ARCHIVED REPORTS_XR0009117 - Laserfiche WebLink

Home Browse Search

chain-of-custody protocol. Well development and sampling event data sheets are presented 3n Appendix 11. ' 3.4 Well Surveying Following construction, the five monitor wells were surveyed for top of casing (TOC) ' elevation and vault box rim elevation relative to Mean Sea Level (MSL) The survey was conducted by Tom O. Morrrow, a state-licensed surveyor. 4.0 INVESTIGATIVE RESULTS ' A description of the soil lithology, hydrogeology and the analytical results of the soil and groundwater sampling are presented below. 4.1 Soil Lithology ' The soil encountered at the subject site during the drilling of the soil borings from the surface to approximately 5 feet below grade was described as a slightly moist, stiff, dark brown silty clay. At approximately 5 feet below grade a dry to slightly moist, hard, brown, clayey silt was encountered grading to a sandy silt at 30 feet below grade. From approximately 30 feet to the bottom of the borings at 40 to 45 feet a saturated, dark brown sand and silty sand was encountered The sands in the bottom of the borings at 30 to 45 feet are predominantly fine sands and poorly to moderately graded 4.2 Hydrogeology ' Groundwater was first encountered during drilling at an approximate depth of 32 feet below grade. The static water Ievels measured during the well development and water sampling events stabilized to approximately 30.5 feet below grade, and are summarized in Table 1. Based on the static depth to water levels relative to the monitor well top of casing elevations, the inferred groundwater flow direction is to the northeast as shown in Figure 3. ' Accordingto Mr. Michael Collins of the PHS EHD, the seasonal highest groundwater � elevations in this region of Stockton occurred in August of 1986, where groundwater ' elevations were approximately 25 feet below ground surface. The seasonal highest groundwater elevations encountered in 1986 were approximately 5 feet higher than groundwater elevations encountered during this investigation. Since petroleum hydrocarbons are slightly soluble in water, the highest hydrocarbon concentrations in soil beneath a source area are expected to be encountered at or immediately above the vadose (unsaturated) and saturated zone contact. As discussed further in Section 4 3, the highest concentrations of Ipetroleum hydrocarbons in soil correspond to the seasonal highest groundwater elevation Historically, groundwater elevations in this area of Stockton, California, have varied by as much as fifteen feet (EA Engineering Science and Technology, pers. comm.). It is assumed liiaicai 9