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i ATC Associates Inc. 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 209-579-2221 CVATC Fax:209-579-2225 ASSOCIATES INC. +,,, The groundwater basin beneath the site and in the vicinity of the site is identified as the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin and is defined by unconsolidated to semi-consolidated sedimentary deposits. Significant water bearing formations in the Eastern San Joaquin Subbasin consist of the Alluvium and Modesto/Riverbank Formations, Flood Basin Deposits, Laguna Formation, and Mehrten Formation. Shallow groundwater beneath the site is believed to be associated with the Alluvium and Modesto/Riverbank Formations. These units are exposed along a band approximately 15 miles wide that extends from the vicinity of Stockton eastward, and consist of Recent to lV. late Pleistocene age sediments consisting primarily of sand and gravel in the fan areas while clay,silt,and sand are more prevalent in inter fan areas. Poor quality groundwater has been moving east on the eastern side of the Delta as a result of declining groundwater table elevation. Elevated saline concentrations are I" evident in the Stockton area and large areas of elevated nitrate are present in groundwater southeast of Lodi and south of Stockton. With regard to the subject site, low concentrations of petroleum hydrocarbon constituents have been detected in groundwater samples collected from MW1 and MW2 located near the former source area. Sporadic detections of some of the analytes of interest have been detected in samples collected from MW3. Petroleum hydrocarbons have not been detected in samples collected from MW4 through MW7, with the exception of a single detection reported in a sample collected from MW4 in June of 2000. Historical groundwater quality data are presented in Table 2. 4.0 EVALUATION AND STABILITY OF CONTAMINATION An evaluation of petroleum hydrocarbon contamination was conducted for the site. The evaluation included non-aqueous phase liquids, vadose zone soil, saturated zone soil, groundwater, and subsurface vapor impacts. Additionally, the stability of the chemicals of concern was evaluated. This evaluation was made using historical subsurface investigation soil data and groundwater analytical data. The L+ following is a summary of the evaluation. 4.1 Non-Aqueous Phase Liquids Non-aqueous phase liquids(NAPLs) are contaminants that remain undiluted as the original bulk liquid in the subsurface. NAPLs including gasoline as free product have never been encountered at the site. NAPLs can be light or dense. Light NAPLs have a specific gravity of less than 1.0. Because the specific ;„ gravity of water is 1.0, most light NAPLs float on top of the water table. Most petroleum hydrocarbons within the gasoline range and fuel oxygenates are considered to be light NAPLs. Since free product gasoline has never been encountered at the site, light NAPLs do not appear to represent a significant risk I+ to human health and/or the environment. Dense NAPLs such as chlorinated hydrocarbon solvents or petroleum fractions with a specific gravity of greater than 1.0 sink through the water column until they reach a confining layer. Because they migrate La to the bottom of aquifers instead of floating on the water table, typical monitoring wells do not indicate their presence. The analytes 1,2-DCA and EDB are the only potential analytes of interest that would meet this criterion. These analytes have never been detected in soil and groundwater samples collected from the site. Dense NAPLs do not appear to represent a significant risk to human health and/or the environment. #r S:Environmental1625771Reports12010 closure summary.doc 4 i.� 160