Evaluating the Potential Benefits of Permeable Pavement on the Quantity and Quality of Stormwater Runoff
Urbanization of the landscape has an appreciable negative impact on the quantity and quality of runoff water entering our lakes and streams (Davis, 2005; Wang and others, 2001; Williamson, 1993). By replacing natural land covers (like grasslands and forests) with impervious surfaces (like parking lots and streets), we lose the water retaining role of the soil and vegetation. Increased runoff from impervious surfaces causes dangerous floods, severe erosion damage to our stream channels, diminished recharge of groundwater, and degraded habitat for our fisheries. These same impervious surfaces can transport the many pollutants deposited in urban areas, such as nutrients, sediment, bacteria, pesticides, and chloride. In the worst cases, the amount of pollutants in urban runoff are high enough to prevent us from being able to swim or fish in our local waters.
Efforts to reduce the impacts of urban runoff have been happening for some time at federal, state, and local levels. The Clean Water Act (CWA) is the primary federal law that regulates the quality of the nation’s water bodies. The CWA, through the National Pollutant Discharge Elimination System (NPDES) program, establishes pollution limits for anyone discharging into streams and lakes, including cities. In Wisconsin, NPDES permits are issued by the Wisconsin Department of Natural Resources (WDNR) which identify performance standards and limits for things like peak flow, runoff volume, phosphorus, and total suspended solids. As part of their permit, each city must prepare a management plan to meet these prescribed limits by implementing best management practices (BMPs). BMPs are practices, treatments, and technologies that can alleviate one or more if these negative effects. Permeable pavement is one of these BMPs that is believed to improve water quality and reduce the impacts of urban runoff.
What is Permeable Pavement?
Permeable pavement is a porous urban surface composed of open pore pavers, concrete, or asphalt with an underlying stone reservoir. Permeable pavement catches precipitation and surface runoff, storing it in the reservoir while slowly allowing it to infiltrate into the soil below or discharge via a drain tile. The most common uses of permeable pavement are parking lots, low-traffic roads, sidewalks, and driveways.
What are the Potential Benefits of Permeable Pavement?
General hydrologic benefits
- Permeable pavements help reestablish a more natural hydrologic balance and reduce runoff volume by trapping and slowly releasing precipitation into the ground instead of allowing it to flow into storm drains and out to receiving waters as effluent. This same process also reduces the peak rates of discharge by preventing large, fast pulses of precipitation through the stormwater system.
- Permeable pavement can reduce the concentration of some pollutants either physically (by trapping it in the pavement or soil), chemically (bacteria and other microbes can break down and utilize some pollutants), or biologically (plants that grow in-between some types of pavers can trap and store pollutants).
- By slowing down the process, permeable pavements can cool down the temperature of urban runoff, reducing the stress and impact on the stream or lake environment.
- By controlling the runoff at the source, such as a parking lot, permeable pavement can also reduce the need for or the required size of a regional BMP, such as a wet detention pond, which saves money and effort.
- Another benefit of permeable pavement is the reduced need to apply road salt for deicing in the winter time. Researchers at the University of New Hampshire have observed that permeable asphalt only needs 0 to 25% of the salt routinely applied to normal asphalt (Houle and others, 2009).
- Other researchers have found that the air trapped in the pavement can store heat and release it to the surface, promoting the melting and thawing of snow and ice (Roseen and others, 2012).
Concerns People Have About Using Permeable Pavement
Here are some of the concerns and questions about permeable pavement:
- Durability – Will permeable pavement last as long as traditional pavement?
- Upkeep and maintenance – Permeable pavement can clog with sediment and pollutants, reducing its permeability and beneficial productivity.
- How much effort is required to keep permeable pavement functioning?
- What is the frequency of maintenance needed to maintain design life of the pavement?
- Water quality - How much pollutant reduction can be expected? Of particular interest, low reductions have been observed for nutrients (phosphorus and nitrogen). This concern has two implications:
- What is the potential for groundwater contamination by infiltrating water treated by permeable pavements?
- What is the quality of water discharged from a permeable pavement drain tile?
- Temperature - What temperature reductions can be expected with permeable pavement?
- Residence time – How long does the runoff need to stay in the storage layer to adequately treat the runoff?
- Model accuracy - How well can existing urban runoff models predict the water quality benefits of permeable pavement?