About Subsurface Gravel Wetland
General Information Performance Data Water Quality Treatment Process Design




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System Performance

Water Quality Treatment
The gravel wetland does an exceptional job of removing nearly all of the pollutants commonly associated with stormwater treatment performance assessment. It consistently exceeds EPA’s recommended level of removal for total suspended solids, and meets regional ambient water quality criteria for nutrients, heavy metals, and petroleum hydrocarbons. Like all other systems monitored at UNHSC, it does not provide chloride removal, but does exhibit an ability to dampen chloride peaks.

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

Water Quantity Control
Like other filtration systems, the subsurface gravel wetland exhibits a tremendous capacity to reduce peak flows of stormwater entering the system. The figure at bottom right illustrates effective peak flow reduction and long lag times for the range of seasons monitored.

Maintenance
No maintenance has been performed on the gravel wetland since it was installed in fall 2004. The system continues to function well and is developing the appearance of a healthy diverse wetland. The majority of the vegetation planted after installation has survived. Trees and shrubs did well, though some wetland species, such as water lily and pickleweed, did poorly. Researchers observed no Phragmites and little purple loosestrife, two common invasive species in the area.

Recommended maintenance mostly involves mowing and replacement of vegetation, as needed. Sediment removal from the forebay, or any pretreatment device installed with this system will reduce maintenance on the treatment basins.

Cold Climate
The subsurface gravel wetland’s water quality treatment and water quantity control capacity remained strong in all seasons, reinforcing the conclusion that filtration systems such as these perform well, even in cold climates. Because the flow is subsurface and enters the system through riser pipes, freezing of the wetland surface does not impact its function. In fact, the subsurface flow results in almost no change to seasonal performance.