By Chester Arnold, Kelly Collins, Deb Caraco, Anne Kitchell, and Lori Lilly
In February 2007, the U.S. EPA entered the next generation of watershed-based pollution control by issuing a Total Maximum Daily Load (TMDL) based not on a specific pollutant but on impervious cover.
The goals for Connecticut's 2.4-square-mile Eagleville Brook Watershed integrate aspects of urban development. Since then, similar TMDLs have been or are being developed across the Northeast, including in Maine, Massachusetts, and North Carolina. In Connecticut, 238 square miles of impervious cover (about 5% of the state) was added between 1985 and 2006. This work is expected to become a national model by which communities can use a framework of impervious cover management to meeting water quality goals.
Typically, TMDLs are managed by local jurisdictions through a waste load allocation established by the state. In this case, the Connecticut Department of Environmental Protection (DEP) determined that a biological impairment — such as low fish densities in some areas and large amounts of aquatic habitat completely unoccupied in others — existed, but couldn't be attributed to one specific pollutant. Instead, the impairment was attributed to an array of pollutants transported by stormwater and linked to urbanization, and — more directly — impervious cover.
The Eagleville Brook TMDL was created to improve the quality of streams impaired by urbanization. Eagleville Brook is a small watershed that drains much of the University of Connecticut campus.
The brook is on the 2008 list of state waterbodies not meeting quality standards due to very low aquatic life use support scores, the causes of which are cited as “unknown.” The watershed flows to an impoundment of the Willimantic River, a tributary of the Thames River basin, which encompasses much of the eastern one-third of the state.
In 2005 – 2006, the DEP conducted statewide research comparing stream health, as indicated by metrics for benthic macroinvertebrate populations, to watershed impervious cover estimates provided by the university's Center for Land Use Education and Research.
As urban watersheds become even more urbanized, runoff causes elevated concentrations of pollutants, altered channel morphology, and reduced biotic integrity. Of the 125 stream segments that were studied, no segment with more than 12% impervious cover in its immediate upstream catchment area met the state's aquatic life criteria for a healthy stream. This became the foundational research supporting the impervious cover TMDL framework and setting the impervious cover goal at 11%.
The university and the Town of Mansfield responded by partnering to evaluate the feasibility of the maximum pollutant level concept and document a general methodology that would allow other communities to implement a similar program. The project team included the university's Center for Land Use Education and Research, the Center for Watershed Protection, and the Horsley-Witten Group engineering firm.