Evaluation of Groundwater Supply Alternatives Within Bernal Subbasin

REGIONAL GROUNDWATER FACILITIES PROJECT FEASIBILITY STUDY 6 Zone 7 Water Agency December, 2025 and the broader regional dataset from existing Zone 7 and City wells, which more accurately represents long-term basin conditions. Zone 7 voluntarily sets water quality targets that in some instances are more stringent than state and federal requirements. For example, under Zone 7’s Water Quality policies, PFAS concentrations in delivered treated water is generally required to remain below 80 percent of primary and secondary federal/state Maximum Contaminant Level (MCL) standards and/or 80 percent below proposed and enforced notification levels. Zone 7’s target water quality goals also include more stringent levels for boron, salinity and hardness, taste and odor, hexavalent chromium and other specific contaminants as compared to state/federal MCL standards. A full list of Zone 7 potable water quality target goals can be found in Appendix D. While treatment for hardness and TDS is not required at this time for state and federal regulatory compliance, as part of this Study’s efforts, a Basis of Design Report and 10 percent design with probable construction and operational costs will be developed to allow Zone 7 (separate from the City) to evaluate the feasibility and cost of treatment of these constituents should Zone 7 choose to pursue it for regional salt management purposes. PFAS Assessment PFAS conditions and potential future risks were evaluated using Zone 7’s basin-wide PFAS fate and transport model (EKI, 2025). For modeling purposes, perfluorooctane sulfonate (PFOS) was used as a representative compound for the PFAS class. The 20-year predictive model simulations indicate that PFOS concentrations at all three proposed well sites remain below Zone 7’s operational water quality goals (80 percent of applicable proposed or enforced notification levels or MCLs) under both the Baseline (No- Project) and Project Alternative scenarios. Simulated PFOS concentrations at the Tennis Park site are the highest of the three, primarily due to its relative proximity to the existing PFAS footprint and high production rate, but still remain below Zone 7’s target levels in all model scenarios, with final predicted concentrations ranging from approximately 0.9 to 1.5 parts per trillion (ppt). Modeling results and assumptions are summarized in Appendix G. The conclusion of the PFAS modeling analysis is that treatment for PFAS is not required for the proposed production wells to meet current regulatory or Zone 7 water quality targets. However, recognizing the evolving nature of PFAS regulations and ongoing regional investigations, Zone 7 is further assessing potential feasible PFAS treatment approaches from a conceptual cost and site-space perspective as part of this Study with development of a Basis of Design Report and 10 percent design with probable construction and operational costs. Implementation of PFAS treatment is not planned at this time and would be pursued separately by Zone 7 if future regulatory or operational conditions warrant. 2.1.2. Groundwater Production Rates Test well drilling and aquifer testing conducted at Tennis Park, Hansen Park, and Del Prado Park confirmed that the Bernal Subbasin has favorable hydrogeologic conditions capable of supporting high-capacity municipal supply wells. Constant-rate pumping tests were performed at each site to evaluate sustainable yield, aquifer response, and long-term production potential. Zone 7’s groundwater modeling was also applied to assess the potential for mutual well interference between the proposed project wells and