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0 <br />v = [(K)(h/1)]/n = [(250 ft/d) (0.003)] / 0.25 = 3 ft/d <br />C <br />Where: v = velocity (cm/sec) <br />K = hydraulic conductivity (cm/sec) <br />n = effective porosity <br />h/1= gradient <br />Finally, the initial concentration of a generic leachate compound was assumed to be 2740 <br />milligrams per liter (mg/L), consistent with the historical average total dissolved solids <br />(TDS) concentration in leachate at the Forward site. <br />As shown on Table 2 in Attachment A, the contaminant concentrations at the base of the <br />vadose zone would be expected to peak at a value of 6.76 mg/L after 133,600 years, and <br />the concentration of the contaminant in groundwater a point 15 feet downgradient from <br />the landfill would be expected to peak at a value of the order of 1.72E-05 mg/L. These <br />results are within the standard deviation of the historical average TDS concentration in <br />leachate at the site, and indicate that the potential changes in groundwater chemistry <br />associated with leachate leakage are statistically insignificant and are, thus, not truly <br />detectable. The low concentrations that are delivered by unsaturated flow through the <br />vadose zone are associated with the low leachate leakage rates that are anticipated <br />through the liner, and the buffer provided by low transport rates through the vadose zone. <br />Sensitivi1y Analysis. <br />Considering the low values that were calculated in the initial analysis, sensitivity analyses <br />were performed assuming that the leakage rate was increased by an order of magnitude to <br />0.017 cubic feet per day per acre in a second round of analyses. The results of this <br />simulation (Attachment A - Table 3) show that, after about 7270 years, contaminant <br />concentrations at the base of the vadose zone are only about 0.7 mg/L. Concentrations <br />peak at a value of 61.9 mg/L after 14,600 years. This is approximately 2% of the orignial <br />contaminant concentration. <br />With respect to groundwater, the second set of analyses indicate that the concentration of <br />the contaminant at a point 15 feet downgraient from the landfill peaks at a value of the <br />order of 1.06E-04 mg/l (0.000004% of the original concentration). <br />CA2002-002\Revised Appx A - Leakageldoc\09/19/02 10 <br />Geologic Associates <br />