ARCHIVED REPORTS 2ND QUARTER 2010

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Beneath the WWTF, water levels in the nine WWTF monitoring wells increased by an average of about 0.5 feet between First Quarter 2010 and Second Quarter 2010. Groundwater elevation contours indicate horizontal groundwater movement is generally to the west and southwest beneath the Land Application Sites and to the north and northwest beneath the storage ponds. Groundwater flow directions are generally similar to flow directions observed during previous monitoring events. The calculated gradient between paired wells was 0.0013 beneath the Land Application Sites and was 0.0030 beneath the storage ponds (Table 5). Gradients beneath both the Land Application Sites and the storage ponds increased relative the First Quarter 2010. 3.2 Groundwater Analytical Results and Interim Limitations Table 6 shows groundwater sampling results for all constituents analyzed, and Figure 4 presents box plots (stem and whisker diagrams) that concisely display the data distributions for the pooled Crossroads site monitoring wells. In the Lathrop area, observed background groundwater quality exhibits substantial spatial and temporal variability and background concentrations typically exceed interim limitations.4 In a report submitted to the RWQCB in 2003, background groundwater quality was tabulated and statistical analyses employed to recommend final groundwater quality limitations for the Crossroads site.5 Subsequently, Lathrop's "Best Practical Treatment and Control Evaluation Work Plan" (BPTC Workplan) submitted to the RWQCB in 2005 concluded that the final limitations recommended in 2003 may not be appropriate.6 The 2005 BPTC Workplan included data suggesting groundwater quality beneath Crossroads Land Application Sites likely exceeded the 2003 recommended limits prior to disposal of recycled water. The BPTC Workplan therefore recommended further study to better understand background groundwater quality beneath the Crossroads site. This additional study has not yet been performed pending RWQCB concurrence with the BPTC Workplan conclusions. As required by the WDRs, the Second Quarter 2010 data are compared to the interim limitations (Table 6). Concentrations of TDS, sodium, and chloride in all the monitoring wells exceeded interim groundwater limitations. However, as described in the BPTC Workplan, the TDS, chloride, and sodium concentrations in all wells exceeded interim groundwater limitations prior to the application of wastewater on the Land Application Sites. Therefore, the current TDS, sodium, and chloride concentration exceedances are likely due, at least partially, to background groundwater quality conditions. 4 City of Lathrop, Background Groundwater Quality Study Report, Membrane Bioreactor Wastewater Recycling Plant, Order No. R5-2006-0094 February 27, 2009, HydroFocus, Inc. 5 City of Lathrop Water Recycling Plant No. 1, Background Groundwater Quality Study Report, November, 2003, Nolte Associates, Inc. 6 City of Lathrop Water Recycling Plant No. 1, Best Practical Treatment and Control Evaluation Workplan, January 2005, Nolte Associates, Inc. - 11 -