Definition
Examples
Effects
Wetlands and NPS
Help Prevent NPS
* Agriculture
* Construction
* Individuals
* Sewage Systems
* Silviculture
* Urban

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.

URBAN BMP OPTIONS	POLLUTANTS TREATED	RELIABILITY FOR POLLUTANT REMOVAL	LONGEVITY	APPLICABLE TO MOST DEVELOPMENTS	ENVIRONMENTAL CONCERNS	COMPARATIVE COST	SPECIAL CONSIDERATIONS
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