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2012 data for the new permanent monitoring wells, and first quarter 2012 data from <br /> the existing site monitoring network to depict the distribution of groundwater impacts <br /> in the shallow aquifer. As a result of the very slow rate of movement in the aquifer as <br /> described in Section 4.4.7 of this report, the use of multiple data sources was deemed <br /> acceptably representative of site conditions. As shown on these figures, groundwater <br /> impacts appear to generally migrate outward from the northern corner of the landfill in <br /> a radial pattern following the flow of groundwater with the largest concentrations <br /> measured at well MW-5. The findings of direction and source of migration from this <br /> data is consistent with the on-site monitoring results presented in previous reports. <br /> With respect to inorganic constituent concentrations, the data suggest a degree of <br /> spatial geochemical variability that is consistent with aquifers of lacustrine or marine <br /> sedimentary origin. Concentrations of nitrate as nitrogen exceed the State of California <br /> and Federal MCL in the samples from wells MW-9A and MW-10A, and total dissolved <br /> solids concentration in the sample from well MW-9A exceeded State of California <br /> secondary drinking water standard. These results are consistent with water quality data <br /> for the regional shallow aquifer in the Tracy Subbasin of the San Joaquin Valley <br /> Groundwater Basin (DWR Bulletin No. 118), and are not considered to be indicative of a <br /> release from the CHSL. In particular, nitrate as nitrogen concentrations typically <br /> decrease in reducing environments associated with landfills, and the regional nitrate <br /> impacts have been associated with the agricultural activities in the Central Valley. <br /> In summary, the findings of this EMP investigation support the conclusion that the <br /> 1 release from the CHSL is characterized by very low concentrations of VOCs that have <br /> migrated approximately 1000 feet to the north and west of the property boundary. <br /> However, there is no evidence that VOC concentrations exceeding State of California or <br /> ' Federal maximum contaminant levels have migrated beyond the property boundaries. <br /> There are no known users of groundwater, residents, or commercial or industrial <br /> facilities within the plume area. <br /> EMP Report—Phase 11 Geo-Log0c Page ■ 19 <br /> 1 Corral Hollow Sanitary Landfill ASS O C I ATE S <br /> August 21,2012 <br />