03.2012
www.greenbuildermag.com
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places where a wetland or natural system of wild grasses and swales might be more appropriate for water management. This is especially true in planned communities and rental properties, where landlords tend to terraform the landscape for lawn mow- ers, not people or gardens. But a less con- ventional hillside with swales, or a natural wetland with 4-ft.-tall grasses, frogs and dragonfl ies, can do a lot more to mitigate runoff damage.
Regional Regulation Pollutants in stormwater runoff have been regulated under the Clean Water Act since the 1970s, but it took until the 1990s for a permit program to be developed by the EPA. Typically, water quality can be com- promised by numerous point-source pollut- ants. Consider the typical water treatment utility, trying to control municipal and
System
Bioretention Dry Well
Infiltration Trench Filter/Buffer Strip Vegetated Swale Infiltration Swale Wet Swale Rain Barrel Cistern
Total Suspended Solids (TSS)
-
80–100 80–-100 20–100 30-65 90 80 NA NA
Total Phosphorus (P)
81
40–60 40–60 0–60 10–25 65 20 NA NA
industrial wastewater treatment plants, outdated, leaking septic systems, fertilizer runoff from farms and urban stormwater laced with heavy metals and petroleum byproducts. It’s clear that that planning must be done more regionally, balancing all sources of pollution likely to enter the near- est watershed. For example, a watershed based ap-
proach has been required in the 64,000-sq.- mile Chesapeake Bay watershed since December 2010, when the EPA developed a Total Maximum Daily Load (TMDL) for nutrients and sediment in the Bay. The TMDL is basically a pollution
“diet” for the Bay—literally measuring the pounds of nitrogen and phosphorus and tons of sediment that can be discharged from all sources per day. The upper thresh- old is the “fi shable, swimmable” water qual- ity requirement of the Clean Water Act.
Total Nitrogen (N)
43
40–60 40–60 0–60 0–15 50 40 NA NA
Zinc 99
80–100 80–100 20–200 20–50 80–90 40–70 NA NA
Lead 99
80–100 80–100 20–200 20–50 80–90 40-70 NA NA
Back in the pre-LID days, developers simply channeled stormwater overflow into pipes and drainage basins, but that method did nothing to remove phosphorous, nitrogen and metals from runoff. LID offers several better alternatives.
Material
Asphalt/Concrete Pervious Concrete Porous Asphalt Turf Block Brick
Natural Stone
Concrete Unit Paver Gravel
Wood Mulch Cobbles
Initial Cost
Medium High High
Medium High High
Medium Low Low Low
Maintenance Cost Low High High High
Medium Medium Medium Medium Medium Medium
Water Quality Benefits Low High High High
Medium Medium Medium High High
Medium
Choosing ground cover with the right level of permeability is key to LID success. The most porous surface, it should be noted, is not always the best choice, depending on the likely overall flow of runoff.
Source: Prince George’s County Bioretention Manual, 2001.
Source: Bay Area Storm Water Management Agencies Association (BASMAA) “Residential Site Planning & Design Guidance Manual for Storm Water Quality Protection, 1997.”
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