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Information: We would like to acknowledge researchers at the University of Minnesota - Jacques Finlay and Ben Janke - who contributed data and shared their expertise in executing this work order.
MPCA executed a work order with LimnoTech to
- estimate the extent and occurrence of constructed stormwater ponds that do not effectively retain phosphorus,
- identify conditions likely to contribute to phosphorus export from constructed stormwater ponds,
- compile information for constructed stormwater ponds identified as potentially exporting phosphorus into a spreadsheet or database, and
- conduct a high level assessment of characteristics, trends, and patterns for ponds that potentially export phosphorus.
To download the final report, click on this link: File:P in ponds final report.docx
This work order was intended to be the first part of a two phase study. The second phase, yet to be started, is to develop guidance or provide a summary of practices for maximizing phosphorus retention in ponds, and develop recommendations for additional research on this topic.
This page provides a brief summary of findings from this first phase. Included are links to other research being done or recently completed on this topic.
Occurrence of constructed stormwater ponds that do not effectively retain phosphorus
Literature reviewed for this task included the following.
In addition, University of Minnesota researchers, Jacques Finlay and Ben Janke, contributed data and expertise to this effort.
Comparison of pond P removal compared to designed removal efficiency (Lake Simcoe Region Conservation Authority)
General conclusions included the following.
- Of the over 500 ponds in this initial compilation, the dataset was narrowed to include 240 ponds in the Upper Midwest or Canada that had at least one surface total phosphorus (TP) measurement
- Of 66 ponds with paired TP measurements (influent and effluent samples collected at approximately the same time), 8 which were in the Midwest, 17 percent of the effluent measurements exceeded the paired influent measurement, and six of the eight ponds in the upper-Midwest had at least one effluent exceeding the influent (International BMP Database). The median effluent exceedance was 13 percent for both the national and upper-Midwest dataset. Higher effluent concentrations do not occur in all paired samples within a pond, highlighting the variable nature of TP export. For the upper-Midwest dataset, these exceedances occurred throughout the year, suggesting multiple possible drivers of TP export. Further, concentration data do not indicate the overall load retained (i.e. difference between influent and effluent loads).
- Riley Purgatory Bluff Creek Watershed District found that in 2013 (the year with the broadest sampling effort), 71 percent of the ponds surveyed had annual average TP concentrations that were equal to or greater than the MPCA’s threshold for effluent water (0.25 mg/L), and suggested that these ponds may not be retaining phosphorus. LimnoTech’s own review of this data found that 34 percent of the ponds had an average TP concentration greater than 0.50 mg/L over the 2010-2013 period, which is the upper 95th percentile confidence limit of Twin Cities stormwater runoff measurements (n=19 sites; Janke et al., 2017).
- From the Lake Simcoe study, of 74 ponds analyzed, 28 were estimated to be meeting design phosphorus removal efficiency. The average removal efficiency was 59 percent, compared to the average design efficiency of 76 percent,. The figure on the right shows that 7 ponds were not retaining phosphorus (i.e. potentially exporting phosphorus).
Overall the literature has not come to a conclusion on the extent of phosphorus release from stormwater ponds, but recent efforts have begun to shed light on the processes. RPBCWD revealed a large proportion of ponds have surface concentrations higher than estimated stormwater influent, and LSRCA identified that many are not operating as efficiently as designed. LSRCA speculated that anoxic conditions (which were documented in nearly half of their 98-pond survey) may be contributing to internal phosphorus loading. This speculation has been supported by recent literature highlighting the conditional influences on internal phosphorus dynamics.
Mechanisms for phosphorus release
This section provides a very brief summary of major biogeochemical and geochemical mechanisms driving phosphorus release from the sediments and retention by the sediments in constructed stormwater ponds. For a detailed discussion of these mechanisms, see the technical report: File:P in ponds final report.docx.
For over three decades, constructed stormwater ponds have been designed and maintained to maximize sedimentation and minimize scour during storm periods. The design of stormwater ponds for water quality has focused nearly entirely on the goal of maximizing sedimentation of phosphorus bound to particles and minimizing scouring of these particles by subsequent storms. Other physical, geochemical, and biological processes have not adequately been considered.
A conceptual box model of a stormwater pond composed of three reservoirs of phosphorus (upper water column, lower column, and sediment)) and seven major fluxes of phosphorus (arrows).
The figure on the right shows a conceptual model illustrating phosphorus reservoirs and fluxes in a typical stormwater pond. While each of the fluxes shown are potentially important, the following are likely to be of greatest importance for ponds that potentially release phosphorus.
- Input/external load. Ponds receiving heavy phosphorus and sediment loads or large quantities of labile phosphorus are more likely to eventually export phosphorus. Large sediment loads ultimately reduce pond efficiency and create a large phosphorus reservoir in benthic sediment. Generally, more labile forms of phosphorus are associated with organic material, such as leaves.
- Dissolution. If phosphorus concentrations in benthic sediment are high and anoxic conditions occur, phosphorus will be released from iron-bearing minerals.
- Mixing and resuspension. Phosphorus released through dissolution can be brought to the upper water column through mixing. It may then be exported from the pond.
Although phosphorus dynamics in ponds are complicated and highly variable, ponds susceptible to phosphorus export are those receiving high inputs, particularly of organic material, and that undergo stratification, which allows anoxic conditions to occur within the pond. The issue of stratification may be exasperated by chemostratification resulting from road deicers accumulating in ponds (Taguchi et al., 2018; McDivitt, 2019)
List of ponds with phosphorus data
A list of over 500 stormwater ponds in the upper Midwest was compiled in order to identify characteristics that may be indicative of phosphorus release in this general region. This list includes approximately 400 ponds in Minnesota and 98 that are located in Ontario, Canada. The dataset was narrowed to 240 ponds that had at least one surface TP measurement.
The following information was targeted for inclusion in the dataset, although much of this information was not available for many of the ponds.
- Pond name
- Pond location
- Year constructed
- Maintenance record
- Design information
- As built information (if available)
- Monitoring data (if available)
- Chemistry (i.e. TP, SRP, iron, DO) data in the pond (if available)
- Chemistry in any inflows or outflows (if available)
- The occurrence of stratification of temperature, conductivity, and DO in the pond (if available)
- Soil/sediment chemistry information (if available)
- Contact information
To access the file with the pond information: File:Pond data.xlsx. The technical report provides some analysis of the data: File:P in ponds final report.docx.
High level assessment of phosphorus release
Analysis of the data attempted to identify patterns or trends that would help understand why some ponds are more susceptible to releasing phosphorus. The analysis is in the technical report: File:P in ponds final report.docx.
Overall, the assessment revealed that the data are not sufficiently detailed to tell the story of internal phosphorus cycling. Linear correlations suggest that ponds built more recently perform more effectively, but positive correlations between estimated P removal efficiency and surface TDP highlight that there is more to be learned about the internal mechanisms beyond estimated efficiency.
Although more rigorous research is needed to gain a fuller understanding of phosphorus dynamics in ponds, some general observations included the following.
- Categorical analysis indicated Minnesota ponds designed to NURP guidance have higher surface TP concentrations than non-NURP ponds.
- While data was limited, ponds in residential and forested drainage areas were among the ponds with highest surface TP concentrations.
- Previously published studies demonstrate that for many stormwater ponds water column phosphorus concentrations are higher than water quality standards or regionally calculated event mean concentrations, and that many ponds appear to operate at removal efficiencies that are lower than targeted during pond design.
- Previously published studies typically do not present sufficiently detailed data to independently assess phosphorus removal performance, as most only present average phosphorus concentrations in the pond water column and sometimes in the influent waters to the pond. Calculations of the difference of these averages between influent and presumed effluent concentrations, especially when sample numbers are not equal and collections known to be paired, can not provide robust estimates of removal due to differing hydrological conditions.
- The design of stormwater ponds over the last three decades has focused nearly entirely on the goal of maximizing water quality via sedimentation of phosphorus bound to particles and minimizing scouring of these particles by subsequent storms.
- The full suite of physical, geochemical, and biological processes, mechanisms, drivers and factors that control phosphorus cycling and fluxes within stormwater ponds, and their temporal dynamics, should be considered when evaluating and managing phosphorus retention by ponds.
- Useful information can be gleaned from the compilation and reanalysis of existing metadata on pond characteristics and summary data on pond phosphorus concentrations and previously estimated removal efficiency. However, these summary datasets are insufficiently detailed to support robust statistical analyses that might best highlight the characteristics of ponds that are best and worst at retaining phosphorus during storms or over an annual cycle.
On-going or recent studies - Minnesota
There are several on-going or recent studies examining phosphorus dynamics in and release from constructed stormwater ponds. Below is a review of some of that work, including links to reports and presentations.
- Characterization of Phosphorus Release from Ponds. This is a study of the P dynamics of stormwater ponds to understand the factors that control retention and release of P. Because ponds rely on sedimentation for P removal, they are most effective when a high percentage of P is bound to suspended solids and less effective when input P is in the soluble form. More generally, the study will represent the first stage in research to understand the functioning of stormwater ponds with respect to P removal. This information is crucial for the development of guidelines for the management of ponds for improved P retention.
- Detecting phosphorus release from stormwater ponds to guide management and design. There is growing concern that aging stormwater retention ponds may become net sources of phosphorus (P) to receiving waters. Release of P previously deposited in sediments (i.e. internal loading) is a major contributor to eutrophication in lakes. Stormwater ponds often have high external P loading, and other characteristics that may increase the likelihood of internal loading as ponds age. However, stormwater ponds have received comparatively little research attention, even though they are widely used with the intended goal of permanent immobilization of phosphorus. The ability of these systems to retain phosphorus over their lifespan is essentially unknown. The proposed research will build understanding necessary to assess the capacity of stormwater ponds to retain or release phosphorus in Minnesota’s stormwater pond infrastructure. The projects aim to develop methods for rapid and efficient identification of pond phosphorus release, to guide management of existing ponds, and to reveal factors that underlie poor performance for P removal. The results of this project will be used to inform and improve pond maintenance, pond design and decision making around construction of new ponds, and to ultimately improve the water quality of our lakes, rivers and wetlands.
- Research team: John Gulliver, Jacques Finlay, Ben Janke, Poornima Natarajan, Shahram Missaghi
- Project duration: January 1, 2019-June 30, 2020
- Pond Treatment with Spent Lime to Control Phosphorus Release from Sediments. Sedimentation ponds that accumulate particles and phosphorus in stormwater runoff are a standard and widely applied storm water best management practice. However, just as internal loading occurs in lakes during warm summer periods when the potential for oxygen depletion is greatest, aging ponds have the potential to release more phosphorus than is captured during summer months (Watershed Protection Techniques, Technical Note 102). Dredging is a potential, but expensive, option to improve pond performance, but phosphorus release may occur long before a pond is filled with sediment. Areal applications of alum and iron can control phosphorus release, but incur raw material production costs. In cooperation with SPRWS, City of White Bear Lake, RWMWD, and VLAWMO staff, Barr Engineering proposes this study to evaluate the application of spent lime (amorphous calcium carbonate from drinking water treatment) to pond sediments to reduce phosphorus release during warm summer months. Spent lime can reduce phosphorus release by forming calcium phosphate and potentially by increasing the pH of the treated sediments to facilitate iron and aluminum phosphate binding. This study includes a laboratory and a field component and is intended to validate large-scale applications. The laboratory component includes the addition of spent lime at a range of doses to phosphorus rich pond sediment to determine optimal spent lime dosing. The field component involves the addition of spent lime to two ponds and monitoring to determine the magnitude of reduced phosphorus release, evaluate cost-effective methods for areal application and quantify the benefits of this water treatment byproduct.
- Research team: Greg Wilson, Keith Pilgrim, Erin Anderson-Wenz, Kevin Menken, Tyler Olsen
- Use of alum in stormwater ponds
Other recent studies
- Mobility and Bioavailability of Sediment Phosphorus in Urban Stormwater Ponds. Paul C. Frost, Clay Prater, Andrew B. Scott, Keunyea Song, Marguerite A. Xenopoulos. 2019. Water Resources Research, Volume55, Issue5, May 2019. Abstract: "Stormwater ponds can serve as retention hotspots for phosphorus (P) moving out of the urban environment. This retention may be reduced by P speciation that reduces the bioavailability of P to primary producers and alters its mobility in sediments. Here we examined the mobility and fate of dissolved P in urban stormwater ponds with a set of complementary field measurements and short‐term laboratory and field experiments. We measured the types and amount of P in water column and sediments of urban stormwater ponds. We further assessed the mobility of different P types in pond sediments in the field and rates of P release from sediment cores maintained under laboratory conditions. Finally, we assessed P uptake rates by pond algal communities using short‐term bioassay experiments. We found that dissolved organic P was highly prevalent in urban pond water and sediments and that this type of P was mobile within sediments and could be released under high or low O2 conditions. We also found highly variable P demand by algae among stormwater ponds and that algal growth responses to P was correlated to water column N:P ratios. Altogether, our results indicate an important role for organic phosphorus cycling in urban stormwater ponds, which likely constrains the overall retention efficiency in these aquatic ecosystems."
- International BMP Database.
- Janke, B.D., J. C. Finlay, and S.E. Hobbie. 2017. Trees and Streets as Drivers of Urban Stormwater Nutrient Pollution. Environ. Sci. Technol.2017, 51, 9569−9579.
- Lake Simcoe Region Conservation Authority. 2011. Stormwater Pond Maintenance and Anoxic Conditions Investigation- FINAL REPORT
- Riley Purgatory Bluff Creek Watershed District. 2014. Stormwater Pond Project - 2013 Report.
- Song, K., Winters, C., Xenopoulos, M. A., Marsalek, J., & Frost, P. C. (2017). Phosphorus cycling in urban aquatic ecosystems: connecting biological processes and water chemistry to sediment P fractions in urban stormwater management ponds. Biogeochemistry, 132(1–2), 203–212. 
- Song, K., Xenopoulos, M. A., Marsalek, J., & Frost, P. C. (2015). The fingerprints of urban nutrients: dynamics of phosphorus speciation in water flowing through developed landscapes. 
- Walker, W. W. (1987). Phosphorus removal by urban runoff detention basins. Lake and Reservoir Management, 3(1), 314–326.