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CLOSURE REQUEST REPORT Section No 4 0 Former American Forest Products Company 07/31/01 2803 West Lane Page 4-3 Stockton,California 16 14. Assumptions, parameters, calculations and model used in risk assessments, and fate and transport modeling. A risk assessment for the site was presented in the FS/RAP (Dames & Moore, 1988), and revisited in the Technical Evaluation (Dames & Moore, 1994c) A basic assumption in evaluating risk was that the residual soil impacts are located at depths greater than 20 feet, which would prevent contact with humans, unless excavation was conducted to remove impacted soil Another important factor which limits the risk of exposure is the distance of the site to active drinking water wells, and the fact that the drinking water wells downgradient from the site are screened in a deeper, semi-confined aquifer, as opposed to the shallow unconfined aquifer Impacts to groundwater derived from the leaching of residual TPH in soil was modeled using VLEACH, as presented in the Draft Amended Closure Report (Dames & Moore, 1994a) The fate and transport model was run using a 25-year time frame, and focused on naphthalene, which was assumed to be the most mobile constituent of diesel (Note naphthalene has not been detected in groundwater samples collected at the site) The maximum naphthalene concentration reported in residual soil samples is 4 0 mg/kg, which is below the FRG of 56 mg/kg The maximum naphthalene concentration reported prior to excavation was 7 8 mg/kg) Conservative assumptions were utilized, such as non-degradation of naphthalene Results of the modeling for three levels of groundwater indicated that if groundwater remained at a constant level of 35 feet bgs for 25 years, groundwater naphthalene concentrations could reach of 36 5 pg/L, which exceeds the MCL of 20 pg/L The rate of naphthalene transport to the nearest water supply well was calculated at 250 years, assuming no degradation (Dames & Moore, 1988) If groundwater remained at levels deeper than 35 feet bgs, then the predicted naphthalene concentrations would be below the MCL (Dames & Moore, 1994a) Because of the potential for naphthalene leaching into groundwater, the 1994 closure request included the stipulation that groundwater should remain below 35 feet bgs 15 Rationale why conditions remaining at the site will not adversely impact groundwater quality,health, or other beneficial uses. The apparent sources of TPH impacts, the USTs, have been removed, and approximately 47% of the impacted soil was excavated and removed in 1990 The residual impacted soil is located at depths where human contact is unlikely The area of soil impacts is capped with asphalt, which functions to reduce TPH leaching from soil to groundwater Impacts to drinking water wells is unlikely due to distance and hydrogeological conditions Available TPH groundwater data collected since 1990 does not indicate the presence of free product, or a widespread distnbution of excessively elevated TPH concentrations TPH concentrations in excess of 780 gg/L have not been reported, indicating that the groundwater impacts are not severe The TPH detected has generally been weathered diesel, indicating that natural degradation is occurring 16 Waste Extraction Test(WET) or TCLP results. WET or TCLP analyses are not available However, treated soil was analyzed for hazardous waste classification parameters specified in the California Code of Regulation Title 22 Article 11 prior to the transport and disposal at the Class II landfill (Dames & Moore, 1992b) Results of the screening indicated that the soil was classified as non-hazardous K 1840191afp1afp_c1usure_rpt doc