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5.0 SITE CHARACTERIZATION <br /> 5A Soil Litho1W <br /> ' Soils encountered beneath the site consist primarily of well graded sand and well graded sand <br /> with silt to approximately 40 feet below grade Below 40 feet interbedded lenses of well to <br /> poorly graded sand with silt and clay, and elastic silt and sandy clay are present to 85 feet, the <br /> total depth explored Soil stratigraphy beneath the site is shown on Plate 4 <br /> 5.2 Local Hydroloe <br /> ' Shallow groundwater beneath the site is encountered at approximately 68 feet below grade and <br /> generally flows to the west with a gradient of 0 002 ft/ft, or approximately 11 ft/mile The <br /> groundwater gradient measured on June 14, 1994 is depicted on Plate 5 <br /> 5.3 Residual Hydrocarbons in Soil <br /> Analytical results of soil samples indicate that residual petroleum hydrocarbons have been <br /> delineated and are limited to the vicinity of the former gasoline tanks and pump island at depths <br /> between 10 and 45 feet below grade Soil impacted with TPHd remains in place beneath <br /> California Street in the vicinity of the former diesel tank At the direction of PHS/EHD no <br /> ' further investigation or remedial action is warranted at this location It is estimated that <br /> approximately 900 cubic yards of soil have been impacted by petroleum hydrocarbons and <br /> ' require remediation The estimated areal extent of residual hydrocarbons is shown on Plate 6 <br /> ' 5.3.1 Toxicity, Persistence and Potential for Contaminant Migration <br /> The toxicity of residual contaminants in soils at the site can be discussed in terms of <br /> ' occupational exposure limits established by National Institute for Occupational Safety and Health <br /> (NIOSH). A short term exposure limit (STEL) is designated by a 15-minute time-weighted <br /> average (TWA) exposure that should not be exceeded at any time during a workday <br /> Immediately dangerous to life or health concentrations (IDLHs) represent the maximum <br /> ' concentration from which, in the event of respirator failure, one could escape within 30 minutes <br /> without a respirator and without experiencing any escape-impairing or irreversible health effects <br /> 1500471RMRMCAP-0644 FNL 5 <br />