SITE INFORMATION AND CORRESPONDENCE_1994-1996

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Shell Distribution Facility, Stockton -5- 23 March 1995 or"atypical"diesel results should be considered in this light: perhaps the diesel plume is degrading along the east side. The consultant should analyze, and submit to us, the chromatograms that should have accompanied these analytical reports.) Similarly, Finding 6 refers to definition of one plume. The data suggest at least two plumes, one of diesel and one of gasoline. The lead and BTEX distributions may also suggest another source, or differential movement and degradation of these gasoline components. Examination of the contaminant distribution maps as given in the 1994 Annual Report suggests that the groundwater flow patterns are not well understood, or, that the gradient is variable. Arrows plotted on Figures 7-10 of the Report clearly indicate "average" gradient for each figure; examination of the flow directions indicated by the arrows, and plotting a few flowlines between liquid surface contours, shows that during 1994, the flow directions actually vary from south with a slight southwesterly component to southeast. Other reports I have reviewed from the Shell and the Navy files indicate that the gradient is occasionally to the southwest. This amount of variation is to be expected and may be related to seasonal, climatic, inadequately assessed tidal, or offsite artificial influences, but it does not by itself explain the offsite distribution of contaminants to the east and north. The apparent variability in flow neither supports nor disputes Shell's suggestion that detections to the southwest are effects from the Navy site. In all cases, it should be remembered that the ground water elevations shown on the maps in these reports are potentiometric surface elevations rather than elevations within a water table aquifer. On the average, according to the Annual Report, gradient at the site is 0.005 ft/ft, which is fairly gentle. Perhaps if the plots showed actual groundwater surface measurements, rather than "liquid" surface measurements (which includes floating hydrocarbons) some difference in pattern might be perceived. Progress of the Remedial Actions The Ground Water Extraction (GWE) system began operation again on November 29, 1994. The Report states (p. 6) that the liquid surface elevation map for the fourth quarter 1994 shows that the GWE has created a hydraulic depression at the site "that appears sufficient to halt or slow" hydrocarbon migration. There are, however, no data presented to support this conclusion. A zero line is not defined, as required by the CAO. Further, the results of the fourth quarter sampling, taken after the GWE was restarted, show continuing presence of hydrocarbons in wells to south and offsite of the extraction system. The system has not been operating very long; therefore a conclusion about its effectiveness may be premature. Differences in groundwater contour configurations are apparent between the second and third quarter maps that are at least equally as significant as the differences between the third and fourth quarter contours. Moreover, two of the three GWE wells (wells MW-1 and MW-23) and an offsite well, MW-27, are not used in contouring the third quarter maps. This is not explained, and the possible significance of the "anomalous"readings in wells 1 and 23 (when the GWE was supposedly not active) are not discussed. At the very least, this circumstance renders the comparability of the quarterly maps questionable. The Annual Report does not attempt to estimate the length of time that will be needed for remediation with the GWE and/or with the SVE..