The Essential Role of Aquitard Boundaries in Geochemical Outcomes for Public Supply Wells

Many public supply wells (PSWs) fail because their water chemistry does not meet regulatory standards, despite pilot hole water quality testing suggesting compliance. This is partly attributed to conventional testing that focuses on the mid-section of permeable zones, excluding low-permeability units or aquitards. The main goal is to prove yield, so groundwater is sampled within the tested interval for efficiency. Clay boundaries are typically excluded from zone testing because of low expected yields, but they may harbor elevated concentrations of constituents of concern. Well discharge concentrations may thus be non-compliant due to the blend of groundwater from permeable, high-yield zones and less permeable, elevated concentration zones. We evaluated flow and chemistry across the screens of 143 wells in California and Nevada, identifying the screen intervals with maximum arsenic, iron, manganese, and nitrate concentrations. We examined the relationship between sediment type, flow contribution, and maximum concentrations, focusing on the influence of aquitard boundaries and interbedded sequences on geochemical outcomes. Maximum concentrations occurred mostly (73-84%) in well screens associated with interbedded or coarse sediments with an aquitard boundary. Intervals with aquitard boundaries had higher arsenic concentrations (p = 0.02). In non-compliant wells, 64-69% of the maximum metal concentrations were sourced from fine-grained and interbedded sediments, warranting their inclusion in water quality zone testing. Approaches that may provide the geochemical resolution to determine the distance between aquitard boundaries and well screens are suggested to minimize the risk of constructing non-compliant PSWs that then require treatment.