SITE INFORMATION AND CORRESPONDENCE

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cbcll� Uy: IJA5tL1NE; 510 420 1707; Nov-9-99 16;27; Page 14129 APPARENT PROBLEM Residual concentrations of contaminants in soil and groundwater associated with historical land uses, and possibly also off-site historical land uses, have affected soil and groundwater quality at.the site. Disposal and handling practices of hazardous substances from former Iand uses are not known; however,petroleum hydrocarbons as gasoline,benzene,toluene,ethylbenzene,and xylenes(BTEX) and methyl tert butyl ether have been released to soil and groundwater at the site. Other volatile organics (e.g., chloromethane, 2-hexanone, cis- 1,2 dichloro ethylene, trichloroethylene; and chloroform)are also present in soil or groundwater at the site and may have been associated with on- site or off-site land uses, In addition,some pesticides,semi-volatile organics, and metals have been reported in soil and/or groundwater underlying the site; the source of these chemicals is ualmown. Other known off-site contamination,which could potentially affect groundwater underlying the site in the future,includes other chlorinated organics and organic compounds(e.g.,vinyl chloride,trans 1,2-dichloroethylene, n-butyl benzene, yee-butyl benzene, and tetrachloroethylene). The primary goal of this PEA is to determine if the documented releases of contarninants to soil and groundwater at the site, or from off-site land uses,present sign ficant hazards to human health-or the environment under current conditions and future redevelopment of the site is a water square. ENVIRONMENTAL SETTING FACTORS .RELATED TO SOIL PA'T'HWAY The site and adjacent sites are relatively flat. The slope of the site has not been estimated,but would not be expected to exceed two percent. The site is located between 10 and 15 above mean sea level (U.S. Geological Survey, 1958). No evidence of stained sail, stressed vegetation, or dead or ill wildlife was observed at the site during the 1999 site reconnaissance,as the site is completely paved with asphaltic concrete and is fenced. This paving prevents direct contact with surface soils and groundwater underlying the site. The parking structure also prevents contact with surface water in the Stockton Channel. Generally, the soils within the project area are unconsolidated sediments, primarily silty clay,and range from brown to dark gray in color. These soils are subject to poor drainage (Paoli and Associates, 1991).. The silty clay extends to at least 20 fect bgs (Black & Veatch, 1998x), the i,II maximum depth investigated.The permeability of silt and clay with mixtures of sand, silt, and clay J range from 0.000328 to 0.328 feet/day(Todd, 1980). Two borings installed during a geotechnical invcstigation of the site had three inches of asphaltic concrete at the surface underlain by six to nine inches of aggregate base;the aggregate base was underlain by approximately two feet of Portland i cement concrete slab (Kleinfelder, 1999). The presence of the concrete may have been related to . former building foundations located on the site;concrete slabs were not observed during the Phase II investigation (Black & Veatch, 1998a). Soil boring logs for the site or adjacent sites were not available for review for borings installed below approximately 20 feet bgs. 97364ick.pca.wpd-1 119/99 S