Mineral formation explains the high retention efficiency of dissolved reactive phosphorus in a residential stormwater pond†

Stormwater ponds (SWPs) alter the export of the macronutrient phosphorus (P) from urban landscapes, hence impacting the trophic state and water quality of downstream aquatic environments. Despite an increasing number of studies reporting P load reduction by SWPs, the mechanisms responsible for P retention remain unclear. We analyzed P chemical speciation and fluxes in the inflow and outflow of a SWP in the Toronto metropolitan area. In addition, we collected sediment cores to determine under what forms P accumulates in the SWP. The resulting P mass balance for the SWP yielded an average annual retention of 62% for total P (TP). Retention efficiencies varied significantly among the various TP fractions, however: 53% for particulate P (PP), 67% for total dissolved P (TDP), 66% for dissolved unreactive P (DUP), and >80% for dissolved reactive P (DRP), with DRP representing the most bioavailable TP fraction. Sequential chemical extractions performed on the sediment cores indicate that, with increasing sediment depth, the concentration of mineral-bound P increases while that of organic P decreases. We therefore attribute the efficient retention of DRP to biosynthesis of P-containing organic compounds followed by their post-depositional degradation and sequestration of the released phosphate ions by in situ precipitation of inorganic phases, primarily calcium (Ca) minerals. The conditions in the SWP are favorable to the formation of common Ca minerals, such as hydroxyapatite and calcite, including near-neutral to moderately alkaline pH values and high dissolved Ca²⁺ concentrations. In areas where urban runoff does not meet these conditions, interventions that stimulate P-containing mineral formation in SWPs may help reduce the export of DRP, hence, protecting receiving water bodies from eutrophication.