Water quality refers to the state of the water, including its chemical, physical, and biological qualities, and is generally measured in terms of its appropriateness for a certain function such as drinking or swimming. Water quality can be assessed by collecting water samples for laboratory examination or by utilizing probes that can capture data at a single point in time or at regular intervals over a long period of time. The Department of Water relies on water quality data to make water resource management choices.

Water quality criteria include dissolved oxygen, turbidity, pH, bioindicators, nitrate compounds, and water temperature, among other things.

Microbes, nutrients, heavy metals, organic compounds, oil, and sediments are major water pollutants; heat, which elevates the temperature of the receiving water, can also be a pollutant. Pollutants are often the source of significant water quality degradation all around the world. Agriculture is the biggest cause of water pollution worldwide. Agricultural pollution is the leading cause of contamination in rivers and streams in the United States, the second-leading source in wetlands, and the third-leading source in lakes. Water quality management often entails the licensing of harmful material discharges, for which monitoring of effluent discharges to surface water is critical.