SITE INFORMATION AND CORRESPONDENCE_1988-1993

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. Mr. Charles B. Kolesar -2- 20 August 1992 Harding and Lawson Associates' report entitled Technical Assessment of Existing Data, Shell Oil Distribution Facility, Stockton, California dated 24 November 1987 provides the well logs for monitoring wells (MWs) 1 through 22. The logs show that the maximum well depth is 28 feet. Thus, there is no information below this depth. The report also states that soil sampling data for MWs 13, 15, 16, and 17 obtained during well construction are inadequate to characterize the distribution of chemical concentrations in the soil on the site. Furthermore, at MW 15, results of soil samples taken below the water table shows higher concentrations of benzene, toluene, xylene, and volatile hydrocarbons than those of soil samples taken above the water table. This indicates that definition of soil contamination below the water table is essential to account for the total amount of contamination. Attached is an article from the Winter 1991 issue of Ground Water Monitoring Review entitled Evaluating the Effectiveness of Ground Water Extraction Systems which evaluated 19 sites undergoing remediation. The article underscores the importance of defining the vertical extent of soil contamination including soils in saturated zones, among other things, in remediating contaminated sites. We have previously stated that since fuel contamination in the deep zone exists at ARCO, which is proximal to Shell , a similar situation may exist at Shell . The pumping depressions east of ARCO appear to be the cause of the vertical gradient at that facility and may be causing the same phenomenon at Shell . The two facilities' proximity to each other notwithstanding, Shell still must perform its own investigation to ascertain site specific conditions. 2. We concur with your statement that mud rotary is not a recommended drilling method for installing monitoring wells. You may use any appropriate drilling method subject to Board approval . 3. The benzene concentrations greater than 18 mg/l referenced in your letter were obtained from MW 8A at ARCO. MW 8A is a shallow monitoring and was drilled using a hollow stem auger. MW 8B, which is a mid-depth well and had benzene levels of 10 and 14 mg/l during the December 1990 and May 1991 sampling events, also was constructed with a hollow stem auger. However, the benzene concentrations in the deep well MW 8C, which was constructed with mud rotary, were <0.003 and 0.0039 mg/l in the April 1991 and May 1991 sampling events. Although mud rotary is not a recommended method for drilling monitoring wells, the preceding data show that the high benzene concentrations in MW 8A and MW 8B were not a consequence of the drilling method used in constructing the monitoring wells. We were unable to obtain information on the solubility of benzene specifically in ground water. In the future, we request that you cite the source of your information. However, we did find information on benzene's solubility in water. U. S. EPA's Water-Related Environmental Fate of 129 Priority Pollutants, Volume II , cites values ranging from 820 mg/l at 10°C to 1,800 mg/l at 25°C. These concentrations are much higher than the 18 to 31 mg/l detected in ARCO' s MW 8A. Furthermore, gasoline typically has about 29. benzene by weight. Thus, with a solubility of about 1 ,800 mg/l in water and approximately 2% in weight, i