The U.S. Environmental Protection Agency (EPA) attempts to reduce pollution of the nation’s rivers and streams by requiring states to test the water in all rivers and streams for pollution. Those rivers and streams with serious pollution are classified as “impaired”. The law requires states to calculate a “Total Maximum Daily Load” or TMDL for each pollutant in an impaired body of water.

Some target pollutants are bacteria, pesticides, other chemicals, low oxygen levels, temperature, acidity, phosphorous, nitrogen, lead, arsenic, mercury, and others. A TMDL tells people who discharge pollutants into the waters how much of a target pollutant can be discharged without making the water unsafe for drinking, recreation, and aquatic life. In practice many states have not completed TMDLs for all impaired bodies of water due to inadequate resources. The law does not require states to develop a plan for enforcing TMDLs.

There are two types of water pollution – pollution from point sources, and pollution from non-point sources. Point sources are more easily measured. An example is pollution that comes out of a pipe at a factory. Non-point sources are hard to measure. Examples are fertilizer runoff from cropland, or temperature in a river. TMDLs for non-point sources are often measurements of the amount of pollution in receiving waters, not the measurement of pollutants entering the water, having the effect that "dilution is the solution to pollution."

CAFOs are point sources, but also apply wastewater to crop fields as fertilizer. TMDLs should have an important role in assigning limits on discharges to impaired bodies of water when authorities write National Pollutant Discharge Elimination System (NPDES) permits for CAFOs. However, CAFOs are often classified as "non-discharging" in TMDL’s and are not assigned a significant role in reducing pollution. Examples of how the TMDL process is not working well to improved surface waster quality in the U.S. are provided in the factsheet that follows.