DEVELOPMENT OF AN APPROACH FOR CALCULATING TMDLS FOR HABITAT/AQUATIC LIFE IMPAIREMENTS CAUSED BY MULTIPLE URBAN STRESSORS

Habitat/aquatic impairments in an urban environment are often associated with multiple stressors, including known and unknown pollutants, storm water runoff, hydrologic modifications, riparian corridor encroachment, and channel alteration. Many of these stressors may be acting either in an individual or cumulative manner. However, it is very difficult to determine the exact role and significance each pollutant/stressor plays in contributing to the impairment to aquatic life. As a result, developing TMDLs for these impairments present unique and complex challenges. For example, it is very difficult to meaningfully identify appropriate loading capacities for each individual stressor and sufficient data/information are rarely available to isolate the relative strength of each stressor and to link each stressor independently to the impairment. As a result, innovative approaches are required to develop TMDLs that will address such impairments and also be the basis for implementing control actions. The impairments, in many cases, are related to excessive development in the watershed and the stressors are largely associated with a single source category such as storm water runoff. Therefore, using storm water as an umbrella surrogate for all stressors contributing to the aquatic life impairment may be appropriate for developing TMDLs if all stressors (pollutant loading, habitat destruction and hydrologic alteration) are related to storm water runoff. Also, this approach provides the necessary information to meaningfully guide storm water management implementation activities since many of the available storm water controls are capable of addressing excessive runoff rates and multiple pollutants. The use of surrogate indicators expressed as quantitative targets is an important tool for developing such TMDLs. This paper presents a simple and innovative approach utilizing flow duration statistics to identify a suitable surrogate hydrology target that supports healthy habitat/aquatic life. Two case studies are presented to highlight the approach, including the Shawsheen River Watershed in Massachusetts and the Penjajawoc Stream in Maine.