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rDiamand Lodi,CA-November 1993 Groundwater Monitoring <br /> Page 6 <br /> result of flushing from potential off-site sources during seasonal changes in regional groundwater table <br /> levels. <br /> The subject site and surrounding properties are located in an area where the groundwater gradient is <br /> extremely flat. These properties including the former Diamond Lumber facility have been developed since <br /> the late 1800's. Adjacent land use is a mixture of commercial and residential. An automotive repair shop <br /> ' including a wrecked vehicle storage area is located west of the site (see Figure 1), and a former farm <br /> implement sales and repair facility is located east of the site, and is currently a discount furniture outlet. <br /> Tracks for the Central Pacific Railroad extend along the north property line. <br /> Based on site-specific static water level data collected during 6 quarters of groundwater monitoring, the <br /> groundwater gradient in this area of Lodi appears to be relatively flat. Due to the lack of significant <br /> differential in gradient, the surrounding properties should be considered as potential sources of lead <br /> I contamination detected during quarterly groundwater monitoring at the site. It is highly significant that lead <br /> was not detected in analytical soil samples collected during monitoring well installation in 1991. Two <br /> analytical soil samples were collected from each soil boring (SRK-MWI, SRK-MW2, and SRK-MW-3) <br /> during monitoring well installation at depths of 30 feet (seasonal high groundwater level) and 50 feet <br /> (capillary fringe during the November 1991 monitoring well installation/soil sampling). The three <br /> monitoring wells were installed under the direct guidance of the San Joaquin County PHS/EHD field <br /> ' representative in accordance with state and local guidelines. Because these wells were drilled in close <br /> proximity to the former UST location, it is likely that lead should have been detected in at least one of the <br /> well borings if the former tank indeed was the source of lead contamination detected in groundwater at the <br /> site. It should also be noted that the constituents of BTEX or TPH have not been detected in 6 quarters <br /> of groundwater monitoring. It is therefore unclear that any lead contamination detected in groundwater <br /> samples is attributable to the former UST. <br /> 1 Based on available data from the City of Lodi, the former Diamond Lumber facility is not located <br /> upgradient of any active water supply well. It should be noted that City Supply Well 44 (located south of <br /> the site)has been on stand-by since 1985, and is sampled quarterly. Lead reportedly has not been detected <br /> in City Supply Well #4, although the well is located downgradient from the subject site. City Supply Well <br /> #2 (located west of the site) has an average lead concentration of less than I part per billion (ppb). <br /> ' Copies of the Geologic Logs and Monitoring Well Construction Diagrams are provided in earlier reports <br /> of groundwater monitoring and in the report addressing well installation dated 1991. <br /> ' We have also included in Appendix F lead data for a number of large water supply systems in the USA <br /> with lead levels in excess of 0.015 mg/l. <br /> l <br /> 3210:.24w4Qvry.iod Steffen Roberuon and Kirsten <br />