RALEIGH, N.C. -- New research sponsored by the University of North Carolina Water Resources Research Institute (WRRI) suggests that for regional stormwater detention ponds to be as effective as advertised in protecting water quality, more data-based approaches to designing, siting, and managing these structures and modeling their performance are needed.
Independent research projects conducted in urban areas of Piedmont North Carolina by scientists at North Carolina State University and the University of North Carolina at Charlotte show that wet detention ponds are dynamic, ecologically complex systems whose effectiveness cannot always be assured by following simple design recommendations. The research also calls into question some of the theoretical assumptions upon which state design guidelines and performance predictions are based.
In one research project, Drs. Robert C. Borden and Sarah K. Liehr of the N.C. State University Department of Civil Engineering compared the performance of two regional stormwater ponds in the city of High Point's City Lake watershed. Both ponds generally conform to state design standards for wet detention ponds.
One of the ponds the researchers studied receives highly polluted runoff from a largely agricultural drainage area. While this pond is effective at removing suspended solids and associated pollutants, it experiences excessive algae growth because of the nutrients it receives. Some of the algae escapes the pond and travels downstream. A portion of the algae settles to the bottom of the pond where it is trapped during summer stratification. When cool weather causes the pond to "turn over," nutrients trapped in the cool anaerobic waters at the bottom of the pond may be released to downstream waters.
Borden says that the tendency of stormwater ponds to stratify and turn over means that they may not be as efficient in permanently trapping suspended solids and reducing nutrients as the state's evaluation criteria assume.
The second pond in the Borden/Liehr study receives runoff from an industrial site where a smaller stormwater basin is located. This pretreated runoff carries only fine-grained soil particles and does not contain as high a concentration of pollutants. Only small percentages of the remaining solids and pollutants are removed by this second stormwater pond.
Borden says these results show that the pollutant removal ability of similar ponds in the same general area can vary greatly, depending upon the quality of the runoff that flows into them and on internal pond dynamics. He suggests that to get the most water quality benefit for the dollar, local governments need to conduct site-specific field monitoring in order to put their regional stormwater ponds in watersheds with the highest pollutant loads.
A separate study of ponds in the Charlotte area underscores the importance of understanding stormwater pond dynamics and knowing environmental conditions in a pond's drainage area. In one phase of this project, Drs. Craig Allen, Randall Forsythe and John Diemer of the UNC-Charlotte (UNCC) Department of Geography and Earth Sciences studied 20 ponds installed primarily for flood control. Most of the ponds are now 32 to 45 years old and have become shallow because of sediment accumulation. Sediment accumulations of 2.6 tons/acre/year are typical in this region of the Piedmont and suggest that the useful life of a wet detention basin without dredging is conservatively 50 years. Much of the accumulated sediment is very fine-grained and would not settle out in sediment fore bays proposed for engineered wet detention basins. Water in many of the older ponds is continually turbid, indicating a failure to settle suspended solids. This points to the potential for future engineered wet detention ponds to become sources of pollution if not maintained or closed on an appropriate time frame.
In another phase of the Charlotte study, researchers monitored the performance of a single newer pond for a full year with detailed analyses of several storms. As in the High Point study, the Charlotte researchers found stratification and exchange of dissolved material between the bottom sediments and the water. They found that, although the pond removed 98 percent of suspended solids, when storm inflow displaced resident pond water, the displaced water had higher concentrations of dissolved pollutants than the incoming stormwater. Overall, 60 percent removal efficiencies were found for biological oxygen demand, chromium, lead, and ammonium. However, some chemical constituents, such as total nitrogen and orthophosphate, showed less than a 20 percent reduction.
Both Borden and the UNCC team emphasize that wet detention ponds can remove large amounts of heavier sediments and adsorbed pollutants where structural controls are needed for water quality protection. However, they also suggest that, to get optimum benefit from stormwater controls, the state may need to rethink some of its design guidelines, and local governments will need to site ponds carefully. They also say that state regulators need to rethink the way they evaluate the performance of regional stormwater ponds to take into account the variability in pond performance.
In addition, the UNCC team says that in watershed- and basin-scale water quality modeling, the state should give attention to the potential for aging flood management structures and private ponds across the state to become chronic sources of pollution. They also say that a remediation program for older structures could be an effective means of maintaining and improving water quality in North Carolina as an alternative to, or in addition to, building new BMPs.
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