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Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page 8 2. Depth-discrete grab samples of groundwater adjacent to existing monitoring wells outside the facility interceptor drain system. These data will allow us to evaluate the stratification of salinity seen in monitoring well data. To correlate well data with other grab sampling, discrete depths should be the same as in Item 1. 3. Grab samples of groundwater from traverses parallel to flow lines running away from the SJR to the east. These data will allow us to evaluate spatial trends in degradation with travel distance east of the river. Data would be used to adjust threshold values of different constituents and should be collected at two discrete depths as above. 4. Shallow well data at MW-1, and MW-2. These data will allow us to compare monitoring data from MW-15 (shallow background well) with similar data at these locations. Currently these wells have much higher salinity than "upgradient" well MW-15, but they also have deeper screens. 5. Shallow and deeper well samples from areas immediately under the ponds. These data will allow us to evaluate the degree to which pond effluent waters mix with underlying groundwater. 6. Additional data on pond effluent geochemistry. These data will allow better characterization of pond effluent and better evaluation of risk to and/or degradation of background groundwater. 7. Establish methods for evaluating the mixing of pond effluent with groundwater. Existing data from monitoring wells MW-15, MW-1, MW-2, MW-5 and MW-6 suggest little mixing of pond effluent with groundwater occurs. Methods to demonstrate that percolated effluent is not mixing with groundwater would be an important part of any BPTC evaluation. Data collected in Items 1 through 6, above, will provide general chemistry of shallowest and deeper groundwater to better understand if these waters are mixed with effluent or not. Water isotope analyses of pond effluent, background groundwater, and monitored groundwater could provide an independent quantitative method to evaluate mixing of pond effluent and groundwater. For isotope analysis to be useful, the different sources of water (e.g., pond effluent, background groundwater, river recharge water, and agricultural drainage water)would have to have distinct S'H and 5180 ratios. 9.0 GROUNDWATER GRAB SAMPLING 9.1 GEOPROBEO GRAB SAMPLING PROGRAM Further information regarding the spatial distribution of groundwater quality is needed before a clear understanding and designation of background water quality can be achieved. To address the fust three data needs identified in Section 8.0, groundwater grab samples will be collected using a Geoprobe's push- rod system. Drilling permits will be obtained from the San Joaquin County Environmental Health Department(SJCEHD)prior to drilling and the areas will be marked for USA 72 hours prior to drilling. A site specific health and safety plan has been prepared(Appendix 5)and will be reviewed and signed by all field workers prior to commencing activities. The work will be completed by a C-57 licensed contractor under the supervision of a Condor field geologist working under the direct supervision of a California Professional Geologist. Sampling equipment will be cleaned or replaced between samples. No cuttings will be generated by the push probes. Sample boreholes will be filled with grout after sampling is complete at each location. The samples will be collected from two discrete depths at each location, the depth of first groundwater, and approximately 20 feet below fust groundwater, if possible. Thirty-four (34) grab sample locations have been specified on Figure 5, resulting in a total of 68 groundwater samples. Groundwater samples for metals will be field filtered through a 0.45-micrometer (µm) pore-size filter prior to collection; unless the field geologist judges that field filtering becomes excessively lengthy. Any unfiltered samples must be noted on field logs and filtering will be done at the laboratory within 24 hours. No acid preservatives will be used on unfiltered field samples. The samples will be analyzed for the constituents listed in Table 5. Sample collection and shipping methods are described in the Sampling and A 1� CONDO