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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 22 iron and phosphates. Groundwater in the Upper Tulare Formation is also affected by rising concentrations of nitrates which in groundwater recovered from man wells in the g g Y neighborhood, are close to the maximum contaminant level (MCL) permissible for potable water. The sources of the nitrates in the neighborhood are agricultural fertilizers } and septic tanks, which provide the only means of sewage disposal in an area not served by a municipal water supply or sewerage system. i Groundwater in the Lower Tulare Formation is of better quality than that in the Upper Tulare Formation and is generally free of significant concentrations of nitrates but; locally, it may contain phosphates, iron or other naturally-occurring minerals that exceed secondary water quality goals. Due to the risk to potable water posed by rising =� concentrations of nitrates in groundwater extracted from the Upper Tulare Formation, at -� several sites ii-i the Tracy area that are not served by a municipal water supply (and for which SJC has served as project managers or consulting engineers) we have directed the =- installation of replacement or new water-supply wells that produce groundwater from the Lower Tulare Formation beneath the Corcoran Clay. Those include two new water- supply wells north across West Eleventh Street in the neighborhood of the subject F=. property. The new 520-ft. deep well at the Carmelo Business Park replaced a shallow well that drew water from above the Corcoran Clay, which well is now used only to produce irrigation water for landscaping. The other is a recently installed 500-ft. deep 'F well at 7601 West Eleventh Street. The locations of both of those water-supply wells are ,_._i shown on Figure 2. 6.2 Groundwater Gradient and Flow Direction The depth to groundwater beneath the site varies seasonally between 7 and 11 ft. Regionally, the general direction of groundwater flow is to the north toward the Old 'r= River anastomosic branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street site. Locally, the direction of groundwater flow is affected by the local sedimentary geology, particularly where continuous or semi-continuous sand strata provide channels for subsurface flow through less permeable facies. The depths to groundwater measured in the 15 groundwater-quality monitoring wells that have been installed to characterize the 7500 West Eleventh Street site were measured on April 11, 2002. The depths varied between 7.02 and 10.56 ft. Together with the surveyed �_J well casing elevations, those measurements were used to calculate the elevations of the groundwater beneath the site. The results are shown in Table 2. - The groundwater elevations presented in Table 2 were then used to generate the groundwater contours shown on Figure 8. However because, as is noted in Table 4, an apparent 0.3 ft of floating product was detected in Monitoring Well MW-7, the groundwater elevation in that well was corrected for the purpose of drawing the groundwater contours to allow for the depression of the water table in the vicinity of that I well by the hydrocarbon fuel products floating on the surface of the water. The correction was derived as follows: SJC J