Fluoride geochemistry in groundwater at regulated industrial sites.

Few studies apply geochemical concepts governing fluoride fate and transport in natural waters to geochemical conditions at contaminated industrial sites. This has negative implications for designing sampling and compliance monitoring programs and informing remediation decision-making. We compiled geochemical data for 566 groundwater samples from industrial waste streams associated with elevated fluoride and that span a range of geochemical conditions, including alkaline spent potliner, near-neutral pH coal combustion, and acidic gypsum stack impoundments. Like natural systems, elevated fluoride (hundreds to thousands of ppm) exists at the pH extremes and is generally tens of ppm at near-neutral pH conditions. Geochemical models identify pH-dependent fluoride complexation at low pH and carbonate stability at high pH as dominant processes controlling fluoride mobility. Limitations in available thermochemical, kinetic rate, and adsorption/desorption data and lack of complete analyses present uncertainties in quantitative models used to assess fluoride mobility at industrial sites. PRACTITIONER POINTS: Geochemical fundamentals of fluoride fate and transport in groundwater are communicated for environmental practitioners. Fluoride is a reactive constituent in groundwater, and factors that govern attenuation are identified. Geochemical models are useful for identifying fluoride attenuation processes, but quantitative use is limited by thermodynamic data uncertainties.