Porous pavement treatment effectiveness is not known with confidence. Siting of porous
pavement is limited to gentle slopes, permeable soils and light traffic and may require street sweeping or
special maintenance to help prevent surface sealing.
Detention Devices. Storm water detention serves as temporary storage or active treatment.
Storage treatment effectiveness is limited and device types include an inlet control at the source, on-site
detention for several parcels of real estate, within the sewer or conveyance network, or off-line storage
when a predetermined flow has been exceeded at a treatment facility.
Dry basins do not retain a permanent pool of water and their primary pollutant removal
mechanism is sedimentation, which limits their effectiveness compared to other devices. Stahre and
Urbonus (1990) estimate total nitrogen and phosphorus nutrient removal efficiency of 10-20%. They
suggest the use of the lower end of the efficiency rating for planning purposes. EPA (1989) provides
estimates of greater efficiency (30%, up to 70%) based on a ratio of basin volume to basin runoff,
however they caution that their estimates are probably high.
Extended detention basins temporarily hold water for an extended period of time which allows
further sedimentation. Normally, extended detention basins are dry and they can be effective for
removing particulate nutrients and for reducing peak discharges (Brach 1989). Pollutant removal
efficiency increases with detention time. Total phosphorus removal at 12 hours of retention is
approximately 40% and increases linearly to about 55% efficiency at 48 hours. Total nitrogen removal
is about 22% removal at 12 hours, increasing to 32 at 24 hours and increasing linearly to 40 at 48 hours
OWML (1983).
Wet basins (ponds) permanently hold a pool of water and their effectiveness is rated relatively
high compared to other urban BMPs. Walker (1990) reports typical wet detention pond treatment
efficiency for total phosphorus ranging between 36%-70% and 31-47% for total nitrogen based on
simulations of the model P8 and assuming a treatment efficiency for total suspended solids of 70% and
85%.
The effects of macrophytes on the removal efficiency of detention ponds has been studied in
Australia. Phillips and Goyen (1987) and Lawrence (1986) found an increase in treatment efficiency of
approximately 5-30% for ponds with macrophytes compared to ponds without macrophytes having the
same hydraulic detention time.
Forestry. Forest land management, practice 409, is a comprehensive RMS that includes
practices for erosion control and production. Forestry practices should be evaluated individually and in
combination to determine their effect on surface and ground water quality (SCS, 1988) and nutrient
runoff.
4.2-6
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