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August 5, 1997 38737 001 Harding Lawson Associates Ms Mary Meays San Joaquin County Public Health Services Page 4 TPHg was detected in four of the soil samples at concentrations ranging from 0 7 mg/kg to 4 6 mg/kg Benzene was detected in four of the soil samples at concentrations ranging from 0 005 to 0 068 mg/kg Based on the methodology used by SECOR to collect the groundwater samples, the analytical results cannot be considered representative of Site conditions Thus is based on the potential for cross- contamination in three ways 1) soil cuttings coming from depths above the sampling depth can contaminate the groundwater in the borehole being sampled, 2) potentially impacted soil exposed in the borehole above the sampling depth can contaminate the groundwater seeping into the borehole, and 3) shallow groundwater can move downward into the deeper groundwater through the open borehole during 1 collection of the deeper groundwater samples Therefore, detections of petroleum hydrocarbons in the groundwater samples collected this way are not necessarily indicative of the concentrations in groundwater at a given sampling depth Nondetectable results on groundwater samples collected in tlus way, do however, provide useful data Both of the groundwater samples collected from boring SB-8 did not contain detectable concentrations of TPHg or BTEX Based on the above, and considering the method of sampling, it is reasonable to assume that petroleum hydrocarbons are present in the soil or groundwater, or both, in the vicinity of boring SB- 7 10,but at uncertain concentrations <42 p SITE CONDITIONS Soil Profile Based on HLA's review of the previous data and the recent SECOR investigation, soils beneath the Site consist of a heterogeneous assemblage of mainly fine-grained sediments with minor amounts coarse- grained sediments Plate 4 is an east-west cross-section (A-A') constructed from previously completed borings, the location of A-A' is shown on the Site Plan (Plate 3) The upper 12 to 15 feet consists mainly of fine-grained sediments including clay, silty clay, and clayey silt Beneath these fine grained sediments are discontinuous horizons of clayey fine grained sand and silty-clayey fine grained sand with occasional lenses of clayey fine- to medium-grained sand Groundwater Gradient and Flow Directions Based on quarterly monitoring data collected from September 1987 to March 1991, groundwater flow directions have varied considerably at the Site from northeast through northwest and towards the southwest and south The dominant flow directions appear to be southwest through northwest Groundwater gradients varied from 0 0023 to 0 018 feet per foot(ft/ft), averaging approximately 0 004 ft/ft In Appendix A, a compilation of data collected prior to beginning of groundwater extraction and treatment operations is included (WaterWork, 1991, Table 1) Depth to groundwater beneath the Site during this period has varied from approximately 7 to 12 feet bgs More recently, water levels about 5 feet bgs have been measured (SECOR, draft) Groundwater Analytical Data Analytical data from the six monitoring wells collected from September 1987 through June 1994 are compiled in Table 1 These data indicate that only groundwater samples from MW-4 have consistently contained detectable concentrations of TPHg, with concentrations that have ranged from 9 1 milligrams ' per liter(mg/1) (September 1987) to 0 24 mg/1 Qune 1993) Well MW-4 is located in a cross-gradient to up-