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Uses of Data
Biocriteria Links
Total Maximum Daily Loads
Bioassessment data can provide compelling evidence of water resource impairment because bioassessments directly measure the aquatic community's response to pollutants or stressors.
A total maximum daily load (TMDL) is a tool used to achieve applicable water quality standards. The TMDL process quantifies the loading capacity of a waterbody for a given stressor and ultimately provides a quantitative scheme for allocating loadings (or external inputs) among pollutant sources. In doing so, the TMDL quantifies the relationships among sources, stressors, recommended controls, and water quality conditions. For example, a TMDL might mathematically show how a specified percent reduction of a pollutant is necessary to reach the pollutant concentration reflected in a water quality standard.
Section 303(d) of the Clean Water Act (CWA) requires each state to identify and establish priority rankings for waters for which existing pollution controls are not adequate to attain and maintain water quality standards. States and Tribes must then establish TMDLs for those waterbodies and submit, from time to time, the list of waters and TMDLs to EPA. Section 303(d) requires EPA to review and approve or disapprove lists and TMDLs.
TMDLs are vital elements of a growing number of state programs. For example, as more permits incorporate water quality-based effluent limits, TMDLs are becoming an increasingly important component of the point-source control program.
The TMDL process is a geographically-based approach to preparing load and wasteload allocations for sources of stress that might impact waterbody integrity. The geographic nature of this process will be complemented and enhanced if ecological regionalization is applied as part of the bioassessment activities. Specifically, similarities among ecosystems can be grouped into homogeneous classes of streams and rivers that provides a geographic framework for more efficient aquatic resource management.