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li n water provided to customers by the City of Stockton. The other 60-70% is produced by City-owned wells from the groundwater basin. Over the past two years, because of exceptional amounts of precipitation provided in the winter months, surface water has been more abundant and comprised the majority of the water supply distributed by the City. Surface water and groundwater are used conjunctively by the urban area water purveyors. Groundwater Currently, the City operates 22 groundwater wells in North Stockton, S groundwater wells in South Stockton, and 3 in the Diamond Walnut Water System. The 22 wells in the north provide groundwater which, combined with surface water from SEWD, makes up the water supply far the North Stockton system. The 5 wells in the South Stockton water system provide the water for that system exclusively. The City's Master Water Plan calls for the continued construction of wells until an adequate long-term supply of surface water can be obtained which will sufficiently meet both base and peaking requirements. `Historically, combined annual groundwater pumping for municipal and agricultural uses 'has exceeded the safe yield of about 13,300 acre-feet for the northern portion of the basin and has caused a lowering of the groundwater level. Although overdrafting has caused a deterioration of water quality in some wells due to the intrusion of poor quality water into the basin, the City's groundwater is generally of good quality, with sequestering of iron and manganese and chlorination being the only treatment required. Iron and manganese sequestering is utilized only at a few wells which exceed the secondary contaminant levels for those constituents. Water from all wells is chlorinated prior to distribution to customers. Saltwater intrusion from connate brines under the Delta into Stockton's western regions threatens groundwater quality, and small annual increases in salinity have been noted during years with low surface water availability. However, due to additional surface water deliveries in recent wet-years, groundwater has risen to pre-drought levels and the saltwater intrusion condition has been somewhat relieved. A study prepared for SEWD indicates that the use of fertilizers and pesticides has also impacted groundwater quality in the Stockton area. The study identified three areas within the Stockton groundwater basin for which there is a significant potential for migration of contaminants within the next 30 years. Two of these three areas are located within the City's service area: one in North Stockton and one in South Stockton. FSurface Water Until 1977, groundwater was the sole source of supply for domestic water users in the Stockton area. A supplemental surface water supply was established in 1977 when the SEWD water 1 F treatment plant began operation. The SEWD plant currently has an approved treatment capacity of t . F-2