Emma Krause, Justine Dauphinais, Chase Vanderbilt - Coon Creek Watershed District
Background
Stormwater ponds are widely used to reduce sediment and phosphorus in urban runoff. Over time, their performance can decline, so it is important to monitor performance over time.
- Epiphany creek pond is a shallow (<1m), large (~5 ac)flow-through stormwater pond constructed in the early 1980s to provide rate control in Coon Rapids, MN
- Historically a Type 3 inland shallow fresh marsh prior to development
- Limited sediment has accumulated since construction, but dense vegetation growth has been observed.
Photos of aerial view of epiphany creek pond in 1985 versus August 2024 with dense lily pad and submerged vegetation.
Photo of Epiphany creek in 1980 prior to construction of epiphany creek pond
Map of pond area
Existing conditions
2024 stream monitoring displayed:
- Periodic elevated total phosphorus (TP) at downstream outlet compared to upstream inlet
Site observations displayed
- Dense submersed and floating leaf vegetation
Research question: Is stratification and/or vegetation contributing to internal phosphorus release
Methods
2025 monitoring included:
- 10 sampling events throughout the season
- Analyzed chloride (Cl), total phosphorus (TP), and ortho phosphorus (OP) in both the epilimnion and hypolimnion to track vertical nutrient and ion distribution
- Continuous water level and fixed-depth temperature monitoring in bothe the epilimnion and hypolimnion with sonde profiles for each sampling event
What did we find?
Graph of Epiphany Pond East temperatures
- Despite the shallow (<1 m) depth, results indicate that thermal stratification developed during portions of the summer
- Temperature logger failure limited continuous data; spot measurements indicated reduced vertical mixing
- Low dissolved oxygen observed in the hypolimnion, especially later in the season
- Hypolimnetic TP and OP concentrations were higher than those at the epilimnion during periods of stratification
Graphy of Epiphany Pond DO Profile
Lab Results
Graph of TP HYPO/SURF
Graph of OP HYPO/SURF
Graph of Chloride HYPO/SURF
- TP: higher in hypolimnion, indicating stratification and internal phosphorus release with at least one mixing event between 7/28 and 8/7
- OP: Hypolimnion spikes observed during low oxygen periods
- Chloride: More uniform than TP but could contribute to stratification when elevated in hypolimnion
Conclusions
Monitoring and managing stormwater ponds over time is essential to identify maintenance needs and offers opportunites to study how vegetation may influence mixing and nutrient cycling.
- Even shallow (<1 m), flow-through ponds can stratify strongely enough to develop anoxia and TP buildup in the hypolimnion. Mid-summer mixing events can then lead to elevated TP in surface water and export downstream.
- Dense submersed and floating-leaf vegetation may reduce wind-driven mixing, promoting hypolimnetic oxygen depletion and internal phosphorus release
- Results highlight the potential for applied research on vegetation management to enhance pond mixing and nutrient capture.
Three photos of Epiphany creek pond 1977, 1985, and 2025.
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Logo for the Minnesota Stormwater Research Council
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