Construction stormwater best management practice – Untreated Stormwater Runoff to Lakes, Streams, and Wetlands
Untreated Stormwater Runoff to Lakes, Streams, and Wetlands
Structural stormwater Best Management Practices (BMPs) are designed to collect sediment and absorb the pollutants, slow the destructively high flow rates, and/or decrease the volume of water before being discharged into the states lakes, streams, and wetlands. The following guidelines are intended to provide clarity on the prohibition of using natural water bodies as stormwater treatment devices for other state agencies and local governments as well as the regulated public.
Using wetlands for stormwater treatment
Stormwater contains high levels of pollutants and sediment and will degrade a wetland over time. Also, the large volumes of stormwater into wetlands create large fluctuations in the water levels and can drown out most desirable wetland plant species, leaving a degraded, monoculture wetland of reed canary grass or cattails. Recent studies show that these unnaturally large fluctuations in wetlands can cause phosphorus releases and methyl mercury formation, further degrading or impairing downstream water bodies.
The issue of when to provide treatment for stormwater before discharging into a wetland is sometimes confusing for local implementation because many communities have had a long history of discharging untreated stormwater to wetlands over the years. Today, communities often label these wetlands as “storm ponds” in their local water inventories. The Minnesota Pollution Control Agency (MPCA) supports projects that remove past discharges to wetlands or provide full stormwater treatment prior to discharging to wetlands.
The NPDES Construction Permit prohibits further degradation of wetlands with new untreated stormwater discharges or significant expansions of existing untreated discharges from new impervious surfaces to wetlands. There may be limited situations or projects where wetlands may be used for stormwater treatment if there are no prudent and feasible alternatives to providing BMPs in upland areas. In these cases, permits can be issued if the full wetland mitigation sequence (avoid, minimize and mitigate) has been satisfied and completed according to MPCA Water Quality Standards in Minn. R. 7050.0186.
Stormwater treatment areas located in-line with streams or lakes
Using existing creeks, streams or lakes to help remove pollutants from storm water runoff is called in-stream treatment. The Clean Water Act expressly prohibits the use of in-stream treatment as a pollution treatment system except in the most extreme situations. All these waters are defined as “Waters of the State” in Minn. Stat. § 115.01, subd. 22 and have varying protections under MPCA Water Quality Standards (Minn. R. ch. 7050) according to their designated uses. This also includes the partial taking of a lake to create a forebay treatment area. In stream treatment systems have poor performance due to the re-suspension of sediment during high flow storm events.