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that contamination may extend to between 75 and 80 feet in that area The low concentrations detected in deeper samples in GT-11 (and also in GT-3 in 1990) may be due to cross-coni amination during sample retrieval, as the samples were pulled upward through contaminated groundwater Evidence for this suggestion includes the fact that samples at different depths have simiIar results, and there is no progressive decrease in concentrations downward Not 4roL , ^iy "e swtefc icrca&,,A w,.-- I g &4-6s r a 9) Measurements of the hydraulic conductivity of beds in the Tupper pari of the Riverbank Formation range from 1.8 x 10-3 cm/sec in the sand beds to 4 4 x1cm/sec in the silt bods These values are characteristic of moderately productive aquifers and moderately restrictive aquitards, respectively Additionally, we must conclude that the groundwater gradient and flow direction are considerably more complicated than was previously thought, and are in doubt at this time Because th-_ possible flow directions are numerous, the direction in which the dissolved-phase contaminant ph.me might migrate is uncertain. The effect of pumping of the many domestic, irrigation, and industrial wells ' P �� ' ' ✓ within 2000 feet of the site cannot be evaluated at this time, because there is i tsufficient information on screened intervals and pumping rates for these wells -� 6.0 RECOMMENDATIONS Based on the data that have been collected in this and previous phases of drilling, and in view of regulatory concerns about the magnitude of diesel contamination in the aquifer, we make the following recommendations 1) Application to aerate the stockpiled soil from borings GT-10 and GT-11 should be rr ade to the San Joaquin Valley Air Quality Management District if granted, the soil should be ma ✓ed to the rear of the Gillies Trucking yard and spread out for several weeks The soil should then tie sampled to determine its suitability for on-site disposal 2) Groundwater sampling should continue on a quarterly basis for the foreseeable futuie At this time, we have been unable to demonstrate that natural attenuation of hydrocarbon concentrations in the aquifer is occurring, or to determine attenuation rates from which predictions abou the long- term fate of contaminants could be made Water samples from GT-10 should be analyzed for gasoline and BTEX as well as diesel,and EPA method 8260 should be used to check the validity of the reported detection of MTBE in water from this well Any other wells in which a diesel concentration in excess of 1 ppm is detected should also be analyzed for gasoline a,id BTEX During the next monitoring event and annually thereafter, the depth to groundwater should be measured in the Gillies and Barbot domestic drinking water wells and these wells should be sampled 3) Due to the uncertainty regarding the direction of groundwater flow, it would also be advisable during the next monitoring event to measure the depth to groundwater in some of ie nearby domestic wells that were identified in the well survey. To do this, it would be necessary to obtain 11 &00(A