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THE CITY OF <br />Chapter 6 <br />Water Supply Characterization PL£ASANTON <br />6.2.2.2.4 Groundwater Quality Monitoring and Protection <br />In general, the Main Basin contains good -quality groundwater that meets all state and federal drinking <br />water standards; groundwater is chloraminated to match the disinfectant residual in the transmission <br />system. Zone 7 has several groundwater wells with naturally -occurring hexavalent chromium (Cr(VI)) <br />concentrations nearthe Maximum Contaminant Level (MCL) and polyfluoroalkyl substances (PFAS) above <br />the notification limit. In response, Zone 7 is actively managing flows from the affected wells. For example, <br />Cr(VI) levels at the Stoneridge well is being managed through system blending and/or blending with other <br />wells. Also, the PFAS levels in the Mocho 2 well currently require blending with the other wells in that <br />wellfield and/or being sent through the MGDP. These conditions are being monitored and may change in <br />the future. <br />Over the last few decades, there has been a slow degradation of groundwater quality as evidenced by <br />rising total dissolved solids (TDS) and hardness levels. To address this problem, Zone 7 developed a Salt <br />Management Plans (SMP), which was approved by the Regional Water Quality Control Board (RWQCB) in <br />2004, satisfying a condition of the Master Water Recycling Permit. The SMP was incorporated into <br />Zone 7's GMP in 2005. Salinity levels are being addressed primarily through groundwater pumping and <br />demineralization6. Zone 7 completed construction of the 6.1 -MGD MGDP in 2009 in the Mocho wellfield. <br />The facility simultaneously allows for the removal and export of concentrated minerals or salts from the <br />Main Basin and the delivery of treated water with reduced TDS and hardness levels to Zone 7's customers. <br />Table 6-1 lists the average TDS and hardness for each year from 2016 through 2020. <br />Table 6-1. Groundwater Quality: TDS and Hardness (2016-2020) <br />2016 <br />2017 — <br />2018 <br />2019 <br />L 2020 <br />685 <br />416 <br />673 <br />395 <br />673 <br />409 <br />687 <br />417 <br />683 <br />433 <br />Zone 7 implements a wastewater and recycled water monitoring program as part of the GMP. In the 2020 <br />water year, about 14 percent (1,036 AF) of the recycled water produced in the Tri -Valley area was applied <br />to landscapes over the Main Basin; the remainder was applied on areas outside of the Main Basin, <br />primarily on areas overlying the Dublin and Camp fringe basins and the Tassajara uplands. There is also a <br />small amount of untreated wastewater (681 AF in the 2020 water year) that is discharged to the Main <br />Basin as leachate from wastewater treatment ponds located in southern Livermore, from onsite domestic <br />wastewater systems (septic systems), and from leaking wastewater and recycled water pipelines that run <br />throughout the Basin. <br />5 Zone 7 Water Agency, 2004. Salt Management Plan. htto://www.zone7water.com/publications-reports/reports-Dlanniniz- <br />documents/158-salt-manaeement-plan-2004 <br />6 The brine concentrate resulting from the treatment system is exported to the San Francisco Bay via a regional wastewater <br />export pipeline. <br />WEST YOST <br />N-680-60-20-04-WP-R-680-2020UWMP <br />6-13 <br />City of Pleasanton <br />2020 Urban Water Management Plan <br />May 2021 <br />