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i _ I i I strata consist of Holocene, Pleistocene and Pliocene sediments, including the Laguna Formation , the Mehrten Formation and the Valley Spring Formation . The sediments are i interbedded mixed-fluvial and deltaic sediments, dominated by medium-grained sand and silts. Generally, thin beds of unconsolidated clayey silt, silty clay and sandy clay are abundant in this upper zone, which are laterally discontinuous over distances of several tens of feet. 2. 2 Hydrogeologic Conditions Ground water is present beneath the site at a depth of approximately 33 feet below ground surface (bgs), as measured in a groundwater monitoring well installed in 1996 (Figure 2) . The elevation of groundwater was calculated by Sierra-Pacific at 18 feet below mean sea level, based upon elevation data from the U .S. Geological Survey Stockton West 7.5- Minute quadrangular map. The northeasterly groundwater flow direction shown herein is based upon several published data sets, notably the Eastern San Joaquin County Groundwater Study prepared by Brown and Caldwell Consulting Engineers in 1985. This shallow aquifer is part of the Eastern San Joaquin County Groundwater Basin , as defined in the California Department of Water Resources Bulletin 118-80, published in 1980. Water levels in this aquifer have been generally declining due to consumptive municipal and i irrigation use since the 1950s. It should be noted that groundwater flow gradients are low in this area, and seasonal changes in water levels can , therefore, have significant effects on the apparent groundwater flow direction . II 3.0 PURPOSE OF INVESTIGATION The purpose of the current investigation is to determine the extent of the reported release of total petroleum hydrocarbons (diesel and waste oil) at the site. Impacts to the soil and groundwater in the vicinity of the former diesel USTs will be assessed. Excavation and sampling of three soil borings is proposed in the vicinity of the former diesel UST excavation . The proposed locations for the borings are shown in Figure 3. Two of these borings will be converted to groundwater monitoring wells which, along with the existing groundwater monitoring well, will allow determination of the direction of groundwater flow. Groundwater flow will direction will be determined by triangulating the groundwater Environeering, Inc. Project No. 98-012.01 2