pH

Water Quality Measurement  | Chemical Parameter

Lake Pend Oreille | Pend Oreille River

The acidity of water is measured using pH, which is affected by the gases, chemicals, and ions dissolved in the water column. The pH scale ranges from 0 to 14, where 0 corresponds with the most acidic, 14 with the most basic, and 7 with a neutral pH. Each number represents a tenfold change in pH. For example, a substance that measures pH 7 is ten times more acidic than a substance that measures pH 8.

Ideally, the pH of a healthy freshwater ecosystem is neutral (pH 7). Acidification can be an indicator of long-term climate change. Very acidic water can be detrimental to fish and other organisms because it increases the toxicity of heavy metals and can affect the form and quantity of important nutrients.

On our graph, there is likely to be overlap of data points due to duplicate measurements for the sites. Try clicking on a month to see all the data for a specific month or click on the “Average” tab to see the average monthly pH of each site.

pH determines the form, solubility, and biological availability of many chemical constituents in water, such as nutrients and trace metals (e.g., low pH can increase the solubility of trace metals, which can increase their bioavailability).

Surface waters tend to have a relatively lower pH in the morning, which corresponds with the circadian rhythm of plant photosynthesis. During this time, the water contains higher amounts of carbon dioxide, which forms carbonic acid. The rate of photosynthesis increases as the sun rises and oxygen levels increase, causing the pH to increase during the day.

Some sources of abnormally-low (acidic) pH

  • Carbon dioxide – Decomposition of organic matter, Atmospheric emissions, and Fertilizers/agricultural runoff
  • Acid rain—nitrogen oxide and sulfur dioxide o Automobile and coal-fired power plant emissions
  • Mine drainage Some sources of abnormally-high (basic) pH

Some sources of abnormally-high (basic) pH

  • Natural erosion of limestone
  • Detergents, cleaners, etc.
  • Fertilizer runoff

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