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Work Plan and Time Schedule for Analyses of Background Gmuadwater Quality City of Stockton Regional Wastewater Control Facility Page 3 SM the groundwater gradient is toward the Stockton groundwater pumping depression to the east. The configuration of the groundwater gradient is consistent with recharge of the shallow water table from the river. 5.0 THE GROUNDWATER QUALITY DATABASE The current water quality database primarily consists of quarterly field parameters and groundwater sample data collected since 2003 from a network of 18 monitoring wells shown on Figure 2.MW-4 is no longer active and there are currently 17 active monitoring wells. The data include salinity parameters, nitrate as nitrogen and ammonia as nitrogen and contain about 20 independent values for each parameter at each site. The data sets for three (3) salinity indicators, field electrical conductivity (EC Field), laboratory electrical conductivity (EC Lab) and total dissolved solids (TDS) have approximately 350 independent measurements for each parameter in all wells. Data for nitrogen parameters included many non-detects. Supplemental testing of groundwater,pond water, and agricultural drainage water for general minerals was performed in 2006. The supplemental data are included in the full database shown in Table 2. The current dataset does not include information on sodium (Na), boron (B), chloride (Cl), dissolved iron (Fe) and dissolved manganese (Mn); these parameters were added to the required monitoring suite for Order No.R5-2008-0154. Normal distributions are required for most useful statistical treatments used to deme exceedance probabilities and background threshold values. The salinity data were analyzed for normal distributions using the Lilliefors test for high sample counts (l) and the Shapiro-Wilk test(N<50)in ProUCL, an EPA statistical program. ProUCL output reports are included in Appendix 1. Each of the three salinity- indicator datasets were combined for all wells and tested for a normal distribution. None of them were normally distributed. This indicates that multiple populations of data are present among the different wells and time periods represented in the data. The preliminary"take home"message from this statistical information is that the site as a whole cannot be well characterized by one set of salinity data from these monitoring points. When each salinity parameter was tested in each well, we found most of the EC Field data fit normal distributions (except MW-17), but that in approximately half of the wells EC Lab and TDS data were not normally distributed. The fact that some wells exhibited normal populations of EC Lab data while others did not could be data variability from sample handling. The EC Field is the preferred parameter if measuring equipment is routinely calibrated, because EC can be affected during transport to the laboratory. The fact that almost all wells had normally distributed populations of EC Field data indicates that parametric (predictable) statistics may be applicable at an intra-well level. This also indicates that some variability between TDS constituents within wells could be due to seasonal or other causes that are non-random and would need further statistical analysis to evaluate suitable methods to evaluate background conditions.Table 3 summarizes the results of the Shapiro-Wilk intra-well testing. 6.0 HYDROCHEMISTRY Information from Report of Groundwater Conditions in the Vicinity of the City of Stockton Regional Wastewater Control Facility Oxidation Ponds prepared by Condor in 2006 was updated through December 2008 (see Table 2). Chemical data are plotted in Appendix 2. Additional analyses, performed for this Work Plan,have led to the following discussions. EC Field and TDS exceed the secondary drinking water standards in all wells except MW-11 and MW-14 located east of, and near the river (see Table 4). The highest values are in samples from MW-1, MW-2, and MW-5 with mean EC values over 2,000 micromhos per centimeter(equivalent to 2,000 µS/cm). The upgradient well MW-15 to the south has a mean field EC of 1,453 µS/cm and the downgradient well MW-16 to the west has a mean field EC of 1,525 µS/cm. EC Field and TDS are both correlated with i� CONDO