Frequent stormwater management pond turnover leads to poor water quality performance

Stormwater management (SWM) ponds are ubiquitous blue spaces throughout urban and urbanizing landscapes designed to reduce flood wave energies and sediment from upstream impervious environments to downstream receiving water bodies. Research suggests SWM ponds become thermally stratified throughout growing season months in temperate regions, which has important implications for biogeochemical cycling, notably the development of anoxic bottom water capable of denitrification but also redox-mediated phosphate release. We present detailed analysis of temperature profiles of a typical SWM pond from southern Ontario, Canada, over a 12-month period, demonstrating daily temperature-driven density inversions capable of mixing the entire water column during summer months. This mixing was accompanied by elevated total phosphorus levels throughout the water column, with output concentrations during steady-state conditions averaging 98 μg L^-1, over three times the provincial water quality guideline for healthy water systems. These levels were on average three times greater than inflow concentrations and occurred on 78 % of sampling events. Much of this enrichment was due to increased dissolved organic phosphorus as the water migrated through the pond. Total suspended solid (TSS) concentrations followed similar patterns, with outflow levels on average 15X greater than inflow levels during steady state conditions, occurring on 88 % of sampling events. In contrast, nitrate and total nitrogen were generally reduced through the pond, due to biologic uptake in the surface waters and probable denitrification in the bottom waters. Despite the temperature-driven mixing, the SWM pond was strongly chemically-stratified during ice breakup and snowmelt due to salinity differences in surface and deep waters, which had important implications for the export of nutrients and TSS. This research demonstrates that while SWM ponds can serve as urban refugia for aquatic, avian, and terrestrial organisms and reduce flood pulses, there may be limited water quality improvements.