decay. ' 2.4 SUMMARY OF SITE-SPECIFIC OBSERVATIONS Information contained in Section 1.2.6.2 Ground Water Investigations, described the methodology and results of the aquifer pumping tests performed between April 13 and 16, 1993. The quantitative results of the pumping tests were used to determine the average linear flow rate ' of ground water across the site according to the following equation: i 4p = K * dh * 1 dl n Where: q, = Average linear pore velocity ' K = Hydraulic conductivity (7.47 x 10') dhldl = Ground water gradient (0.01 foot/foot) ' n = Porosity (Assumed 35%) ' Based on this equation, the estimated average linear ground water ve!:)city is approximately 11.2 feet per year. This velocity is equivalent to the conservative transr)ort rate of a nonreactive solute. However, considering this velocity estimate ignores sorNtion,�degradation, and ' volatilization processes, the effective rate of dissolved-phase hydrocar'z,n transport is expected to be lower. ' The approximate distance between onsite monitoring wells and the nc.,-.fcst offsite wells located downgradient from the site is approximately 110 to 125 feet. At an av e.uge linear ground water 30-0136-11 2-3 1