Grassed swales can treat storm water runoff before it enters a structural device. Studies on
grassed swales do not provide consistent results to estimate nutrient removal efficiency. Moderate
removal of particulate nutrients can be expected if grassed swales are built with low slopes, erosion
resistant grass species, highly permeable soil and check dams (Schueler 1987).
Buffer strips are designed to accept and treat runoff in overland sheet flow. Overland sheet flow
can be enhanced by building a level spreader (e.g. stone trench or low gravel berm) to distribute runoff
entering the buffer. Walker (1990) estimates total Kjeldahl nitrogen removal of approximately 30-60%
and total phosphorus removal of 36-67% based on simulations with the model P8 for a case study in
Rhode Island. Given that the design and maintenance of buffer strips dramatically effect their
performance, conservative estimates would be at the lower end the reported effectiveness ratings.
Natural or created wetlands may be used to treat runoff. Schueler (1987) provides references
for design specifications and planting requirements for wetlands and shallow marshes.
Infiltration Devices. Infiltration (or retention) devices include infiltration basins, trenches and
porous pavement. The physical process of infiltration through porous media removes nutrients and
controls storm runoff depending on the design and maintenance. If storm water is treated in a basin or
other structure to remove most of the large sand and colloidal particles additional sediment-attached
phosphorus treatment can occur as water percolates downward through soil (EPA 1989). Up to 90%
of the dissolved nutrient fraction may be removed by infiltration as evidenced by the relatively low
concentration of nutrients in ground water below infiltration devices (Walker 1990). However, some of
the nutrients in percolate may reach surface water. Extensive pretreatment is required to remove
coarse-grained sediment before storm water enters an infiltration device to prevent clogging and device
failure (Schueler 1987; EPA 1989).
Infiltration basins are typically large natural or excavated depressions designed to remove
dissolved and fine-grained sediment-attached nutrients from areas up to 20 ha. The basin holds
stormwater as it percolates through the bottom or sides of the basin. Detention basins can be designed
to control peak discharges based on the volume of a large design storm and for maximum storage
(Schueler 1987). Basins effectiveness reach 60-75% of influent total nitrogen and total phosphorus
removed for a 2-year design storm (EPA 1989; Walker, 1990). Siting considerations include soils with
sufficient hydraulic conductivity and depth to ground water and bedrock. Detention basins have a high
rate of failure compared to other urban BMPs (Schueler 1987) due to clogging with coarse-grained
sediment.
Infiltration trenches and dry wells can treat nutrients from areas of up to 4 ha. Trenches have
similar site restrictions as basins and both are prone to clogging. When designed to treat 1.3 cm (one
half inch) of stormwater per impervious acre, infiltration trenches can remove 45-55% of influent total
nitrogen and phosphorus. EPA (1989) found greater efficiencies of up to 70% have been reported for
treating a 2.5 cm storm.
4.2-5
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