Stormwater retention ponds are ubiquitous in many urban landscapes, but are not given much consideration post-construction. High total phosphorus (TP) concentrations from pond surface water sample data for 98 ponds in Minnesota suggest that urban stormwater ponds may be releasing phosphorus (P) to receiving water bodies due to high sediment phosphorus release. This is alarming because retention ponds are one of the most frequently implemented stormwater control measures (SCMs) targeting phosphorus removal. Laboratory incubations of intact sediment and water cores from eight ponds suggest that mobile-P concentrations (iron-bound P, loosely-bound P, and labile organic P fractions) in pond sediments and sediment oxygen demand (Smax) are indicators of P release potential. Actual P release was measured under anoxic (dissolved oxygen (DO) &lt;1 mg/L) conditions and was observed to be negligible under oxic conditions (DO &gt;1 mg/L). However, in-situ field monitoring revealed that several shallow stormwater ponds in the Twin Cities Metro Area are so strongly stratified during the spring and summer months as to prevent diurnal mixing and reoxygenation of the water column from periodic storm events. This is largely attributed to wind sheltering from mature trees. Additionally, conductivity profile measurements suggest that chemostratification also plays an important role and results from chloride applied as road salt during the winter months. The resulting anoxic conditions at the sediment-water interface throughout much of the year would facilitate P release into the water column and subsequently downstream receiving water bodies.

Sediment phosphorus release in stormwater ponds

2019 Fall National Meeting

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