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' Simulation time (t) • ' Run times of 10, 20, 30 years Source Thickness ' Assumed thickness of 20 feet ' Source Zone Width Assumed width of 100 feet ' Source Zone Concentration ' Value of 28 mg/L from MW-2 in 1988. Soluble Mass in NAPL, soil Calculated from site data of 100 kg. RESULTS Benzene was used as the modeled chemical because it is the most soluble and mobile of the BTEX compounds Benzene concentration predications were made for 10, 20 and 30 years after release of the chemical The simulation was started in 1988, when the maximum concentration of benzene dissolved in groundwater was 28 mg/L. The BIQSCREEN model input screen is shown in Figure A-1, the plume depicted on this screen is a generic plume and does not represent actual site conditions Model output for the 10-, 20-, and 30-year simulations is shown in Figures A-2 through A-4 The no degradation line has been plotted for purposes of comparison, as discussed in Section 2 2 7, biodegradation is clearly occurring The purpose of the modeling was to determine if benzene concentrations would exceed regulatory criteria down gradient of the source area and how far downgradient the plume will extend The modeling assumptions are conservative The Domenico model assumes the source is infinite, i e , the source concentrations are constant. Because biodegradation is occurring at the site, the source term would actually be expected to decline rather than remain constant; this is confirmed by the decline in concentrations in well MW-2A which indicate that the source term is declining This source term assumption results in overly conservative predicted concentrations At the site, because the source also declines rather than remaining constant as assumed by the model, the model will over predict downgradient benzene concentrations The results of modeling as shown by the instantaneous reaction line or by the first order decay lines indicate that the BTEX plume will not extend 300 feet from the source area and that it is very unlikely that the plume will pose a risk to potential downgradient receptors, because concentrations at the leading edge decrease below the MCL for benzene. L.P1tOJECrS%5ANJOQMA\ACIAPPA A-3