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How Wetlands Control Nonpoint Source Pollution


Wetlands have an innate ability to reduce pollution. The earliest modifications that were made to Restoration Park concentrated on enhancing this ability. Efforts to control nonpoint source pollution fall into two categories: reducing run-off and keeping run-off clean.

When storm water and other forms of run-off enters Restoration Park, the wetland detains the water, preventing it from continuing elsewhere. By stopping the run-off, the land gets an opportunity to absorb the water. Only a tiny trickle of water flows out of Restoration Park under normal conditions. Although approximately 1.25 square miles (3.25 square kilometers) of urbanized area drains into Restoration Park, several days of heavy rain are necessary to increase that outflow by a significant amount. Furthermore, the man-made earthen dam is capable of withstanding "100-year floods."

In addition to stopping run-off, wetlands can clean the run-off. Vegetation in the path of run-off reduces water velocity, which allows sediment to precipitate and settle. Many nutrients and contaminants are attached to fine sediment particles, so the deposition of sediment also removes nutrients and contaminants from the water. Wetland plants removing many nutrients from the water column and the sediments through direct uptake for growth. The dynamic biogeochemical environment of wetlands also allows for the transformation and removal of many nutrients and contaminants. The active microbial environments of the sediments allow for the decomposition of some contaminants. Water leaves the wetland in a cleaner condition.

Best Management Practices (BMP's), as a general term, designate any method for controlling the quantity and quality of stormwater runoff. Typically, a BMP is considered to be either (1) a practice (routine procedure) that reduces the pollutants available for transport by the normal rainfall-runoff process, or (2) a device that reduces the amount of pollutants in the runoff before it is discharged to a surface waterbody. Urban BMP's are a set of controls designed to reduce the following pollutants in urban runoff: sediment, nutrients, heavy metals, bacteria, pesticides/fertilizers, oxygen-demanding substances which deplete dissolved oxygen (DO) levels in receiving waters (COD, BOD), oil and grease, and others.

The table below lists the some common BMP's to illustrate the capabilities of wetlands as compared to other urban run-off BMP's. Although they are listed individually, BMP's are normally employed in combination for greater pollutant reduction. Specific site characteristics such as soil permeability and drainage area will determine the BMP types and combinations that are most appropriate for each case.

Extended Detention Ponds Total Phosphorus, Total Nitrogen, Total Suspended Solids, Trace Metals, Hydrocarbon, Bacteria Moderate, but not always reliable 20 or more years, but frequent clogging and short detention common widely applicable Possible streams warming and habitat destruction Lowest cost alternative in size range Recommended with design improvements and with the use of micropools and wetlands
Wet Pond Total Suspended Solids, Total Phosphorus, Total Nitrogen, Trace Metals-both particulate and dissolved Moderate to high 20 or more years widely applicable Possible streams warming, trophic shifts, habitat destruction, safety hazards, sacrifice of upstream channels Moderate to high compared with conventional stormwater detention Recommended, with careful site evaluation
Stormwater Wetlands Total Suspended Solids, Nutrients, Oil and Grease, Bacteria, Trace Metals Moderate to high 20 or more years Space may be limiting Stream warming, natural wetland alteration Marginally higher than wet ponds Recommended
Multiple Pond Systems Total Suspended Solids, Total Phosphorus, Total Nitrogen, Trace Metals-both particulate and dissolved Moderate to high; redundancy increases reliability 20 or more years Many ponds options Selection of appropriate pond option minimizes overall environmental impacts Most expensive pond option Recommended
Infiltration Trenches Suspended Sediments, Trace Metals, Total Nitrogen,Total Phosphorus, BOD, Bacteria Presumed moderate 50% failure rate within five years Highly restricted (soils, groundwater, slope, area, sediment input). Slight risk of groundwater contamination Cost-effective on smaller Rehab costs can be considerable Recommended with pretreatment and geotechnical evaluation
Infiltration Basins Suspended Sediments, Trace Metals, Total Nitrogen,Total Phosphorus, BOD, Bacteria Presumed moderate, if working 60 - 100% failure within five years Highly restricted (see filtration trench). Slight risk of groundwater contamination Construction cost moderate, but rehab cost high Not widely recommended until longevity is approved
Grassed Swales Total Suspended Solids, Organic Materials, Nutrients, Trace Metals Low to moderate, but reliable 20 or more years Low density development and roads Minor Low compared with curb and gutter Recommended, with checkdams, as one element of a BMP system
Filter Strips Total Suspended Solids, Organic Materials, Nutrients, Trace Metals Unreliable in Urban settings Unknown, but may be limited Restricted to low density areas Minor Low Recommended as one element of a BMP system
Water Quality Inlet/Catch Basins Total Suspended Solids, Litter, Oil and Grease, Trace Metals Presumed low 20 or more years Small, highly impervious catchments (less than 2 acres) Resuspension of hydrocarbon loadings, disposal of hydrocarbon and toxic residual High, compare with trenches and sand filters Not currently recommended as a primary BMP option


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Last updated Monday, November 17, 2003 7:02.