Internal loading in stormwater ponds as a phosphorus source to downstream waters

We assessed the prevalence and causes of sediment phosphorus (P) release within urban stormwater ponds, a process that may reduce P removal by sedimentation. Data collected from surface water of 98 urban stormwater ponds in Minnesota showed that nearly 40% had median summer total P concentrations in excess of average stormwater runoff (0.38 mg L⁻¹), implying effects of internal loading. We sampled seven ponds more intensively and found four were strongly stratified with persistent hypolimnetic anoxia, despite mean depths <2 m. Sediment core incubations revealed that, unlike in most lakes, both labile organic P (NaOH minus persulfate extractions) and redox-sensitive P (NH₄Cl and Na₂S₂O₄ extractions) contribute to P release. Together, these analyses suggest P accumulated in stormwater ponds is highly susceptible to internal release and potentially contributes to downstream eutrophication. Understanding how frequently these conditions occur and how they affect different P forms is vital to improving pond design and management.