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Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page 5 7.1.1 How will the background values be used? At the RWCF,background values will be used to answer two fundamental questions. 1. What constituent concentrations can be compared to new data from compliance sampling points to detect degradation of groundwater quality downgradient of the facility? 2. Has the groundwater quality downgradient of the facility been degraded as a result of past practices? In either the forward-looking (Question No. 1) or backward-looking (Question No. 2) sense, threshold values can be used to evaluate Best Practicable Treatment and Control (BPTC) for minimizing off-site water quality degradation. 7.1.2 Do we need a temporal or spatial background? We will need both temporal and spatial background thresholds. To answer Question No. 1 above we can apply a present-day background concept to stable data from existing monitoring points to detect future changes with time. This is a temporal type comparison. Spatial background data are used to compare upgradient and downgradient water quality to detect degradation. This is best applied in a linear flow situation, such as east of the SJR, where water near the river will flow east under the facility. West of the SJR the RWCF ponds are at higher hydrostatic head than surrounding groundwater and true upgradient groundwater sample sites do not exist west of the river. While groundwater at MW-15 is at higher hydrostatic head than wells surrounding the RWCF ponds, groundwater at MW-15 will most likely flow south of the RWCF ponds to agricultural drainage ditches and will not flow under or through the areas affected by pond discharges. In these areas, spatial background can only be used by employing surrogates for what the water quality upgradient of the facility would have been if the facility did not exist. To answer the backward looking question (Question No. 2 above) we are interested in what the water quality at our monitoring points was before any discharges from the facility. Historical data to provide background from before facility operation are lacking. Available regional datasets are limited to the eastern side of the SJR (Figure 3) and do not include the new constituents of concern (e.g., Na, B, Cl, Fe and Mn)<.5 We must therefore rely on present-day sampling from analogous areas to represent the range of water quality that otherwise would have been expected at the site had the facility not operated. This approach uses a spatial surrogate for the previous groundwater quality under the facility, but also carries the"baggage"of other ongoing uses that may have had their own impacts to water quality. In addition, spatial background analysis is required for other reasons. We know that monitored groundwater can be divided into at least three (3) groups based on water quality. The use of"one-size- fits-all" background values compared across these groups could result in constituent background thresholds that are too lax for some constituents and too stringent for others. For example, the highest nitrate-N concentration west of the river is 22.2 mg/L, well above the water quality objective of 10 mg/L. This concentration occurs at the "upgradient" well MW-15, which has desirable characteristics of a background well (i.e., upgradient of discharges, and similar land use as before the ponds were installed). However, nitrate-N concentrations at most monitoring wells are well under the water quality objective of 10 mg/L. A background nitrate threshold based on MW-15 data (or even the water quality objective) would not provide adequate detection of incipient nitrate degradation at wells where nitrate is historically 4 San Joaquin County Department of Public Works, 2007, San Joaquin County Flood Control and Water Conservation District Groundwater Report Fa112007 s USGS,USGS California Water Science Center,http://c.water.usgs.gov,gmapsiwq_map.html A �/� CONDOR