Prediction of Stormwater Pond TP Removal Performance by Limnological Investigation and Rapid Field Assessment Methods

Stormwater ponds are designed to remove phosphorus (P) from stormwater via sedimentation of the particulate-P fraction per criteria established by the US EPA Nationwide Urban Runoff Program (NURP). They are ubiquitous features of the urban landscape in many states, and especially in northern tier of states. Ponds thermally stratify in the summer and experience sediment anoxia. Ponds typically go anoxic in the winter. Because sediment P is bound largely to ferric iron, reduction to ferrous iron releases P. Therefore, the dead pool P concentration of solubilized P determines pond performance, not sedimentation. A study of 72 stormwater ponds in the western suburbs of Minneapolis, Minnesota USA confirmed that a substantial fraction of particulate P solubilizes because of sediment anoxia induced by thermal stratification. Only 26% of ponds were mostly compliant to NURP performance criteria (median TP ‰¤150 µg/L), 21% partially compliant (median TP 150 to 280 µg/L), and 52% were uncompliant (median TP 290 to 2500 µg/L) (Figure 1). From a limnological perspective, poor compliance is not surprising. From a management perspective understanding performance of ponds across the landscape is problematic. Determination of NURP compliance by direct sampling is largely impractical because there are too many ponds. Therefore, we investigated a rapid assessment protocol (RAP) to predict non-compliance with NURP performance criteria using field observations on ponds that were sampled for phosphorus at least 48 hours after the last significant precipitation event to obtain dead pool TP concentrations. Weight of field parameters to predict dead pool TP was determined with a random forest algorithm. Four RAP score ranges were determined to have significant (Wilcoxon-Mann-Whitney, 0.04 < p < 0.0001) differences in median TP ranges. The most highly weighted parameter predicting dead pool TP values was presence of emergent macrophytes in ponds. The study demonstrated that many RAP parameters could be discarded in future assessments. It appears that other parameters (e.g., pond age) may prove useful in a RAP method. Improvement of stormwater pond performance is an active area of investigation demonstrating practical methods to reduce pond TP discharge or dead pool TP concentrations. A RAP methodology would allow municipalities to efficiently assess stormwater pond performance across urban landscape to prioritize application of methods to improve stormwater P performance.