Conceptual framework for incorporating urban watershed functions into Maryland’s TMDL program

Background/Question/Methods

The TMDL program of the Maryland Department of the Environment (MDE) must address all state waters on the Clean Water Act 303(d) list of impaired waters. To date, TMDLs have been developed for specific, identified pollutants such as PCBs, bacteria, pH, and nutrients. In Maryland, however, 8-digit watersheds are also listed on the 303(d) list as impaired based on the proportion of streams that possess degraded biological communities. This biological approach allows MDE to identify impairments from unmeasured stressors and comprehensively address watershed degradation across the state. The challenge is to apply a TMDL program designed for specific pollutants to unidentified or multiple stressors that may be causing watershed-scale impairment. In 2006, MDE developed a threshold for sediment impairments based on the normalized sediment load (beyond the all-forest condition) that correlated with degraded instream biological and instream habitat conditions.  Following in 2009, MDE developed a Biological Stressor Identification (BSID) methodology that used a case-control, risk-based approach to identify categories of stressors (sedimentation, habitat conditions, and water chemistry). While the BSID makes maximum use of field monitoring data obtained statewide by the Maryland Biological Stream Survey (MBSS), it cannot address all stressors present in urban environments, as stressors such as flow are not adequately characterized by the MBSS.  

Results/Conclusions

This project synthesizes the literature on the “urban stream syndrome” with the latest research on surrogate urban stressors, such as impervious cover, into a conceptual framework that provides a consistent and comprehensive approach to addressing all “limiting factors” affecting Maryland streams. Specifically, the framework addresses each potential limiting factor through a series of management metrics. The primary management metric (in this case the core TMDL target) is the amount of “effective impervious area” (EIA) in the subwatershed. EIA is the amount of imperious area that produces runoff after subtractions are made for attenuation and treatment of runoff. Additional metrics address (1) condition of the riparian area, (2) physical habitat in the stream channel, (3) transportation-related contaminants, and (4) invasive species effects. Ultimately, the urban TMDL would provide targets for each limiting factor to be addressed in a watershed management plan.