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422 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
Pollutant Removal. Constructed wetland systems perform a series of pollutant removal
mechanisms including sedimentation, filtration, adsorption, microbial decomposition, and
vegetative uptake to remove sediment, nutrients, oil and grease, bacteria, and metals.
Wetland systems reduce runoff velocity, thereby promoting settling of suspended solids.
Plant uptake accounts for removal of dissolved constituents. In addition plant material can
serve as an effective filter medium, and denitrification in the wetland can remove nitrogen.
Specific wetland vegetation species remove specific pollutants from stormwater runoff.
Some of the most commonly used wetland vegetation includes cattails, bulrushes, and ca-
nary grass.

Design Considerations. General guidelines recognized as important in the design of wet-
land systems include maximizing the detention time of runoff in the wetland system, max-
imizing the distance between the inlet and outlet, and providing some form of pretreat-
ment for sediment removal. Travel time can be increased in a wetland by reducing the
gradient over which the flow travels or by making the flow travel over a greater distance
before being discharged. In either case, some designers recommend a 24-hour detention
time during the 1-year, 24-hour storm. Wetland design should also take into account that
sediment accumulation in wetland systems can greatly shorten their effective life and that
some suspended solids should be removed from the runoff before it enters the wetland sys-
tem. The design should include sloped sides to allow easy removal of accumulated sedi-
ments and harvesting of plants.

Maintenance Requirements. Like most stormwater quality controls, constructed wet-
lands require regular maintenance. In addition to regularly scheduled sediment removal,
wetland systems should be periodically cleared of dead vegetation. Harvesting of plants in
the wetland might be appropriate for pollutant removal purposes; if so, disposal of re-
moved material must be planned.

Limitations on Use. While constructed wetland systems can treat stormwater runoff ef-
fectively, they do require large areas of undeveloped land, which can make siting of wet-
land systems difficult especially in urban areas. For this reason, incorporating wetland
systems into new development is usually more feasible than retrofitting them into exist-
ing developments. Achieving proper soil conditions and groundwater levels can also
present difficulties. To maintain a wetland environment, soils must be resistant to infil-
tration (i.e., have low permeability) and a water supply must be constant. In general,
soils in the system must be saturated throughout the growing season so the desired veg-
etation will survive.

12.5.4 Infiltration Facilities
Unlike detention facilities that capture and eventually release stormwater runoff to a sur-
face water body, infiltration facilities permanently capture runoff so that it soaks into the
groundwater (Fig. 12.10). Because they do not release the runoff to surface water, infiltra-
tion facilities are sometimes called retention facilities. Pollutant removal in these BMPs
occurs primarily through infiltration, which eliminates the runoff volume or lowers it by
the capacity of the facility. The three different types of facilities commonly used to pro-
mote infiltration and remove pollutants from stormwater runoff are infiltration basins, in-
filtration trenches/dry wells, and porous pavement (swales, which also promote infiltra-
tion, are addressed later under vegetative practices).