Detecting phosphorus release from stormwater ponds to guide management and design

Research findings

Phosphorus Concentration in the Water Column

• Dissolved oxygen (DO) concentrations play a key role in mediating stormwater pond phosphorus concentrations.
• In anoxic conditions (low DO), the phosphorus held in pond sediments is released into the water column.
• We found that DO concentration in ponds is controlled by a number of factors:
  • In deep ponds (> 1 meter), high road salt concentrations at the bottom of ponds lead to chemical stratification and low DO (anoxia).
  • Tree canopy cover reduces wind-driven mixing of pond waters, leading to thermal stratification and anoxia.
  • Ponds with duckweed (Lemna spp.) and watermeal (Wolffia spp.) showed extremely low DO during summer, because sheets of free-floating plants on the surface of ponds act to suppress aeration. Because these anoxic conditions promote sedimentary phosphorus release, and because plants thrive on high-nutrient waters, duckweed or watermeal may create a positive feedback mechanism that promotes high phosphorus levels, which further stimulate plant growth.  

Phosphorus Release from Stormwater Ponds

It’s often assumed that the amount of water flowing into and out of stormwater ponds is approximately equal and that there is little loss to evapotranspiration or groundwater. However, our investigation revealed that water balances differ significantly across the seven ponds we studied, independent of precipitation patterns. 

• We observed significant differences in pond volume response to periods of drought.
  • A small number of sites maintained steady and near-capacity volumes during dry periods, while at the same time a majority of sites lost water volume via non-outflow water losses (via evaporation or groundwater losses).
  • This reduction in pond water volume between rain events, due to non-outflow water losses, increases a pond’s storage capacity.
• Because phosphorus release is more likely to occur when stormwater ponds overflow during or after a rain event, water level data can help managers assess the likelihood of specific ponds retaining or releasing phosphorus.