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'%., .,i <br /> 3.0 NATURE AND EXTENT OF CONTAMINATION <br /> 3.1 Soil <br /> A summaryofhistorical soil analytical data is presented in Table 3-1. Soil sample locations and the lateral <br /> .. extent of soil contamination are shown in Figure 3-1. Based on data collected during the Phase III and IV <br /> investigations,residual contamination is present in the unsaturated zone outside the northwest and southwest <br /> boundaries of the UST excavation. <br /> 3.2 Groundwater <br /> A summary ofhistorical and current groundwater analytical data is presented in Table 3-2. Groundwater <br /> contaminant levels have decreased significantly over the monitoring period. In the I 1 years between the <br /> ` initial sampling ofmonitoring well MW-2A(July 1995)and the most recent sampling event(December <br /> 2005),benzene concentrations have declined from 7,000 micrograms per liter(gg/L)to 110 gg/L and total <br /> petroleum hydrocarbons as gasoline(TPH-g)concentrations have decreased from 79,000 gg/L to 4,300 <br /> µg/L. In monitoring well MW-2,first sampled on December 30, 1988,benzene has decreased from <br /> 28,000 gg/L to 6,500 gg/L and TPH-g concentrations have decreased from 59,000 gg/L to 15,000 gg/L. <br /> The estimated extent of contaminated groundwater, based on the most recent complete round of <br /> groundwater samples, is presented in Figure 3-2 (WESTON, 2006). <br /> 3.3 Physical and Chemical Characteristics of Contaminants <br /> Gasoline is the primary contaminant at the site. Although gasoline is composed ofover 200 compounds, <br /> BTEX compounds are the constituents ofprimary concern as they are toxic to most organisms,even in <br /> small quantities and over short periods oftime. BTEX compounds are generally volatile,highly soluble in <br /> water,and adsorb less strongly to soil than other compounds in gasoline. These characteristics allow <br /> BTEX compounds to leach more rapidly from soil into groundwater and migrate with groundwater flow. <br /> The migration ofBTEX compounds in groundwater is controlled by serval processes,including advection, <br /> dispersion,diffusion,sorption,volatilization,and biodegradation. Advection is the bulk movement of <br /> groundwater and is the primary driving force ofcontaminant movement. Dispersion,diffusion,and sorption <br /> tend to slow contaminant migration. Biodegradation is the most important process in reducing contaminant <br /> mass. <br /> 8 <br />