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Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional wastewater Control Facility Page 10 10.1.1 Two new shallow wells MW-1S and MW-2S New wells are required to resolve the anomalously high salinity reported from wells MW-1 and MW-2, east of Pond 1. These wells have relatively high salinity when compared with data from upgradient well MW-15, and data from effluent waters. These data suggest the existing monitoring wells draw from deep groundwater that has not minced with infiltrating pond water. To test this hypothesis, short-screened, 4- inch diameter,shallow wells, similar to MW-15 will be installed adjacent to the existing wells and will be designated MW-1S and MW-2S, correspondingly. The locations for the proposed new monitoring wells are shown on Figure 7. Also shown on Figure 7 is a cut out of an aerial photograph showing former land use at the location of MW-1 (circa 1940). An alternative hypothesis for explaining the higher apparent salinity, though less likely, would be this different former land use. The new wells will be monitored in addition to and on the same schedule as MW-1 and MW-2. Data from the new wells will be compared to data from the deeper wells, Pond 1 effluent data, and upgradient well MW-15 to develop appropriate background values for these monitoring points. 10.1.2 Two new monitoring wells MW-19 and MW-19S The degree to which pond water mixes with groundwater is important to determining BPTC. It is reasonable to conclude from existing data that little or no mixing occurs. Effluent water is generally less saline (i.e., less dense), and likely warmer (most of the year) than underlying groundwater, so unstable density gradients are not expected to occur. There are no significant hydrostatic gradients, and the stratigraphy indicates horizontal barriers to vertical flow are present. Impacts from chemical diffusion would be expected to be low because of the high organic content of the soil that provides adsorption sites for constituent molecules. If no significant mixing occurs, groundwater beneath the facility is effectively isolated and BPTC are in place. To demonstrate if this isolation occurs, paired monitoring wells,MW-19 and MW-19S will be installed on John Turk Road in the middle of the causeway between Pond 1 and Pond 2 (Figure 7). Data from these wells will be used to differentiate, the water quality of the uppermost groundwater and groundwater deeper in the geologic section. 10.1.3 Close Well MW4 Well MW-4 has been removed from the monitoring network and will be closed in accordance with San Joaquin County regulations. 10.2 EVALUATION OF BPTC IN THE VICINITY OF MW-10 Monitoring data from MW-10 exhibit degradation with respect to nitrate-N. The data have trended upward since initial values of 8.1 mg/L in 2003.This coincided with construction of the wetlands.Recent data show groundwater contains 23 mg/L nitrate-N with previous spikes as high as 64 mg/L. This well is located adjacent to the SJR and the Raw Water Channel, currently used for storage of wetlands effluent water prior to tertiary treatment. The river is a losing stream at this location, with stage elevation higher than ground surface. Depth to water in MW-10 averages 11.22 feet, so the groundwater gradient is landward, toward the Raw Water Channel. Average screen submergence is 4.22 feet and the screen length is five(5)fed, indicating the nitrate plume may extend nine(9) feet below the water table.Nitrate impact appears to be well contained by the hydrogeologic system and unlikely to migrate to the river or off-site. Gradients under the oxidation ponds and wetlands will keep the nitrate phone cornered in the vicinity of the Raw Water Channel. The extent of the nitrate plume is unknown; however, the risk of escape is small, as is the potential to compromise secondary beneficial uses off-site. A BPTC analysis of the plant operations at the Raw Water Channel will be performed and an analysis of BPTC for addressing this groundwater degradation will be completed. w t.l� CONDO