Stormwater Inlet Study Provides Key Information for Determining Credits for Compliance

Poster describes how inlet cleaning will be used.Everyone understands that stormwater runoff adds pollutants to waterways. But how much pollution? And what types, exactly?

Last year, researchers at Morgan State University developed a model to measure stormwater pollutant loading, and now they are characterizing the gross solids found in storm inlets.

Completed in December of 2016, Mitigating Pollutants from Highway Infrastructure for Total Maximum Daily Load (TMDL) Compliance: Monitoring Efficacy of Best Management Practices and Advancing Decision Support, conducted at Morgan through the Mid-Atlantic Transportation Sustainability University Transportation Center, determined stormwater pollutant loading by using an amended framework for the Long-Hydrologic Impact Assessment Model (L-THIA).

Building on that, for a new study Characterization of Gross Solids from Highway Storm Inlet Cleaning for TMDL Compliance, funded by the Maryland State Highway Administration, MSU will partner with the Center for Watershed Protection to extract the solids washed into storm drains and separate it into organic debris, litter, and coarse sediments. The size distribution of the sediments will be determined and chemical analysis will test for levels of total nitrogen and total phosphorus.

"Stormwater runoff from roads and highways carries nutrients, debris, oil, heavy metals, solids and other components and often runs directly into waterways," Dr. James Hunter, an author of both studies, says. "Because of this, many state Departments of Transportation are considered to be Municipal Separate Storm Sewer Systems, known as MS4s, and must obtain appropriate discharge permits and develop stormwater management programs."

Under the Clean Water Act, states must establish a Total Maximum Daily Load (TMDL) - how much pollution a body of water can safely receive and still meet water quality standards - for every body of water. The ability to measure pollution mitigation efforts is key.

The amended model in the first study used data about impervious surfaces, event mean concentrations, and precipitation from 11 MS4 counties in Maryland to estimate the pollutant load on an average annual basis. The model also can measure before and after values for best management practices, such as tree planting and removing pavement, to see how much these efforts reduce pollution.

The second study will be used to determine appropriate crediting of stormwater inlet practices for TMDL compliance. Federal and state water regulations require characterization of the accumulation of pollutants at the highway surface, stormwater attributes and the effectiveness of best management practices.

"The current study will help provide SHA with the data needed to credit the anticipated nutrient and sediment reduction from their current inlet cleaning program, while also providing guidance on cleaning frequency and resources dedicated this maintenance," Dr. Hunter says.

That first study, which concluded in December 2016, is available at The second study is expected to be completed in May 2018.