Pulses of elevated fluoride (up to 58 μM) and filterable reactive phosphorus (FRP) (up to 55 μM) were observed were observed during a large scale groundwater replenishment trial where 3.9GL of highly treated, deionised wastewater (average TDS 33 mg/L) was injected into siliciclastic Leederville aquifer of the Perth Basin. Previous experimental work identified that the elevated fluoride and phosphate concentrations are due primarily to the dissolution of the carbonate-rich fluorapatite (CFA: Ca10(PO4)5(CO3F)F2) which was found to occur in the Leederville aquifer sediments. A reactive transport model has been developed using MODFLOW (Harbarough, 2005) and PHT3D (Prommer et al. 2003) for groundwater flow and reactive transport processes respectively. It was calibrated to geochemical analysis collected approximately monthly from 20 monitoring bores over the four year period of the field trial. The model incorporates the incongruent dissolution CFA as the primary source of fluoride and includes all geochemical processes identified during previous experimental study and modeling work. The primary motivation for the current modelling study is to assess whether fluoride concentrations may eventually exceed drinking water limits (1.5 mg/L = ~79 μM) with continued large scale injection.
The model simulations identified that fluoride is initially mobilized due to acidity generation near the injection well where the low ionic strength but oxic deionized wasterwater is triggering pyrite oxidation. Further dissolution of CFA is triggered by the very low calcium concentrations of the deionized wastewater injectate. Under the low ionic strength conditions calcium initially preferentially partitions on aquifer exchanger sites and elevated pulses of fluoride concentration occur when calcium concentrations in solution remain low. Sorption of fluoride under the moderately alkaline conditions (pH 7.6-7.9) to the aluminium-rich Leederville aquifer sediments was found to be limited. Maximum fluoride concentrations were inferred to be controlled by equilibrium with CFA occurring in the aquifer and are not expected to exceed the elevated concentrations that were observed under post breakthrough low calcium conditions. However a long tapering pulse of moderately elevated fluoride is expected to remain in the aquifer. Scenario modelling of mitigation strategies involving the amendment of CaCl2 and CaO (quicklime) in the injectate to further reduce fluoride and phosphate mobilization during managed aquifer recharge were also assessed. Insights from this study may be broadly applicable to understanding fluoride release and mobilization from CFA and similar fluoride-bearing calcium phosphate minerals both during managed aquifer recharge (MAR) operations as well as due to natural processes.