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8.0 DISCUSSION AND CONCLUSIONS - PHASE 11 SITE INVESTIGATION u 8.1 Discussion of 30 to 50 feet bgs. As discussed above based on the seven borings in this study, the silty gravels are underlain by clayey silts with and without 8.1.1 Existing Above Ground Storage gravel and groundwater was measured beneath Tanks Location the site at a depth of 65 feet bgs. Based on the above, and the RWQCB Guidance document, the nitrate concentration reported in the soil sample y One 550-gallon gasoline and one 550-gallon would not require mitigation. diesel farm ASTs have reportedly been present on Site for the last 5 or 6 years since the removal of DDE, a breakdown product of DDT, was detected the two former gasoline USTs. Initial interviews at a concentration of 33 ug/kg. As previously with Mr. Lopez(foreman) had indicated that the discussed, use of DDT on the Site was diesel AST had previously been present at the discontinued when the federal E.P.A.banned its r.., current location of the secondary containment use in 1973. To evaluate if the detected structure. Surficial soils in the immediate concentration of DDE was a concern, the vicinity of the former AST were concealed by the concentration of DDE detected in the composite installation of the secondary containment soil sample was compared to risk-based soil structure; any potential spillage or leakage stains screening concentrations developed by Region IX if they were present are no longer visible. of the United States Environmental Protection Analytical testing of soil samples collected from Agency(U.S. EPA) (EPA, 1996). These soil boring SB-1, drilled adjacent to the existing ASTs, screening concentrations or Preliminary did not report detectable concentrations of TPHg, Remediation Goals (PRGs) are conservative (i.e., TPHd, or BTEX compounds in any of the health-protective), multipathway values samples. Based on the analytical and field developed by EPA to protect the health of evidence the existing ASTs and former AST at humans potentially exposed to soil through this location are eliminated as a REC. Further ingestion, dermal (skin) contact, and dust inquiries with Mr. Lopez and Mr. Selover, inhalation. Standard risk equations, EPA toxicity v indicated that the diesel AST cited above had values, and conservative exposure assumptions also been previously located near former USTs (e.g., soil ingestion rate and exposure duration) for at least twenty years. This area was were used to develop the PRGs, which are investigated as part of the borings drilled in the regularly updated. For a residential land use ` vicinity of the former USTs. A discussion of the scenario, the PRG for DDE is 1.3 mg/kg; for an UST area is presented in Section 8.1.3. industrial scenario, the PRG is 5.6 mg/kg. The detected concentration of 0.033 mg/kg (33 ug/kg) r., is substantially below both PRGs, suggesting that 8.1.2 Agricultural Chemical Mixing human health risks are unlikely to result from Areas - Composite Soil Sampling exposure to DDE at the three agricultural mixing areas onsite. V Only three of the analytes(nitrate, DDE, and Diuron was detected at a concentration of 0.37 diuron) were present at detectable levels in the mg/kg near the method reporting limit of 0.20 composite sample collected at the agricultural mg/kg. Diuron(trade names e.g., Karamex and chemicals mixing areas. Nitrate was detected in Krovar) is a pre-emergent herbicide commonly the composite sample (reported as NO3) at a used to control weeds prior to germination. concentration of 278 mg/kg. The Regional Water Diuron, as one of the phenylureas, is strongly Quality Control Board, Central Valley Region adsorbed to soils(Ware, 1989) It is therefore (RWQCB) guidance for remediation of nitrate likely to be concentrated in the surficial soils and impacted soils at retail fertilizer facilities unlikely to pose any threat to groundwater (RWQCB, 1996) suggests a cleanup level of beneath the Site. As previously discussed, approximately 900 mg/kg of nitrate as NO3 for interviews with Mr. Lopez indicated that several sites with loamy soils and a depth to groundwater herbicides had been used to control weeds r+ G:\WP\GDH\GH36371Z.151 Harding Lawson Associates 16 Y.r