Water Quality Parameters: Conductivity & Hardness

Two other important chemical parameters to understand in water quality monitoring are conductivity and hardness.

Conductivity is a measure of a solution’s ability to conduct electricity, and therefore a measure of the water’s ionic activity and content. The more dissolved salts in water, the higher its ionic content, or conductivity.  Conductivity is a good measure to reflect the amount of dissolved solids (total dissolved solids or TDS) or salinity of a water source.

Common factors affecting conductivity include low flow conditions (less dilution of high salt content input sources), temperature and pH, especially if there are significant amounts of metals in the local watershed geology. This latter phenomenon is manifested best by looking at Acid Rock (or Mine) Drainage, which produces sulfuric acid through the oxidation of iron pyrite. Most metals’ solubility increases as the acidity of solution increases (as the pH decreases).

Hardness. If you look up hardness in a textbook or on-line, you might see it defined as the concentration of multivalent cations. This definition isn’t quite specific enough or clear in what hardness really means to a water’s quality.

When measured with any of the tests available on the market, hardness is a measure of the concentration of magnesium (Mg) and calcium (Ca) ions in solution, expressed in mg/L or parts per million (ppm). Both of these metals (that’s right: calcium is a metal!) carry an ionic charge of 2+, thus making them multivalent cations.

While “hard water” (>320 mg/L) might cause scaling or the build up of white precipitates on faucets, pipes and other water fixtures, it is not a human health concern and hardness, particularly the calcium fraction, decreases the toxicity of most metals in solution to aquatic organisms.


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The MSP project is funded by an ESEA, Title II Part B Mathematics and Science Partnership Grant through the Montana Office of Public Instruction. MSP was developed by the Clark Fork Watershed Education Program and faculty from Montana Tech of The University of Montana and Montana State University, with support from other Montana University System Faculty.