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• (0.0002 %). This is a very low concentration. All three samples indicate that the soil contains very little organic carbon that could serve as either a bacterial food source or as a nucleation site for natural attenuation of the hydrocarbons. 5.2.3 Hydraulic Conductivity Five samples were tested by Sierra Testing Laboratories to determine their hydraulic conductivity, porosity, bulk density, and moisture content. These parameters are useful in distinguishing aquifers from aquitards, estimating groundwater flow rates, and performing a health risk assessment. The laboratory report is contained in Appendix C. One sample of the upper silty clay bed, two samples. of the middle sand bed, one sample of the lower clay bed, and one sample of the sand bed that underlies it in boring GP-3 were analyzed. The conductivity of each of the three sand samples is typical of sandy aquifers, and the conductivity of both clay samples is about two orders of magnitude less. The conductivity-of the sand beds ranges from 1.1 x 10-4 to 1.5 x 10-3. The conductivity of the clay beds is 1.1 x 10.6 and 3.7 x 10.6. The mean porosity of the two samples of the middle sand bed is 32.5%, which is within the range of most sand beds. The lower sand bed, being somewhat finer grained, has a correspondingly higher porosity(46%), and the mean porosity of the clay beds is similar(43%). 5.2.4 Bac!erial Abundance 0 Cytoculture, Inc. prepared agar plates for three soil samples: GP-3-24.5', GP-4-17', and GP-5-26'. Two of these samples were from the coarse-grained aquifer sand and one was frons the overlying clayey silt layer, and all exhibited slight to moderate hydrocarbon odors. After preparation, the samples were cultured for eight days before a microbiologist analyzed them for the growth of hydrocarbon-degrading bacteria. The laboratory report is included in Appendix C. All three samples showed extremely low growth, with less than 1.5 colony-forming units per gram in two out of three samples. These results indicate that natural bioremediation of the contaminants is negligible or very slow at the present time. It may be that soil conditions are unfavorable for bacterial growth, perhaps due to inadequate oxygen, a high water table, lack of sufficient nutrients, or absence of a hydrocarbon food source. Another possibility is that bacteria were present at an earlier time and were acting on the petroleum hydrocarbons, but have since declined in abundance as the food source was depleted. 5.3 Depth to Groundwater Samples of the upper silty clay layer were damp, but not saturated. Saturated conditions were encountered during drilling when the sand layer was reached at approximately 20 feet. However, the depth to groundwater in the monitoring wells is less than 6 feet. This indicates that the upper Silty clay layer is a significant confining bed that prevents upward movement of groundwater except 6