Water quality would be determined by weathering of bedrock minerals, atmospheric processes of evapotranspiration and dust, salt deposition by wind, natural leaching of organic matter and nutrients from soil, hydrological factors that lead to runoff, and biological processes within the aquatic environment that can alter the physical and chemical composition of water if there were no human influences.

Water quality is usually measured by comparing a water sample’s physical and chemical characteristics to water quality guidelines or standards. Drinking water quality rules and regulations are intended to ensure that clean and safe water is available for human consumption, preserving human health. These are usually based on permissible limits of toxicity for humans or aquatic species as determined by science.

As human populations grow, industrial and agricultural activities expand, and climate change threatens to disrupt the hydrological cycle, declining water quality has become a global problem.

Eutrophication is the most common water quality issue worldwide, caused by high nutrient loads (mostly phosphorus and nitrogen), which severely limits beneficial water usage. Agricultural runoff, residential sewage (also a source of microbiological pollution), industrial effluents, and air inputs from fossil fuel burning and bushfires are also major nutrient sources.

Because of their complex dynamics, extended water residence durations, and position as an integrating sink for pollutants from their drainage basins, lakes and reservoirs are particularly vulnerable to the harmful effects of eutrophication. Nitrogen concentrations of more than 5 milligrams per litre of water are frequently indicative of pollution from human and animal waste, as well as fertilizer runoff from agricultural areas.

The influence of personal care goods and pharmaceuticals, such as birth control pills, painkillers, and antibiotics, on aquatic habitats is a rising water quality concern. Although some are thought to mimic natural hormones in humans and other species, little is known about their long-term human or environmental implications.

Poor water quality has a number of direct effects on water quantity. Polluted water that can’t be utilized for drinking, bathing, industry, or agriculture limits the amount of water that can be used in a particular area.