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I l .. 22 November 2006 AGE-NC Project No . 95-0167 Page 5 of 15 I - - HU4 - Extending from 55 feet to approximately 70 feet bsg, HU4 is composed primarily of thin, alternating layers of silty material (silt, and silt with clay). HU5 - Extending from 70 feet to approximately 80 feet bsg, HU5 is composed primarily of layers of sandy material (silty sand, sand, and fine- to coarse-grained sand). This zone exhibits uniformly low pore pressures and may be considered a flow unit. 4.0. POTENTIAL IMPACT ON NEARBY SURFACE AND GROUND WATER i_- Typically, ground water would represent the migration medium for contaminants over significant distances. Sites that are located in up-gradient or equi-gradient (cross-gradient) directions from the site are not likely to be negatively impacted by the site. Since the Deep Water Channel is approximately 400 feet up-gradient of the site, impacted water at the site does not pose a threat to beneficial uses of the Stockton Deep Water Channel. Currently the Stockton Deep Water Channel is impacted with semi-volatile organic compounds and suffers from secondary water quality objective threats : dissolved oxygen, salt water and pesticides. F1Drinking water near the site is supplied by the City of Stockton and Cal Water from municipal wells. The first encountered ground water is not used for drinking or irrigation, although all ground water in the Stockton area is considered to be of potential beneficial use. The Cal Water well at Lindsey Street and Edison Street is located at a considerable distance in an equi-gradient direction from the site, and therefore is not likely to be effected by site conditions. The nearest down-gradient municipal well is located approximately 2,400 feet northeast of the site. The distance of this well from the site ! limits its potential to be adversely impacted by the site (Lawrence Livermore National Laboratory, 16 October 1995 and 16 November 1995). 5.0. SOIL REMEDIATION ALTERNATIVES —I Based on the distribution and concentrations of petroleum hydrocarbons in soil at the site and on site-specific conditions, AGE believes excavation or soil vapor extraction with natural attenuation are appropriate remedial strategies to consider to address hydrocarbon-impacted soil at the site. 5 . 1 . SOIL VAPOR EXTRACTION Soil vapor extraction (SVE) is a well-accepted method of treatment for hydrocarbon-impacted soil and has been used at numerous sites with an excellent rate of success. The method is most effective on volatile hydrocarbons in permeable soil. I` _ Advanced GeoEnvironmental, Inc.