Stormwater runoff driven phosphorus transport in an urban residential catchment: Implications for protecting water quality in urban watersheds.

Increased stormwater runoff in urban watersheds is a leading cause of nonpoint phosphorus (P) pollution. We investigated the concentrations, forms, and temporal trends of P in stormwater runoff from a residential catchment (31 low-density residential homes; 0.11 km2 drainage area) in Florida. Unfiltered runoff samples were collected at 5 min intervals over 29 storm events with an autosampler installed at the stormwater outflow pipe. Mean concentrations of orthophosphate (PO4-P) were 0.18 ± 0.065 mg/L and total P (TP) were  0.28 ± 0.062 mg/L in all runoff samples. The PO4-P was the dominant form in >90% of storm events and other-P (combination of organic P and particulate P) was dominant after a longer antecedent dry period. We hypothesize that in the stormwater runoff, PO4-P likely originated from soluble and desorbed pool of eroded soil and other-P likely originated from decomposing plant materials i.e. leaves and grass clippings and eroded soil. We found that the runoff was co-limited with nitrogen (N) and P in 34% of storm events and only N limited in 66% of storm events, implicating that management strategies focusing on curtailing both P and N transport would be more effective than focussing on only N or P in protecting water quality in residential catchments.