title: Within-river phosphorus retention: accounting for a missing piece in the watershed phosphorus puzzle.
abstract: The prevailing "puzzle" in watershed phosphorus (P) management is how to account for the nonconservative behavior (retention and remobilization) of P along the land-freshwater continuum. This often hinders our attempts to directly link watershed P sources with their water quality impacts. Here, we examine aspects of within-river retention of wastewater effluent P and its remobilization under high flows. Most source apportionment methods attribute P loads mobilized under high flows (including retained and remobilized effluent P) as nonpoint agricultural sources. We present a new simple empirical method which uses chloride as a conservative tracer of wastewater effluent, to quantify within-river retention of effluent P, and its contribution to river P loads, when remobilized under high flows. We demonstrate that within-river P retention can effectively mask the presence of effluent P inputs in the water quality record. Moreover, we highlight that by not accounting for the contributions of retained and remobilized effluent P to river storm-flow P loads, existing source apportionment methods may significantly overestimate the nonpoint agricultural sources and underestimate wastewater sources in mixed land-use watersheds. This has important implications for developing effective watershed remediation strategies, where remediation needs to be equitably and accurately apportioned among point and nonpoint P contributors.
authors.affiliation: Centre for Ecology & Hydrology, Wallingford OX10 8BB, United Kingdom. hpj@ceh.ac.uk
authors.forename: Helen P
authors.initials: HP
authors.lastname: Jarvie
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Sharpley
authors.forename: J Thad
authors.initials: JT
authors.lastname: Scott
authors.forename: Brian E
authors.initials: BE
authors.lastname: Haggard
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bowes
authors.forename: Lesley B
authors.initials: LB
authors.lastname: Massey
mesh_terms: D004784:Environmental Monitoring; D052918:Environmental Restoration and Remediation; D008956:Models, Chemical; D009825:Oklahoma; D010758:Phosphorus / Q000032:analysis*; D045483:Rivers / Q000737:chemistry*; D013997:Time Factors; D014865:Waste Disposal, Fluid; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D010758:Phosphorus