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e , SOIL CONTAMINATION ASSESSMENT Lockeford Mini Mart NGA Project No. 10-90-104 Page 5 3.3 Installation of Soil venting System In the event that soil sample analytical results would indicate concentrations of petroleum constituents in native soils sufficient to pose a threat to ground water, a decision was made to install the below-ground components of a soil venting system in the tank excavation prior to the installation of new USTs at the site. Below-ground components of the soil venting system consist of two 30-foot length of 4-inch-diameter, schedule 40 PVC well screen (0.020 slot) lain in a horizontal configuration at the base of the tank basin (Figure 4) . The PVC well screen was connected by means of a 20-foot length of 4-inch--diameter, schedule 40 PVC blank casing. The PVC blank casing was manifolded to the surface (via a shallow trench) to an area immediately behind the mini mart building. Following installation, the PVC well screen and blank casing was overlain by approximately two feet of clean backfill material (silty sand) , which was in turn overlain by 6-mill plastic sheeting. The plastic sheeting will serve as a barrier to air flow between the soil venting system (should it be utilized) and the new USTs (and associated clean backfill) which were installed immediately above the soil venting system. 4.0 DISCUSSION OF RESULTS 4.1 Soil Analytical Results The concentrations of petroleum constituents found in soil samples taken from beneath the removed USTs and along product lines are, - with one exception, very low. Soil sample T3N was the only sample analyzed that contained TPH at a concentration exceeding 15 ppm. The TPH concentration of T3N was 390 ppm. while there are no regulations governing concentrations of BTXE constituents or TPH in soil that may be left in place, the California Water Resources }uv Control Board has published guidelines in the Leaking Underground Fuel Tank (LUFT) field manual pertaining to concentrations of BTXE and TPH in soil that may be left in place without threatening ground water. Table 3 (reproduced from Table 2--1 in the LUFT manual) is recommended for use in estimating BTXE and TPH concentrations in soil that can be left in place. Table 3 is used by assigning a score based on the answer to questions concerning site features. Depending upon site conditions a score of 10, 9, 5, or 0 points is assigned for each site feature. The total number of points for a site is then compared with the range of total points shown for the various suggested maximum allowable concentrations of BTXE and TPH that could be left in the -- - - -- .-.--soil,-..- --.-.-.