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Projoct Status t1 <br /> -� 1 Update Report.Subsurface Investigation <br /> Banner Island, Stockton, CA <br /> September 17, 1992 <br /> w0602.1 C <br /> • Recent sampling of well MW-5 at Weber Point, located approximately 1/4-mile east of <br /> the Banner Island Property, showed detectable concentrations of the same metals <br /> (antimony, arsenic, barium, and zinc) reported in the samples collected at Banner Island. <br /> • A review of available data bases (from the California Department of Water Resources, <br /> Regional Water Quality Control Board, United States Geological Survey, and the San <br /> Joaquin County Flood Control District) was performed to characterize local groundwater <br /> conditions in the Stockton area. Information obtained indicated that groundwater in the <br /> Stockton area is of poor quality, exceeding drinking water standards for a number of <br /> analytes including chloride, iron, manganese, TDS, arsenic, barium, chromium, and <br /> selenium. Several sources have been identified for the observed poor water quality <br /> including deep connate water from marine sediments, infiltration from the tidally <br /> influenced deep water channel, and influx of water through dredge spoils adjacent to the <br /> channel (RESNA Industries, Report, Second Phase Subsurface Investigation at Banner <br /> Island Property, Stockton, California, May 29, 1992). <br /> • Considerable variation has been observed between sample analyses. Although a total of <br /> 6 different metals have been detected above State MCLS during the two sampling rounds, <br /> only 3 metals were detected above MCLS during both sampling rounds. Most <br /> importantly, only two wells have shown an analyte above State MCLs during both sample <br /> rounds; well BI-MW-4 (78 and 71 yg/t of arsenic), and BI-MW-1 (13 and 19 mg/t of <br /> selenium). When the two samples taken to date from each of the wells on the site are <br /> averaged, the average value for each analyte is well below State MCLs. A comparison <br /> of the two sampling events shown on Table 2 clearly shows these affects. Well BI-MW- <br /> 9 recorded <5.0 gg/t of antimony in March, and 520 pg/t in August. This same well <br /> had 90 µg/l of zinc in March and 7200 µg/t in August. Similar large differences were <br /> noted for certain other wells, however the two wells, mentioned above which recorded <br /> analytes (arsenic and selenium) above State MCLS during both sample rounds showed <br /> consistent values. Moreover, the arsenic and selenium analyses for all of the wells <br /> during the two sampling events were relatively consistent. This consistency provides a <br /> degree of confidence that the values are representative of prevailing groundwater <br /> conditions at the site and can be compared to other hydraulically distant sites in <br /> addressing the question of background metals levels in the Stockton area. <br /> Analysis of water samples from 3 other sites in the downtown Stockton area that are not <br /> in hydraulic communication with the subject site are shown on Table 3. These analyses <br /> show a pattern of somewhat elevated metals levels, including some analytes above <br /> MCLS, which is very similar to the results of sampling at the subject site. For the two <br /> most consistent analytes recorded on the subject property (arsenic and selenium) the <br /> levels are similar to those documented at the distant sites. One of the distant sites has <br /> R 0RTSU-R0VM.PML 4 <br />