What is a TMDL?

A TMDL, or total maximum daily load, is the maximum amount (pounds per year) of any pollutant that a particular stream, lake, or other water body can handle without violating state water quality standards. It has been mandated since the 1972 passage of the Clean Water Act that all waterways have a TMDL. The benefit to having a TMDL on a watershed is that it assess the current situation of a watershed in terms of where pollutants are coming from (point source and non-point source), and then defines how much pollutant (nitrogen, phosphorus, sediment, etc.) can come from each sector (agriculture, urban, etc.) of the watershed. This allows goals for specific nutrients to be set for specific sectors in the watershed, which is useful because in order to reach your goals you first have to know what they are. For clarification, “point source” means pollutants that come from a specific location, like a pipe or smoke stack, and “nonpoint source” refers to pollutants that enter from a non-specific location, like agriculture and urban surface runoff. The TMDL limits are determined by the actual water quality and how the water body is used. The Clean Water Act sets the minimum goal for any water body as “fishable” and “swimmable”, which means that the water quality needs to be good enough to support fish populations and is safe for people to be in the water. Computer models that factor in weather conditions and land use in the watershed are used to determine the current pollutant load, and then the watershed in question is compared to a watershed that has similar weather conditions and land use but is not impaired. Based on the pollutant values in the unimpaired watershed the maximum pollutant values for the impaired watershed are set.