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/.)

Microbial Source Tracking (MST), also commonly referred to as bacterial source tracking (BST), is a method used to determine the sources of fecal bacteria and establish whether fecal bacteria are being introduced into water bodies through human, wildlife, agricultural, or pet wastes. MST is considered to be a novel technology still in developmental stages. However, the use of MST is rapidly becoming widespread as more researchers and states are recognizing its potential.

A water-quality "impairment" exists if a body of water is unable to support its designated uses. (Italicized terms are defined in the boxes at the bottom of each page.) Virginia's water-quality standards specify that surface waters are either designated for "recreational use" (e.g., swimming, fishing, and boating) or "aquatic life use" (e.g., viable fishing populations). To support the "recreational use," the state sets numeric waterquality criteria for the maximum amount of bacteria in surface waters (Escherichia coli (E. coli)) for fresh water and enterococci for marine waters). When the concentration of bacteria exceeds the state-specified water-quality criteria, the water does not support the designated recreational use and is deemed to have a bacteria or pathogen impairment. E. coli and enterococci bacteria are found in the intestinal tracts and feces of warm-blooded animals, including humans. High counts of these bacteria indicate the presence of fecal contamination in water.