Managed aquifer recharge (MAR) of purified wastewater can create geochemical disequilibrium which may trigger various water-rock interactions, including the mobilisation of geogenic fluoride. A comprehensive MAR trial has been conducted to investigate the feasibility of recharging recycled, deionised wastewater into the Cretaceous silici-clastic Leederville aquifer of the Perth Basin, Western Australia. During the injection trial, simultaneous pulses of fluoride (up to 1.1 mg/L) and phosphate (up to 1.7 mg/L) were observed to occur rapidly upon breakthrough of the deionised injectate. As fluoride concentrations above 1.5 mg/L are considered detrimental to human health it is important to determine the geochemical mechanisms causing the fluoride release.
Saturation indices for a suite of phosphate and fluoride bearing minerals performed on comprehensive pre-injection groundwater analyses indicated that the fluorapatite-water interface layer of dicalcium phosphate composition (CaHPO4.nH2O) [1] was the closest phase to saturation. Other fluoride-bearing minerals, such as fluorite, were significantly undersaturated and presumably absent. Phosphatic nodules sourced from Leederville aquifer core material were analysed and found to contain significant carbonate-fluorapatite (CFA =Ca10(PO4)5(CO3F)F2), a variety of fluorapatite which is by far the most common autochthonous phosphate mineral in sedimentary environments [2]. Anaerobic batch experiments with powdered CFA rich nodules produced a similar release pattern for fluoride and phosphate to that observed during the MAR field trial. Fluoride extraction experiments with Leederville sediments of low phosphate content yielded minimal fluoride release. The observed fluoride and phosphate pulses can be primarily attributed to incongruous dissolution of CFA mediated by calcium preferential removal onto exchange sites under low ionic strength conditions. Elevated fluoride and phosphate concentrations were found to recede once a new equilibrium for Na-Ca exchange was established under low ionic strength conditions.
[1] Chaïrat, C., et al., GCA, 2007. 71(24): p. 5901-5912.
[2] Föllmi, K.B., Earth-Sci. Rev., 1996. 40(1–2): p. 55-124.