Fluoride and boron as geogenic groundwater contaminants in a Cretaceous aquifer (Germany)

Besides arsenic, fluoride is considered to be one of the most widespread geogenic contaminants worldwide. While being essential for the human bones and teeth, elevated fluoride concentrations can lead to severe inverse health... [ view full abstract ]

Besides arsenic, fluoride is considered to be one of the most widespread geogenic contaminants worldwide. While being essential for the human bones and teeth, elevated fluoride concentrations can lead to severe inverse health effects like dental and skeletal fluorosis. Several cases of dental fluorosis were detected in the Münsterland Cretaceous Basin (northwestern Germany), probably associated to elevated concentrations in groundwater from the Cretaceous Emscher Fm. Despite low hydraulic conductivities, this marly stratum is used for drinking water supply by numerous individual wells on farms etc.
The hydrochemistry and geochemistry of the Emscher Fm. were investigated. Groundwater data (760 analyses of individual house wells, mainly from several dentistry PhD studies from the early 2000s) were collected and evaluated. Contents of Corg, Cinorg, Stot and Spyrite were analyzed in 160 rock samples.
Beneath a weathered oxidized zone, unweathered pyrite containing rocks were found. Groundwater partly contains high fluoride and boron concentrations of up to 10 mg/L. Regionally, up to 30 % of house and farm wells show fluoride concentrations above 1.5 mg/L, and up to 50 % above 1 mg/L boron. The spatial distribution depends on the presence or absence of Quaternary cover sediments. Groundwater with high fluoride concentration displays low Ca2+, and vice versa, indicating control by the mineral fluorite (CaF2). Concentrations above drinking water guidelines almost exclusively occur in ion exchange waters of the Na-HCO3-(Cl) type with pH>7.5. The main mobilization mechanism of the two well correlating contaminants appears to be pH-triggered desorption from mineral surfaces.