Speciation and Transport of Phosphorus in Source Area Rainfall–Runoff

Phosphorus (P) delivered by urban rainfall–runoff partitions and speciates during the transport process. This study examines transport and speciation of P in rainfall and runoff across 15 wet weather events from a paved source area dominated by biogenic loads and to a lesser degree, anthropogenic loads. The mean and median event-based source area total phosphorus is 3.6 and 3.5 mg/l, respectively. The mean and median event-based source area dissolved fraction (f _d) are 0.31 and 0.32 illustrating that P is predominately bound to particulate matter fractions. The majority of events across the monitoring campaign produce a weak mass-limited transport of dissolved phosphorus (DP). With respect to the DP fraction in runoff (pH range of 6.4 to 8.6), the dominant species are orthophosphates (HPO₄ ^?2 and H₂PO₄ ^?) which account for more than 90% of DP mass. The order of species predominance is consistently HPO₄ ^?2?≈?H₂PO₄ ^??>>?CaHPO₄?>?MgHPO₄. With rainfall pH ranging from 4.2 to 4.9 and a f _d?≈?1.0, H₂PO₄ ^? accounts for 95% to 99% of DP in rainfall. Despite the inherent variability of a large dataset (362 samples across 15 events) the speciation of DP is influenced primarily by pH, with a range from 4.2 (rainfall) up to 8.6 (runoff) that results in an order of magnitude change in P species concentration and determines the order between the dominant orthophosphate species. For this source area, the role of alkalinity, dissolved organic carbon and partitioning on DP speciation are minor compared to the influence of pH.