Water Quality Management: Storm Water Management

The NPDES Stormwater Program

  • Under the Clean Water Act, the program regulates stormwater discharges from three sources:
    • Municipal Separate Storm Sewer Systems (MS4s)
    • Industrial activities
    • Construction activities
  • The goal is to prevent stormwater runoff from washing harmful pollutants into local surface waters, such as streams, rivers, lakes, and coastal waters.
  • Most states are authorized to implement and administer the NPDES Stormwater Program.

Municipal Separate Storm Sewer System (MS4)

  • An MS4 is a public conveyance of stormwater, including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, and storm drains.
  • An NPDES permit is required for stormwater discharge.
    • Phase I (started in 1990): Medium to large MS4s in cities/counties with > 100,000 population.
    • Phase II (started in 1999): Small MS4s in urbanized areas with a population of at least 50,000 by the last census.
  • Must reduce pollutant discharge to the "maximum extent practicable" (MEP) to protect water quality.
  • This is done by developing a stormwater management plan that addresses the six minimum control measures with measurable goals.

Stormwater Management Plan

  • The plan includes:
    • Public outreach & education on stormwater impact.
      • Educational materials distributed to schools and businesses.
      • Workshops held for teachers on stormwater education.
      • Homeowner workshops for lawn/garden care best management practices.
      • Meetings to educate citizens and developers about low-impact development.
    • Public involvement / participation.
      • Participation in Adopt-A-Stream programs to remove stream trash and debris.
      • Planting riparian trees and wetland plants.
    • Illicit discharge detection and elimination.
      • Identify and eliminate failing septic systems, illegal waste dumps, and improper disposal of used oil, radiator fluid, car washwater, and laundry washwater.
    • Construction site stormwater runoff control.
      • Construction, inspection, and maintenance of best management practices for construction sites.
    • Post-construction stormwater runoff control for new development.
      • Maintenance of best management practices after construction (i.e., riparian buffers) and the testing of water quality of runoff from buffers.
    • Pollution prevention and good housekeeping for municipal operations.
      • Municipal employees trained in proper hazardous material storage.
      • Collection and recycling of used oil.
      • Minimal fertilizer, pesticide, herbicide, and road salt application.
      • Integrated pest management.
      • Parking lot cleaning.
      • Ordinance created to address pet waste.

Best Management Practices (BMPs) for Stormwater Management

  • Non-Structural BMPs
    • Planning & Zoning to provide low impact development that preserves water quality.
      • Conservation zoning decreases lot size & maximizes the shared vegetated space along streams.
    • Preserve Natural Vegetation to slow runoff, minimize stormwater pollutant runoff & provide nutrient uptake.
      • Riparian buffers or vegetated strips along streams with 90 meters of grass or 47 meters of grass/forest remove up to 90% of nitrogen.
  • Structural BMPs
    • Stormwater Detention: Gather runoff in wet ponds or dry basins and slowly release it to waters or drainage systems to control stormwater volume & settle out particulates for pollutant removal.
    • Infiltration BMPs increase the percolation of runoff into the soil to reduce stormwater runoff quantity & reduce mobilization of pollutants.
      • Examples: infiltration basins, porous pavement, silt fence.
    • Vegetative BMPs: Landscaping vegetation & porous soil to slow flow, remove pollutants, & increase percolation of runoff.
      • Examples: grassy swales, artificial wetlands, and rain gardens.

Industrial Activities

  • The EPA identifies six types of industrial activities with the potential to be major sources of pollutants in stormwater, requiring an NPDES stormwater discharge permit:
    • Loading and unloading operations
      • A spill of material can be washed away during a storm.
    • Outdoor storage & Outdoor processing
      • May expose materials to rainfall and/or runoff and contribute pollutants to stormwater.
    • Dust or particulate generating processes
      • Dust/particulates mobilized in stormwater runoff.
    • Illicit connections of process wastes to stormwater collection systems instead of to sanitary sewers.
    • Waste management practices including everything from landfills to waste piles to trash containment.

Construction Activities

  • Need coverage under an NPDES permit for stormwater discharge if engaged in clearing, grading, and excavating that disturbs 1 acre or more, including smaller sites in a larger common plan of development or sale.

Effects of Land Development

  • Construction site soil disturbance increases sediment runoff rates:
    • 10 to 20 times greater than agricultural lands
    • 1,000 to 2,000 times greater than forest lands
  • Paving the earth decreases infiltration by the soil & thus increases the volume of surface runoff.
  • Peak runoff flow rate & flow time increases over the watershed.
  • Overall results:
    • The rate & volume of stormwater flow increases
    • Greater erosion occurs
    • More sediments & pollutants are moved
    • The chance of flooding increases
  • Typical erosion rates for land-based activities (soil loss in tons/acre/year):
    • Farm Land (row crop):  4.7~4.7
    • Farm Land (active pasture):  1~1
    • Forest Land: <1
    • Bare Soil (e.g., unmanaged construction sites):  35−45~35-45

Erosion Control & Minimizing the Impact of Construction Sites

  • Minimize the amount of disturbed area & protect natural features.
    • Natural vegetation is the best and cheapest erosion control BMP.
    • Preserve topsoil; if exposed, the subsoil is more prone to erosion, has less infiltration capacity, & is less able to support plant life.
  • Phase construction activities.
    • Schedule construction work to minimize the amount of soil exposed to the elements at any given time.
  • Control stormwater flow onto & off the site.
    • Construct a diversion ditch or berm to direct runoff away from unprotected slopes & direct sediment-laden runoff to a sediment-trapping structure.
  • Stabilize soils promptly.
    • Have stabilization measures in place after grading activities have ceased.
    • Temporary-cover BMPs include temporary seeding, mulches, mats, and chemical soil binders.
    • Permanent-cover BMPs include permanent seeding, planting, and sodding.
  • Protect slopes.
    • Steeper or highly erodible slopes require more complex control combinations.
      • Examples: Blankets or mats with diversion channel.
    • Silt fence or fiber rolls can be used on moderate slopes.
      • Installed on level contours, spaced at 10- to 20-foot intervals.

Sediment Controls (the 2nd line of defense) for Construction Sites

  • Protect storm drain inlets.
    • Install inlet protection before soil-disturbing activities begin & until the soil stabilization is finalized.
  • Establish perimeter controls.
    • Silt fence or fiber rolls placed on contour at the down slope edge of a disturbed area.
      • 6 inch x 6 inch trench on uphill side of silt fence.
      • Stakes on downhill side of silt fence or roll.
      • Curve the ends of the fence or roll up-gradient.
      • Don’t cover the entire perimeter of a large lot, create small catchments of ¼ acre or less with several J hook shaped silt fences.
  • Retain sediment on-site & control dewatering practices.
    • Use a sediment basin if 10 or more acres is disturbed, large enough to provide storage for a 2-year, 24-hour storm.
    • If a large sediment basin is not practical, use smaller sediment basins or, at a minimum, use silt fences and vegetative buffer strips.
    • Dewatering = removal of accumulated water from excavated areas; pump to sediment basin or to a flat vegetated area enclosed by silt fence where discharges can infiltrate.
  • Establish stabilized construction exits.
    • A 50 ft. rock aggregate construction exit reduces mud transported to roadways by vehicles tires.
  • Inspect & maintain controls.
    • NPDES permit requires maintenance of BMPs at all times; must establish an inspection & maintenance strategy.