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the U.S . EPA RCRA Technical Enforcement Guidance Document. <br /> Additional samples have been collected and analyzed for <br /> total lead in and around the surface impoundment for com- <br /> parison to the ambient levels. Since no organic constitu- <br /> ents were previously detected, only total lead was analyzed. <br /> Total lead concentrations to ND or background have been <br /> established for the surface impoundment and its immediate <br /> vicinity. Re-Manufacturing proposes to completely remove <br /> all soils containing total lead in excess of background <br /> levels. <br /> 7/8 . A three-dimensional representation (plan view and cross- <br /> section) will be provided for sample locations around the <br /> impoundment identifying the extent of lead contamination <br /> exceeding background. <br /> 9 . It is unlikely that the inconsistencies in the previous data <br /> can be conclusively explained to the satisfaction of the <br /> Board. It is not uncommon for results to be sporadic or <br /> non-recurring due to anomalies in sampling or analytical <br /> procedures. It is possible that the increase in TEH concen- <br /> trations is due to sluffing of cuttings from upper soil <br /> zones or due to accumulation on a less permeable horizon. <br /> Isolated occurrences of phenol detection may very likely be <br /> due to laboratory contamination or error. In any case, <br /> confirmation samples will be collected during closure activ- <br /> ities to address analytical inconsistencies observed during <br /> previous sampling. <br /> 10. "Background" lead concentrations must be established for the <br /> immediate vicinity reflecting levels present in soils which <br /> have not been affected by discharges to or emanating from <br /> the surface impoundment. In this area, because of historic <br /> activities throughout the entire Stockton area, this concen- <br /> tration could be substantially different than naturally <br /> occurring concentrations . Ambient lead concentrations are <br /> elevated area-wide due to many years of manufacturing and <br /> ship-related activities in the area. Samples have been <br /> collected upgradient of the site in Mormon Slough and in the <br /> general area which record nonhazardous, but elevated concen- <br /> trations of total lead. These results can be evaluated <br /> statistically using the Average Replicate t-test to deter- <br /> mine a range of lead concentrations which more accurately <br /> reflect current "background. " The locations of the samples <br /> will be provided for review by staff. <br /> Conclusion: <br /> A determination will be provided by a qualified professional <br /> (California Registered Civil Engineer and/or Registered <br /> Geologist ) pursuant to Section 25208 . 8 of the Health and <br /> Safety Code indicating whether constituents have migrated <br /> into the vadose zone, surface water bodies, perched water, <br /> or water-bearing strata upon completion of sampling activi- <br /> ties . <br /> 3 <br />