Nonpoint Source Water Pollution 247Infiltration systems reduce the total amount of surface runoff and the amount
of sediment transport by increasing the amount of precipitation that infiltrates into
the soil. Detention systems reduce pollutant transport by retarding the rate of runoff
and by encouraging the settling of suspended solids. Most retention systems are
wet ponds that are sized to hold a typical (e.g., 6-month) storm event. The ponds
may be lined or unlined. The unlined ponds increase infiltration and can help with
groundwater recharge; however, unlined ponds are not appropriate if the surface
runoff contains pollutants that could contaminate a drinking water aquifer or in
areas where a high water table would result in groundwater flowing into the pond.
Constructed wetlands are similar to retention systems and wet ponds in that they
have permanent standing water, but have the added advantage that they incorpo-
rate biotic functions such as nutrient uptake and microbial degradation of pollutants.
Filtration systems use sand, soil, organic material, carbon, or other materials to filter
out pollutants. Filter systems can be incorporated into existing storm drainage struc-
tures by adding underground vaults. Biofilters and other vegetated systems such as
grassy swales and filter strips can be used to treat shallow flow or sheet flow by
increasing infiltration and reducing sediment transport. Large bioretention systems
add biological treatment such as nutrient uptake and microbial degradation of pol-
lutants. There are also a large number of relatively new, commercially developed
storm water treatment systems that incorporate one or more of the above BMPs. The
USEPA's Environmental Technology Verification Program is currently evaluating the
performance of a number of these commercial systems; their results are posted at
http://ww w.epa.gov/etv/prg m.
Most of the structural BMPs are designed to remove sediments and pollutants that
are transported in surface runoff along with sediments (e.g., phosphorus, pathogens,
metals). Many of the nonstructural BMPs focus on reducing pesticides, hydrocarbons,
commercial and industrial chemicals such as detergents and solvents, pet wastes, and
fertilizers. Public education programs, along with appropriate enforcement of pollution
control regulations, can be very effective at keeping these pollutants out of urban storm
runoff.
Pollutant removal efficiencies vary widely (Table 11-7) depending on the treatment
technology, site-specific considerations such as local weather patterns, maintenance
of the system, and design constraints. In some instances, expensive treatment sys-
tems that offer no measurable reduction in storm water pollutant transport have been
installed. The reasons for such failures are occasionally predictable. An undersized
retention basin or wet pond may not have a long enough retention time to allow set-
tling of suspended sediments. Poorly maintained systems may accumulate so much
sediment that filters become clogged, sedimentation basins fill, and plants in artificial
wetlands smother as their roots are covered with silt. Sometimes, however, a BMP
may fail to perform for no apparent reason. Urban storm water treatment technology
has grown rapidly in the past 20 years. Many of the systems that have been installed
have not had follow-up testing to determine the level of effectiveness at that site or
to develop local criteria for improved pollutant removal. This will undoubtedly be an
active research area for environmental engineers and water quality professionals for
the next decade.