The stormwater management regulations of the last two decades have focused on control of peak runoff rates. It is common practice to detain stormwater runoff in detention facilities in an effort to avoid downstream impacts. However, recent changes to the National Pollutant Discharge Elimination System Regulations (NPDES) are painting stormwater management with a broader brush. In addition to addressing the peak rate of runoff, new regulations seek to mitigate development’s impact on the quality of runoff. The goal of the new regulations is to prevent pollutant-laden stormwater runoff from degrading waterways and aquifers. Currently, the Pennsylvania Department of Environmental Protection (PADEP) suggests the primary vehicle for reaching this goal is infiltration. Ideally, infiltration provides a measure of filtration as runoff percolates through natural earth or an engineered media. Another benefit of infiltration is that it results in a greater recharge of groundwater aquifers, which become starved when natural ground is covered by pavement or buildings.

As a result of the new regulations, developers and engineers are now faced with the challenge of integrating infiltration facilities into their designs. NPDES regulations mandate the infiltration of a computed volume of runoff for all sites with a net disturbance greater than one acre. For many, this may seem burdensome. However, with a greater investment in planning during the concept stage of a development, infiltration facilities can be integrated into a design far more effectively than if they are added as an afterthought.

Key factors that must be understood at the outset of design include, but are not limited to, the permeability of soils, depth to bedrock or groundwater, and evidence of karst geology (i.e. limestone). For infiltration facilities to function, they must be located in permeable soils of adequate depth; consequently, these areas must be identified and earmarked early in the design process. The location of a site’s suitable soils will dictate where infiltration facilities must go and, therefore, where other site features cannot be placed. Without this level of planning, one could invest significant resources in the design of a site only to be unable to obtain the necessary NPDES permit because infiltration is not adequately addressed.

In addition to identifying suitable locations for infiltration, other measures can be taken to reduce the burden that these regulations place on developers. Because the amount of infiltration required is based on the relative amount of runoff before and after development, the best way to reduce the volume of infiltration is to reduce the amount of impervious cover on the site. Consideration should be given to reducing street widths and parking lots where practical and permissible by local zoning regulations. Not only will this practice save in construction costs, it will reduce runoff and therefore the need for infiltration facilities.

Recent changes to the NPDES regulations seek to improve the quality of stormwater runoff. To accomplish this goal, these regulations mandate that infiltration facilities be integrated into most new land development projects. While these more stringent regulations will undoubtedly add to design costs there are measures that developers can take to mitigate these costs. Adopting a design approach that considers infiltration during the early stages and focuses on limiting impervious surfaces will serve the developer well in obtaining an NPDES permit.

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