University of Minnesota:
“There is evidence that aging stormwater ponds can become net sources of phosphorus to receiving waters during high flow events. For example, the total phosphorus (TP) concentration in the pond should be equal to or less than the typical inflow TP concentration (due to settling). To evaluate this, we examined data from the Riley Purgatory Bluff Creek Watershed District (RPBCWD) in Minnesota. Between 2010 and 2012, the RPBCWD surveyed 98 ponds and observed high TP concentrations in a number of them...
All of the pond sediments released ortho-P under low DO conditions (< 0.5 mg/L) , as shown in Table 1 and Figure 5). By contrast, ortho-P release was negligible under high DO conditions (not shown). We used the calculated P flux rates for each pond to approximate the potential impact on the overall TP concentration in the pond (Table 1)... Stratification during the summer with turnover and mixing in the fall and spring are normal in large lakes, but small ponds were thought to turn over diurnally or at least during large storm events. Instead, regular manual temperature profiles throughout 2017 revealed evidence of stratification throughout most of the year, with DO concentration below 0.5 mg/L at the bottom sediment surface.
Phosphorus concentration is unusually high in roughly 1/3 of urban stormwater ponds, indicating that they are not performing as designed. Rather than capturing P, these ponds seem to be releasing P and polluting downstream waters. We believe the reason is the sediments are releasing ortho-P to the water column, a process known as internal loading. Phosphorus release from the sediments of ponds may be facilitated by low DO concentration. It appears that many ponds in the Twin Cities Metropolitan Area are stratified through most of the summer by accumulation of road salt in winter and spring snowmelt, therefore leading to low DO concentration that promotes internal loading. ”
