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sectional lines are shown on Figure 5 Soil stratigraphy beneath the site is shown on cross- sections A-A' and B-B' (Figures 6 and 7, respectively) 2.3 Local Hydrageology ' Groundwater beneath the site was first encountered during initial investigations at depths of approximately 45 feet Groundwater beneath the site was encountered at a depth of approximately 65 feet bgs during subsequent investigations performed by other consultants i ' from 1989 through 1994 Ground Zero encountered groundwater beneath the site at a depth of approximately 60 feet bgs during drilling of the vapor extraction test wells in October i 1995 The materials above the saturated zone are largely silty and clayey A silty sand zone generally exists from approximately 50 to 60 feet bgs The saturated zone has been characterized as sand and sandy silty gravel to a depth of approximately 78 feet The sediments logged in the investigations performed at the site to date are consistent with the recent alluvium and Pleistocene Victor Formation alluvial deposits which occur ' throughout the Stockton Area Regional gradients in the area are variable due to the various pumping influences The localized groundwater flow direction is generally to the east Surface water near the site includes Mormon Slough approximately 1/z mile south of the site I 2.4 Residual Hydrocarbons in Soil Analytical results of soil samples collected from the site indicate that residual petroleum hydrocarbons have been delineated and are limited to the northern portion of the property near the former gasoline tanks and pump island at depths ranging from 15 to 60 feet bgs north of the excavation, and approximately 25 to 60 feet to the south, and west of the excavation No residual gasoline hydrocarbons were detected in soil samples collected from borings to the east of the excavation It is estimated that approximately 2,670 cubic yards of soil have been impacted by gasoline hydrocarbons beneath the site and require remediation The estimated lateral extent of gasoline hydrocarbons in soil beneath the site is shown on Figure 8 2.5 Toxicity, Persistence, and Potential for Contaminant Migration The toxicity of residual contaminants in soils at the site can be discussed in terms of occupational exposure limits established by National Institute for Occupational Safety and Health (NIOSH) A short term exposure limit (STEL) is designated by a 15-minute time- weighted average (TWA) exposure that should not be exceeded at any time during a workday Immediately dangerous to life or health concentrations (IDLHs) represent the maximum concentration from which, in the event of respirator failure, one could escape within 30 minutes without a respirator and without experiencing any escape-impairing or irreversible health effects Benzene has been classified by the American Conference of Governmental Industrial Hygienists (ACGIH) as an A2 carcinogen (suspected human carcinogen) A suspected human carcinogen is carcinogenic in experimental animals at dose ground76T0eklcap 8 Y