Rates of hydroxyapatite formation and dissolution in a sandstone aquifer: Implications for understanding dynamic phosphate behaviour within an agricultural catchment

Abstract Eutrophication, caused by excessive nutrient concentrations, is a major environmental issue and has significant impacts on both aquatic ecosystems and human health. Phosphorus (P) is a key element that contributes to this eutrophication response. As such, P concentrations are regulated under both the European Union Water Framework Directive (EU WFD) and Habitats Directive (EU HD). While P export to rivers from point sources is well-understood, diffuse sources, particularly that route through connected aquifer systems are less well-characterised. Based on data from a catchment in southern England where an Upper Greensand aquifer controls river baseflow, we hypothesise that dynamic precipitation and dissolution of phosphorus in the form of hydroxyapatite occurs along the groundwater flow path, affecting P storage and export. In addition, P may also occur as the less-reactive mineral fluorapatite or sorb onto, or substitute into, Fe-oxides, which also occur in the aquifer. To investigate hydroxyapatite kinetics in this context, batch experiments were conducted to precipitate the mineral onto sand grains, and then dissolved in a continuous-flow reactor under various pH and concentration conditions. The reactive transport of PO43- along a 1D flow path was modelled to simulate hydroxyapatite precipitation and dissolution. Fe-oxides were modelled as sorbing surfaces for phosphate under chemical conditions representative of the aquifer. Hydroxyapatite rapidly precipitated onto quartz sand grains from a solution that was supersaturated with respect to hydroxyapatite, thus demonstrating that an input of P to Ca-rich, alkaline porewaters can result in the precipitation of secondary hydroxyapatite. The dissolution rate of hydroxyapatite is strongly pH dependent, however within the aquifer porewaters, the solution is often close to equilibrium with respect to hydroxyapatite, which causes the dissolution rate to have a greater dependence on the PO43- concentration. Within the range of measured pH and PO43- concentrations, the dissolution rate ranges between 10-12.25 to 10-9.15 mol kg−1 s−1, contributing P to the catchment headwaters. Over a 1D flow path, there is dynamic precipitation and dissolution of hydroxyapatite, as well as PO43-sorption and desorption, over relatively short timescales. These results indicate that hydroxyapatite is likely to precipitate within the pore space of the aquifer, but can then dissolve and re-precipitate, adsorb and desorb, under expected spatial and temporal variations in pH and water chemistry within the aquifer. Hydroxyapatite may therefore represent temporary pool of anthropogenic P, sourced from P-fertilisers, which represents a previously unrecognised pathway for anthropogenic P transfer between the soil surface and surface waters.