Evaluation of Groundwater Supply Alternatives Within Bernal Subbasin

REGIONAL GROUNDWATER FACILITIES PROJECT FEASIBILITY STUDY 5 Zone 7 Water Agency December, 2025 2. GROUNDWATER SUPPLY This section provides the hydrogeologic and water quality context for the Bernal Subbasin of the Livermore Valley Groundwater Basin, including aquifer conditions, PFAS occurrence, and TDS trends relevant to the siting of new production wells. The information presented is based on Zone 7’s Alt GSP, subsequent annual reports, and the results of recent test well drilling and aquifer testing performed as part of this Study. As the GSA, Zone 7 prepared the Alt GSP for the Livermore Valley Basin in 2016, submitted a five-year update to the Alt GSP in 2021, and has prepared Annual Reports for the Basin since implementation of the Alt GSP. These planning and reporting efforts provide the technical and regulatory foundation for this Study and for Zone 7’s ongoing role in managing basin sustainability. 2.1. Hydrogeologic Setting The Main Basin of the Livermore Valley Groundwater Basin, a portion of which underlies the City, is comprised of the Upper Aquifer, Lower Aquifer, and the Livermore Gravels. The base of the Upper Aquifer occurs at depths ranging from approximately 70 to 190 feet below ground surface (bgs) across the Main Basin, with greater thicknesses observed in the southeastern portion. Beneath the Upper Aquifer is a relatively continuous regional aquitard up to 50 feet thick. Cross-sections indicate that the Lower Aquifer extends to depths of about 550 feet bgs in the Pleasanton area. Below this, the upper 200 to 300 feet of the older Livermore Formation also exhibits relatively permeable intervals that may contribute to groundwater production (EKI, 2021). Groundwater quality in the Bernal Subbasin reflects both natural conditions and anthropogenic influences. Maps from the most recent Alt GSP Annual Report show PFOS concentrations in the Lower Aquifer exceeding 4 nanograms per liter (ng/L) across a continuous area extending from near the intersection of Black Avenue and Hopyard Road in Pleasanton eastward to the quarry pits, and from just south of I-580 to Stanley Boulevard. This area encompasses the City’s existing Wells 5, 6, and 8, which have been taken offline due to PFAS detections. The western edge of this continuous PFAS-impacted zone lies between the existing City wells and the proposed test well sites (Zone 7, WY 2024 AR). 2.1.1. Water Quality As part of this Study, exploratory test well drilling, step-drawdown testing, and water quality sampling were conducted at Tennis Park, Hansen Park, and Del Prado Park to characterize source water quality at the three potential production well sites. Separate Test Well Investigation Summary and Well Design Recommendations for each of the three well sites can be found in Appendix A, B, and C which provide detailed analytical water quality results, including concentrations of regulated and unregulated constituents, variability by aquifer depth, and explanations for why certain constituents detected in test well samples above state regulatory or Zone 7 water quality target thresholds are not expected to persist at similar levels in production wells. These differences reflect the inherent limitations of test well sampling, which can produce conservative or non-representative results for these certain constituents,