Reduce negative impacts to aquatic ecosystems, channel morphology, and dry weather base flow by replicating natural hydrologic conditions and retaining precipitation on site.
- Retain the precipitation volume from the 60th percentile precipitation event as defined by the U.S. EPA in the Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act (or local equivalent for projects outside the United States).
- Retain precipitation volume through on-site infiltration, evapotranspiration, and reuse.
- Implement runoff-reduction strategies (e.g., biofiltration through plants, soil) that also improve water quality.
- Cisterns, if used, must be implemented in combination with other approaches to meet the requirements of this prerequisite.
- Ensure the section of the site maintenance plan (see O+M P8.1: Plan for sustainable site maintenance) is complete and includes the maintenance activities used to ensure long-term effectiveness of stormwater features.
Note: On sites where the retention of the precipitation volume from the 60th percentile precipitation event is not feasible due to site constraints (such as clay soils, high groundwater elevations, geotechnical issues, below-ground contamination, underground utilities or transportation systems, watershed water balance considerations, low evapotranspiration rates, or lack of water use potential), retain the maximum precipitation volume possible on the site up to the 60th percentile precipitation event.
- Ensure that discharge volumes and rates do not increase the natural rate of erosion in receiving water channels or negatively affect a receiving channel’s ecological flows or natural groundwater replenishment rates and volumes.
- Implement strategies to reduce precipitation runoff volumes, peak flows, and pollutant discharges.
- Implement strategies to increase evapotranspiration, filtration and infiltration, and mitigate elevated water temperatures caused by contact with impervious surfaces. Use the following strategies and design approaches:
- Design to minimize impervious surfaces; specify permeable materials for hard surfaces, including permeable concrete, asphalt, and pavers.
- Design hard surfaces to drain into localized, on-site landscape areas, and design landscape areas to accept precipitation runoff from hard surfaces.
- Provide infiltration opportunities in the landscape that use plants and healthy soils as filters, such as bioswales, raingardens, water quality ponds, constructed wetlands, or vegetated buffers, that filter, infiltrate, evapotranspirate, and retain precipitation, recharge groundwater, and reduce pollutant loadings, runoff volumes, and rates.
- Where runoff conveyance systems are needed, use vegetated channels when possible. Where vegetated channels are not practical, the use of hard non-erosive materials to convey surface water should take precedence over using pipes, culverts, or underground channels.
- Create living landscapes using soil and vegetation features, such as vegetated roofs, walls, or facades; raingardens; or tree canopies.
- Select appropriate vegetation features that can tolerate periodic inundation and soil saturation without harming the growth or vigor of the plant.
- When selecting vegetation for managing precipitation runoff, select plants that are resistant or less susceptible to pollutants commonly found in precipitation runoff.
- Where possible, select plants appropriate for the site and climate based on their capacity to reduce pollutant loadings for specific pollutants of concern in the receiving watershed.
- Improve the water-retention capacity of the soil by increasing the organic matter content of the soil through the addition of compost or other organic soil amendments.
- Use rainwater-harvesting systems to reduce precipitation runoff volumes and rates. Design rainwater harvesting and use systems to maintain the ecological flows of receiving waters and historical groundwater recharge rates.
- Avoid or minimize the use of materials used in buildings, hardscape, and landscape construction that can be a source of pollutants in stormwater, such as:
- Copper and zinc roofs, roof gutters, downspouts, and siding
- Galvanized materials (e.g. fences, fence posts, guardrails, signposts)
- Treated lumber
- Use integrated pest management (IPM) practices to control pests
- Minimize the use of fertilizers on site and implement practices to reduce nutrient runoff (e.g., slow-release fertilizers, optimized application timing for plant uptake).
- Plan for and implement maintenance activities designed to reduce the exposure of pollutants to stormwater, such as:
- Minimizing exposure of stored materials to precipitation to minimize the chance of pollutants running off the site or entering groundwater both on and off site
- Developing and implementing a contaminated/chemical spill response plan
- Minimizing the use of salt or other potentially harmful de-icing chemicals
- Avoiding on-site maintenance of construction equipment to reduce pollutant loadings of oils, grease, or hydraulic fluids
- Avoiding on-site fueling of vehicles to the maximum extent practicable
- Where appropriate, implement systems of practices in a treatment train to provide multiple pollutant removal processes (e.g., runoff reduction through evapotranspiration and infiltration, sedimentation, filtration, adsorption, biological degradation or uptake) to reduce the concentrations of pollutants in precipitation runoff and to provide redundancy in the system.
- Use soil and vegetation-based controls based on their capacity to reduce precipitation runoff and pollutant loadings through evapotranspiration and phytoremediation.
- Maintain infiltration rates, and regenerate the adsorption capacity of the soils.