The Challenge of Documenting the Effectiveness of MS4 Programs: Moving Toward Quantifying Pollutant Load Reduction from the MS4 Program

Municipal Separate Storm Sewer Systems (MS4s) are required to develop iterative stormwater management programs to reduce the discharge of pollutants to protect receiving water bodies. However, evaluating the effectiveness of these MS4 programs is challenging because of the large MS4 area, multiple outfalls and pollutant sources, and intermittent nature of stormwater discharges. EPA and others have developed guidance to assist in this MS4 program evaluation, but this is still an evolving science. Because evaluating the effectiveness of the entire MS4 program is challenging, many MS4s focus on documenting the progress or effectiveness of specific activities or program elements. This paper discusses the challenges of MS4 effectiveness, presents some examples of effective MS4 activities, and discusses a performance metric for estimating effectiveness.