What controls natural water quality?

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Alaska Mountain Range, Credit: USGS/Photo by John J. Mosesso

Material adapted from: Vandas, S.J., Winter, T.C., and Battaglin, W.A. 2002. Water and the Environment, p. 28-29. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.

The fundamental controls on natural water quality, water not impacted by the activities of humans, are the types of organic and geologic materials it contacts and the duration of this contact. As water moves through organic materials like leaves and roots, it reacts with them and with the living things associated with them, such as soil bacteria and algae. As water moves through geologic materials, it dissolves them.

The processes of rock weathering on the Earth’s surface are strongly influenced by climatic factors such as temperature and the quantity and distribution of precipitation. Climatic patterns and environmental conditions affect plant communities and soil types, causing the waters that flow from these areas to have a certain chemical signature. The influence of climate and geology on water quality is indicated by the quantity and kinds of dissolved materials contributed from an area and the amount of sediment carried by streams.

Natural water can vary greatly in the dissolved materials that it carries. Natural springs that flow through salt-bearing geologic formations can have as much as 200,000 parts per million (PPM) of dissolved materials. Some streams that flow over rocks with low solubility can have as little as 50 parts per million (PPM) of dissolved materials. For drinking water purposes it is recommended that waters contain less than 500 parts per million of dissolved materials.

Natural events such as droughts and floods may cause substantial changes in stream water quality. Reduced flow resulting from droughts can cause an increase in the concentrations of dissolved materials and a decrease in the load or amount of solid material carried by a stream. The reverse is true of floods; high flows generally dilute the concentrations of dissolved materials, and flush new sediments from flood plains, increasing the sediment load.

Biological factors can have a major effect on the quality of natural waters. Changes to any of the environmental factors that make up ecosystems can result in changes to the ecosystem as a whole. Through the process of photosynthesis, aquatic plants produce oxygen and consume carbon dioxide, nitrogen, and phosphorous in the water. The decay of plant materials consumes oxygen and produces carbon dioxide. Change in the balance between growth and decay can result in a change in the ecosystem and its water quality.

Learn More

  • A Primer on Water Quality (Webpage and PDF), U.S. Geological Survey
    A basic overview of water quality, how it is measured, and how natural and human processes affect it.
  • National Water-Quality Assessment (NAWQA) Program (Website), U.S. Geological Survey
    Website providing a large amount of information on water quality across the United States, including pesticides, nutrients, trace elements, corrosivity, pharmaceuticals, and more, with data, maps, and reports.
  • How's My Waterway? (Website), U.S. Environmental Protection Agency
    Web service that allows users to find recent water quality assessments for a huge number of water bodies and waterways across the United States.