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

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