After more than 30 years of the Clean Water Act, our rivers, lakes, and estuaries are generally cleaner than they have been since before the age of industrialization. We have successfully cleaned up most of the industrial and municipal sewage sources of water pollution, but our water bodies still suffer from diffuse sources of pollution which originate in the way we use land. In particular, stormwater runoff from urban and suburban land development with impervious (hard, non-absorptive) surfaces is currently the largest contributor to the impairment of water quality in New England, as well as in many other parts of the country. EPA requires states to develop total maximum daily load (TMDL) targets for water bodies that are classified as impaired for uses they are supposed to, but are not, supporting (recreation, aquatic life, shellfishing, for example). TMDLs are calculations of the maximum amount of each pollutant that can be released into a polluted water body while still allowing it to meet its water quality standards (i.e., clean enough to meet the Clean Water Act's fishable and swimmable goals). TMDLs may be expressed through surrogate measures if the key pollutants causing the problem are difficult to determine.