Challenging Assumptions: Per- and Polyfluoroalkyl Substances Detected in Deep Groundwater across Southeastern Australia

Per- and polyfluoroalkyl substances (PFAS) are increasingly detected in groundwater globally, yet their occurrence in deep or confined aquifers remains poorly characterized. We conducted the first regional-scale survey of PFAS in groundwater systems across southeastern Australia, encompassing 54 bores from four locations to establish baseline concentrations. Fifteen PFAS were detected in both shallow and deep wells, with ∑PFAS concentrations ranging from below detection to 78.8 ng/L. PFOS and PFOA were the most frequently detected legacy compounds, including bores screened at >50 m depth and up to 106 m. PFAS were found across a range of land uses, including sites without known contamination sources, indicating potential long-range lateral transport, vertical migration through bore infrastructure or diffuse legacy inputs. These findings provide chemically based evidence that supports the hypothesis of modern groundwater reaching deeper aquifers, previously thought to be isolated from surface contamination. Random forests modeling identified dissolved organic carbon and nutrient concentrations as key predictors of PFAS presence, suggesting links between geochemical conditions and contaminant occurrence. This baseline data set contributes new evidence that deep groundwater systems are not immune to PFAS intrusion and underscores the need to incorporate subsurface transport processes into monitoring and management strategies, particularly for groundwater-dependent ecosystems and water security planning, worldwide.