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i 4) Man Made Conduits which increase vertical migration of leachate: None are known to be present beneath the subject site. Such conduits might consist of vertical piping or wells f F` directly beneath the former tank or piping installations. Based on evidence from the site excavations, there are no vertical conduits beneath the former tank or piping locations. Because fluids migrate essentially vertically downward in unconsolidated material, the off-site water well would not be considered a potential vertical conduit. Even if it was located near the hydrocarbon leak, the cement/grout seal which is mandatory in all wells would prevent fluid migration��ti:?e.,',, - downward along the well casing. ^ 5) Unique site features-.recharge area, coarse soil, nearby wells, ° etc: There are no recharge areas near the subject site. The soil has been described as silty clay from eight to 30 feet BGS, with a dense sticky gravelly clay '(75% clay by volume) from 30 to 40 feet BGS, underlain by hard brown clay from 40 to 130 feet BGS. This type of soil would most lik:ly create a low permeability barrier to .downward fluid migration rather than a fluid "pathway. Although i:here is an irrigation well near the site, it would not act as' a pathway for fluids due to the reasons stated in Item 4, above. Based on this analysis, the site specific score on the L'JFT form is 49 points and the Maximum Allowable Benzene/Toluene/Xylene/Ethyl- Benzene levels are 1, 50, 50 and 50 ppm, respectively. Therefore, the toluene residues on the site may be left in place. As shown previously, the TPH concentrations on-site are all below the analyticel. detection level and thus well below the TPH action level kof 1000 ppm, The .total lead analyses showed a range of lead concentrations in the soil, all below 20 ppm, which is within the range of natural t background lead in soil of 15-30 mg/Kg (ppm) (U,s.G,s. Professional Paper 574--F) . Upoii request of the EHD, the two samples with the highest total lead concentration were. further analyzed for soluble lead by the CAM WET method. . Because the soil is a solid, the Soluble Threshold Limit Concentration or STLC is the correct F regulatory criteria to.apply to the analytical results according to Title 22 of the California Administrative Code. The STLC for lead is 5.0 mg/L, which represents the concentration at which the material could be considered hazardous. The soluble lead concen- tration of C.13 mg/L in S-2 is well below this regulatory limit. Likewise, the soluble lead concentration in S-6 was below the detection limit for the analysis;,and is thus below the STLC for lead. Ing Finally, from a strictly hydrogeolcgic standpoint, _the.re__is_no -physical-or-chemical-mechariU at the Linden site to_transport the existing low conre;,tration of toluene into the deeper subsurface soil. To mobilize such residue would require total saturation of the soil column with an overlying hydraulic head to create downward fluid flow, as . if a lake or reservoir were overlying the former N- : tank and piping locations, 8 -