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Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page 7 groundwater flow. Therefore, to set background values east of the SJR, one-time grab samples of groundwater along traverses through the facility and elsewhere will be used to evaluate spatial variability of groundwater quality in down flow directions. If spatial variability can be quantified from results of the grab sampling, background threshold values can be realistically adjusted for downgradient travel; otherwise, intra-well comparisons will be used to track degradation from a present-day condition. In cases such as MW-13, where water quality objectives for nitrate-N have already been exceeded and downward- trended data exist, intra-well comparisons are the only realistic option for tracking degradation threat. 7.1.4 What does the background value actually represent (e.g., pristine vs. present-day vs. contaminated upgradient)? The background values represent"present-day"conditions at the facility and may reflect acceptable levels of incremental degradation of water quality at specific places downgradient of the facility. 7.1.5 Is this background value appropriate for the area? Yes, if the spatial variability is successfully addressed. There will be no valid single set of background values for all constituents of concern at all areas of the facility. It is probable that more than one and possibly many sets of values will be needed; otherwise the background thresholds would not perform the functions described in Section 7.1.1. 7.1.6 Are there potential sources of the contamination in addition to the source of concern? Yes,there are many sources of salinity and nitrate in the geologic materials underlying and around the site west of the river. Industrial land uses east of the SIR could also contribute to groundwater degradation. The issue of regulatory concern is that these other sources do not mask RWCF activities contributing to degradation that otherwise could be detected. 7.1.7 Are there physiobiochemical processes that may affect the contaminant concentrations within the hydrologic environment? Yes, nitrogen in particular is susceptible to changes underlying the site. Nitrogen in groundwater can either increase or decrease; either by formation from decay of naturally occurring organic matter (soil peat)or by removal during denitrification and off-gassing in chemically reduced environments. There is also significant evidence for physically isolating conditions. Confining or semi-confining groundwater conditions in shallow delta deposits are found around the facility. Therefore, groundwater at some locations may be substantially isolated from nearby groundwater elsewhere, affecting the mixing and transport of constituents. 8.0 DATA NEEDS FOR DETERMINING USEFUL BACKGROUND GROUNDWATER VALUES This section lists the additional data needed to specify useful numerical background values for both the previously-monitored constituents of concern (COC) (total nitrogen, salinity indicators), and the new parameters(B,Na,Cl, dissolved Fe, dissolved Mn). 1. A suite of groundwater quality data (grab samples) from an analogous but unimpacted area to the south of the facility and from areas downgradient of the facility. These data will allow us to estimate the background groundwater quality had the facility not operated, and to estimate if degradation has occurred downgradient as a result of past discharge practices. The correlation of monitoring data with screen depths (Figure 4) suggests groundwater salinity is stratified, so grab samples should be collected from two depths at each location. A iJ CONDO