A TMDL, or total maximum daily load defines the total pollutant loading a water body can receive and still meet applicable water quality standards. (Italicized terms are defined in the boxes at the bottom of each page.) A TMDL equation is developed from a study that identifies the sources of a particular pollutant in a watershed, the pollutant contribution from each source, and the pollutant reduction required to attain and maintain water quality standards. In TMDL calculations, all identified sources of the particular pollutant are quantified, including both point and nonpoint sources of pollution. Because some TMDL calculations involve assumptions and professional judgment, TMDLs also include a margin of safety to account for uncertainty. (See TMDLs [Total Maximum Daily Loads]: Terms and Definitions, Virginia Cooperative Extension publication 442-550, http://pubs.ext.vt.edu/442-550/.)