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17. Background Concentrations, Section 4 and Section 5.1 <br /> Figure 5-1 and Tables 5-1 and 5-2 provide locations and analytical results <br /> (respectively) for some background soil borings and wells. However, it is understood <br /> from paragraph 4.1.2.0.2 that additional Phase 1 data were used to calculate <br /> background concentrations. It is difficult to determine which additional Phase 1 wells <br /> and boreholes were used for this purpose. <br /> • Please add a table which provides a listing of all boreholes and wells that were <br /> used in the calculation of background concentrations and the associated <br /> analytical results. <br /> 18. Background metal concentrations, Section 4.1.2 (page 4-4) and Paragraph 5.1.2.0.2 <br /> (page 5-2) <br /> Although EPA has approved "Amendments to the Comprehensive RI/FS Work Plan, <br /> 1993" which allows obtaining background data from areas of known contamination, <br /> limitations using this approach for metals should be recognized. The use of metal data <br /> from locations contaminated with organics as background may not be suitable in some <br /> cases. The following is an example demonstrating that metal contamination may be <br /> triggered by the release of organic contaminants. <br /> Certain metals are known to be adsorbed by iron and manganese oxides which are <br /> typical in soil-water environments. Iron and manganese oxides are stable under <br /> aerobic conditions. When certain organic contaminants are released to an oxygenated <br /> soil-water system, the microbial degradation of the contaminants eventually will <br /> deplete the dissolved oxygen. This will lower the redox potential of the system and <br /> bring the system to an anaerobic environment. When the redox potential is lowered to <br /> a certain level, iron and manganese oxides will dissolve. The dissolution of iron and <br /> manganese oxides will lead to the release of any adsorbed heavy metals. If <br /> background samples are taken from locations with elevated metals released from the <br /> dissolution of iron and manganese oxides, then the calculated background levels are <br /> misleading. Metals known to be associated with iron oxides include arsenic, cadmium, <br /> chromium, cobalt, copper, lead, mercury, nickel, selenium, uranium, and zinc. <br /> As indicated on page 5-2, soil samples from background locations contained several <br /> metals, including arsenic, beryllium, chromium, and lead. A maximum chromium <br /> concentration from these background locations was as high as 739 mg/kg, which <br /> seems very high. This raises a concern if the background sampling locations were <br /> improperly selected, e.g., they could have been selected in an area having organic <br /> contamination. As explained above, metal data from such locations are not suitable <br /> for background calculation. A comparison of the background soil metal levels with <br /> published data representative of the Tracy area may be useful to determine the validity <br /> of the background soil metal concentrations. <br /> EPA 14FEB94 610 <br />