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A A.' "k WOODARD &CURRAN proposed changes are not included in the water supply reliability evaluation of this WSA. However,Table 26 in Section 5.2 shows how future supply would be affected based on preliminary estimates made by SSJID and as noted above. 2.3.1.2 Water Quality Impacts on Reliability The SCWSP receives water from the Stanislaus River,which is considered to be of high quality prior to treatment and excellent quality following treatment at the DGWTP. The 2011 Stanislaus River Watershed Sanitary Survey, prepared by the Stanislaus/Calaveras River Group and Stockton East Water District, identified several potential sources of contamination in the upper Stanislaus River watershed (Stanislaus/Calaveras River Group and Stockton East Water District, 2011).These sources included recreational activities at Woodward Reservoir, confined animal facilities, cattle grazing, old septic systems, and wastewater disposal. Source control measures have been implemented to mitigate for these potential water quality threats. If water quality begins to degrade, SSJID will evaluate watershed management programs or modifications to the treatment process at the DGWTP. 2.3.2 Groundwater Supply Reliability The City currently relies upon groundwater produced from its four active wells (Wells 6, 7, 8 and 10) to meet its demands. In the future, the City is reviewing potential increased groundwater production by bringing Well 21 and Well 9 back online. As discussed in Section 2.1.2, the reliability of the City's groundwater supplies has historically been limited by water quality constraints.While groundwater quality constrains contribute to supply uncertainties,they are primarily an issue of treatability, rather than supply limitation. Further, the City has demonstrated the ability to modify its groundwater operations to adapt to changing water quality conditions. Therefore, the City is projecting to receive 100 percent of its current groundwater supplies in all year types.As SGMA is implemented in the Tracy Subbasin,the City's groundwater supply reliability may need to be re-evaluated. GSP developments can be evaluated during water supply verification if restrictions to groundwater supply are proposed. 2.3.2.1 Water Quality Impacts on Reliability One of the biggest threats to the City's ability to use groundwater to meet potable water demands is water quality.The primary water quality concerns in the City's groundwater are arsenic, manganese, uranium, TDS, groundwater contamination from industrial processes, and perfluorinated constituents of emerging concern. 2.3.2.1.1 Arsenic and Uranium Wells 6 through 8 and 10 are currently treated for arsenic at LAWTF, and the City is considering improving and expanding the Well 21 WTF to include treatment for arsenic.Well 21 has also experienced elevated concentrations of uranium. The presence of naturally occurring arsenic and uranium in the groundwater underlying the City impacts the reliability of the City's groundwater supply. Expansion of groundwater production in the future is limited by the costs associated with treatment and the availability of adequate supplies to conduct blending. 2.3.2.1.2 Total Dissolved Solids The City's groundwater supply reliability is also impacted by the migration of groundwater with TDS concentrations in excess of the secondary MCL of 500 mg/L(the recommended limit for aesthetic considerations such as odor and taste), a factor which may make the planned wellfield near Well 21 infeasible due to the potential for expanded pumping to induce the migration of high TDS water of greater concentration than currently present at the well site.Wells 6 through 10 are located immediately east of groundwater with high TDS concentrations, based on water quality data from City wells and sampling and analysis data from both shallow and deep monitoring wells collected by private entities (SGI, Califia(0011533.00) 28 Woodard&Curran, Inc. River Islands Phase 2 Development Water Supply Assessment September 2020