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Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page screen submergence as shown on Figure 4. The variability of salinity parameters, EC Field and TDS from place to place pose a problem for background characterization because the average salinity in the upgradient well MW-15 is lower than salinities in wells surrounding the pond, even though the pond water salinity appears to be lower than both the background well and the surrounding monitoring wells. The screen submergence described in Section 2.0 likely plays a part as does the local stratigraphy, which is known for sand boils and pressurized confining layers which segregate water into different layers. Nitrate-N has exceeded the primary drinking water standards at MW-10, MW-13, and MW-15. Nitrate-N concentrations at MW-10 are on an overall increasing trend with high spikes in spring and summer. The water quality degradation at MW-10 is likely due to circulation of effluent in a nearby unlined canal. Nitrate-N concentrations are trending down at MW-13; the high of 38.6 milligrams per liter (mg/L) in 2003 likely was the result of a known leak at the foul air duct (since repaired). At the upgradient well MW-15, Nitrate-N test results range from non-detect to 22.2 mg/1. Nitrate-N results appear to spike high in the winter months at this well which monitors areas of agricultural use south (upgradient) of the oxidation ponds. Total coliform organisms (TCO) have been detected sporadically in every monitoring well. At MW-15 TCO has been detected in every sample analyzed.MW-15 is upgradient of the oxidation ponds, indicating that TCO occurs in local shallow groundwater regardless of RWCF discharges. Samples collected in June 2006 from monitoring wells and a variety of surface water sites including Pond 1 were analyzed for general minerals. Although it is risky to draw too many conclusions from the results of a single sampling event, cluster analysis of the general minerals data produced three distinguishable groups. Group 1 is composed of samples from monitoring wells installed close to the recirculation canals and interceptor drains around the east, south, and west sides of the ponds. This grouping of wells receives water from the treatment ponds. Group 2 waters are the poorest quality water and include the upgradient well MW-15, samples of agricultural drain water and water from wells farthest removed from the river (e.g., MW-16 and MW-17). Group 3 waters include the wastewater in the ponds and recirculation canals and wells receiving river recharge. 7.0 BACKGROUND GROUNDWATER QUALITY 7.1 THE BACKGROUND CONCEPT In order to measure groundwater degradation or pollution, samples from wells upgradient from a site of concern often are used for comparison with data from on-site and downgradient wells. Based on upgradient sampling, background threshold values are set for each parameter of concern, and if a sample has a concentration above the threshold value, this indicates degradation resulting from site activities. While relatively simple in concept, there are a host of complexities related to setting threshold values, Panno, et al., 2006,' Kelley and Panno, 20083 and references therein point out how poor remediation/policy decisions can result from its misuse. They propose the concept of "present-day background," which includes local anthropogenic influences on water quality. This present-day background concept is consistent with the language of Order No. R5-2008-0154, which acknowledges pre-existing groundwater conditions when it limits groundwater impacts to "water quality objectives or background groundwater quality, whichever is greater." Kelley and Panno offer a series of questions to guide background determinations, which we answer as follows. = Panno et al., 2006, Estimating Background and Threshold Nitrate Concentrations Using Probability Graphs. Ground Water,44,5,pp 697-709 3 Kelley, W.R., and S.V. Panno, 2008. Some Considerations in Applying Background Concentrations to Ground Water Studies. Ground Water,46,5,pp 790-792 A +i 1� COND044