ARCHIVED REPORTS XR0013107 - Laserfiche WebLink

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that the soil gas survey detects the smaller percentage of the lighter volatile fraction of the diesel hydrocarbons. The presence of significant detectable concentrations of volatile fraction hydrocarbons and the BTEX compounds detected in the soil samples suggests that the diesel may not have undergone extensive degradation. Soil gas detections were not always associated with detectable soil concentrations. However, ,.. detections of petroleum hydrocarbons in the soil at a location were consistently associated with higher soil gas concentrations at that location. Insufficient soil data were collected to establish whether a quantitative relationship exists. The soil gas sample and the soil sample did not agree at locations 2, G, and J, although the hydrocarbon levels at locations 2 and J were relatively low and location G had soil concentrations at a greater depth. The soil gas sample.at location G was found to contain about 2,000 ppm TPH while the soil sample at the LW closest depth to the soil gas sample did not contain detectable quantities of TPH. One ' possibility for this disagreement is that soil samples represent only the discrete volume of soil collected in the sample whereas soil gas samples represent a larger volume of soil because �- they are obtained by inducing vapor flow through the soil matrix. With greater concentrations at depth, vapor flow would also be naturally upward. The correlation between the soil gas samples and the soil samples for the BTEX compounds was less distinct at lower concentration levels. The soil gas test is specifically a screening method, consequently the detection limits achieved by the soil gas survey are higher than the �- limits achieved by the soil analyses. In summary, it appears that the soil gas survey was successfully used as a qualitative screening tool for the detection of light fraction hydrocarbons from an area of relatively non- degraded diesel contamination. The effectiveness of the soil gas method as a screening tool for heavily degraded diesel contamination was not demonstrated in this investigation. 6.2 Recommendations Based on the results of the investigation, the following recommendations should be Y considered: L • Prepare a report to the county presenting the results of this investigation and supporting the applicability of the soil gas survey technique as an initial survey method. • The soil gas survey should be used to screen the soils adjacent to the V remainder of the pipeline. The survey should be supplemented with a low number of soil samples collected periodically over the length of the pipeline for laboratory analysis to validate the soil gas results. In addition, soil sample �. collection should be focused in areas of known or suspected pipeline leaks; • Upon completion of the entire soil gas survey over the four-mile length of the pipeline, soil and groundwater investigations should be performed in areas where significant hydrocarbons have been detected by the survey, including the 4 �+ E. C&%TOCKTON.SG 6 it