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 to reduce the crowding of data points.

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.

We measure the pH of the water each time we collect data – once a month for five months out of the year (June – October). The bars represent the upper quartile (75% of the data taken at this site falls below this line), the median (or mid-point of the data) for each site, first quartile (25% of the data taken at this site falls below this line), and the box represents the middle 50% of the data.

Use our interactive graph to check out all the data we have collected since 2012. Hover over a data point to see more information or try clicking on the name of a month to see all the data for just one month! 

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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