In the past, the most common phrase was "the solution to pollution is dilution" and engineers tried to calculate what volume of contaminated liquid they could dump into a river or estuary. Ecological studies have proven this phrase inadequate. Some pollutants persist and bio-accumulate, causing harm to health; some microorganisms thrive and multiply when released into an estuary; other polluted effluents change the long-term acidity, the oxygen concentrations, and temperature of rivers. Multiple releases of pollutants interact and cause problems that overwhelm a watershed's ability to self-cleanse. The reverse ideal is now "zero discharge" of any pollutants. The point sources (like an outfall pipe of an industry) of pollutants are more easy to correct than nonpoint sources such as runoff from a feedlot or city street.

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