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RESIDUAL MASS BALANCE CALCULATION <br /> The residual mass balances in groundwater were calculated for diesel based on the analytical <br /> results of the groundwater samples collected by Apex on April 30, 1998 Figure 5 shows the <br /> estimated impacted area Since diesel was not detected in the soil samples collected at 15 and 20 <br /> feet below ground surface(bgs) during the monitoring well installations, it is possible that the <br /> groundwater diesel concentrations were at least partly the result of false positive diesel detection <br /> due to the diesel weathering or biodegradation byproducts in the groundwater Apex however, <br /> assumes diesel exists in groundwater as reported, and assumes diesel is not present in soil as the <br /> soil sample results indicated <br /> ASSUMPTIONS <br /> Because of the uncertainty in estimating the diesel concentration distribution in groundwater, <br /> following assumptions were made <br /> 1 The vertical extent of the impacted groundwater is 15 feet (from 10 feet bgs to 25 bgs) based <br /> on the depth to groundwater measurements collected and the maximum total well depth <br /> recorded for monitoring well MW-4 , <br /> 2 The lateral extent of the groundwater diesel impact is 940 square feet at the former diesel tank <br /> basin(Figure 5) <br /> 3 The saturated soil contains 25% of groundwater by volume <br /> 4 The diesel concentration used in the calculation was the well MW-4 diesel analytical result for <br /> April 30, 1998 (900 ppb) <br /> 6 Concentrations are uniform in the groundwater saturated zone <br /> i <br /> CALCULATION 41 <br /> Residual diesel in groundwater 900 (ug/L) x-9 0-(ft 15 (ft) x 28 3 (L/ft) x 10-9 (kg/ug) <br /> = 0 4 (kg) 0 88 (lb) <br />