About Retention Ponds
General Information Performance Data Water Quality Treatment Process Design




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System Performance

Water Quality Treatment
During the first year of operation, the retention pond was reasonably effective in removing many of the pollutants commonly found in runoff. It consistently met EPA’s recommended level of removal for total suspended solids, as well as regional ambient water quality criteria for petroleum products, metals, and nutrients. However, during its second year, researchers observed a 25 percent reduction in its TSS median removal efficiency—from 81 percent down to 71 percent. This indicates that while the pond still effectively treats most contaminants, its performance may continue to diminish. Like the other systems evaluated at UNHSC, it does not provide chloride removal, but can dampen chloride peaks.

The chart at top right reflects the system’s performance in removing total suspended solids, total petroleum hydrocarbons, dissolved inorganic nitrogen, total phosphorus, and zinc. Values represent results recorded over a two-year monitoring period, with the data further divided into summer and winter components.

Water Quantity Control
Retention ponds exhibit a tremendous capacity to reduce peak flows, retain channel protection volume, and provide flood protection for up to 48 hours. In the figure at bottom right, the retention pond demonstrates effective peak flow reduction and long lag times, regardless of season. However, in general, these systems do not reduce runoff volume. Research indicates that the extended duration effluent flows typical of retention ponds negatively impact receiving streams, particularly when post-development runoff subjects streams to erosive flows for long periods. This phenomenon is observed in urban areas, where it leads to channel instability and lost ecological value and function.

Maintenance
Minimal need for maintenance contributes to the popularity of retention ponds. However, while little maintenance may be required to support their ability to manage peak flow and floods, more frequent attention is critical for effective water quality treatment. Previous research has demonstrated that erosion and resuspension of benthic sediments in these systems leads to sediment export. Since sedimentation is the main water quality treatment mechanism, inspections are critical to maintaining performance in sites with heavy sediment loads. Dredging for debris and trash is also needed. While not necessary for these systems to function, the establishment of a viable pond ecosystem can enhance treatment, prolong the system’s lifespan, and increase aesthetic appeal.

Cold Climate
The system’s ability to treat water quality and manage water quantity remained effective during cold winter months. While some variation in both kinds of performance does occur in cold conditions, it does not warrant significant alterations to system design to compensate.