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(V! ATC A S S O C I A T E S I N C on the nature of the release and the groundwater flow direction. Dissolved petroleum hydrocarbons beneath the site appear to attenuate with distance from the central portion of the site. Historically, MTBE has been detected in groundwater samples collected from the site's borings/wells at concentrations up to 40 ug/L by laboratory EPA method 8020. However, the highest MTBE concentration detected by EPA method 8260, a more reliable method, did not exceed 7.6 ug/L which was observed in September 1996 in a sample collected from MW-2. The higher MTBE concentrations detected by EPA method 8020 appear to be the result of laboratory chromatogram interference which is often inherent when BTEX of often present in the sample undergoing analysis, resulting in measurements unrepresentative of the sample. A review of June 1999 groundwater analytical data indicated MTBE was not present in the dissolved hydrocarbon plume beneath the site. In accordance with California Heath and Safety Code, Title 22, the on-site domestic well is sampled quarterly under a monitoring program administered by PHS/EHD. According to Dr. Glover, the property owner, no gasoline constituents have been detected in this well above laboratory method detection limits. CONTAMINANT FATE AND TRANSPORT MODELING Utilizing EPA's BIOSCREEN (v.1.4) software program, ATC evaluated the contaminant transport potential of petroleum-affected soil and groundwater at the site. Furthermore, the model was used as a tool to estimate the date of arrival of the contamination plume at the on-site facility well. The BIOSCREEN Natural Attenuation software is based on the Domenico three-dimensional analytical solute transport model. The original model assumes a fully penetrating vertical plane source oriented perpendicular to the groundwater flow to simulate the release of organics to moving groundwater. In BIOSCREEN, the Domenico solution has been adopted to provide three model types representing 1) transport with no decay, 2) transport with first order decay, and 3) transport with"instantaneous" biodegradation reaction. Site specific data collected to date have indicated the predominant groundwater flow direction beneath the site is generally to the northwest to north-northwest with a magnitude of approximately 0.003 ft/ft. Historical groundwater monitoring data is included in Table 4. To assess groundwater transport under representative conditions, ATC incorporated groundwater data from the site's December 31, 1997 quarterly monitoring and sampling event in the parameters selected for the BIOSCREEN model. Hydrogeologic data collected on this date in December 1997 is fairly representative of the site over time including the most recent data from u June 2002, and is close to the date of collection of the geochemical parameters from MW 1, MW2, and MW3. The geochemical parameters collected from MW-1, the predominantly w:117861\repor \modehn&doc 5