Wet Detention Pond Phosphorus Cycling Dynamics: Evaluation of Pond Performance and Application of the General Lake Model

Anthropogenic activities degrade surface water quality by increasing nutrient input to the watershed which is transported via stormwater runoff and accumulated in waterbodies leading to eutrophication. Wet detention ponds are a means of mitigating the impacts of these non-point source pollutants, such as phosphorus (P), through treatment mechanisms such as sediment settling and biological uptake. While these wet detention ponds are designed to serve as P sinks, they have the potential to serve as P sources as well due to release mechanisms such as sediment resuspension and algal decay. Many existing models used in the design and performance evaluation of wet detention ponds do not consider algal release or sediment resuspension of P. This gap between modeled P cycling and the state-of-the-science understanding of P cycling may explain the widespread occurrence of unexpectedly high effluent P concentrations. To advance our understanding of stormwater P dynamics to ultimately improve the models used in the design and evaluation of wet detention ponds, we studied the temporal variation and biogeochemical controls of P cycling in two wet detention ponds over a four-month period. Across the sampling duration, total P concentration was reduced, on average, by 52.7%. Cumulative total P load reduction was 89.5% across the sampling period. In all observed storms, total P concentration was reduced. Total P load decreased in 7 events; however, in 2 storms, total P load increased from influent to effluent. Elevated loads were associated with higher total suspended solids and particulate P indicating algal washout or sediment resuspension and marked variability in pond performance. A sensitivity analysis was performed on pond P removal using the General Lake Model which showed that P removal was most sensitive to pond volume and maximum flux of P into the sediment which supports the value placed in dredging as a wet detention pond maintenance strategy. This knowledge is important for optimizing wet detention pond P removal through design and maintenance strategies that will mitigate water quality degradation