General water hardness is related to the dissolved minerals in the water. General hardness is a misleading term often confused with carbonate hardness or temporary hardness, which is related to alkalinity and relates to the "buffering capacity" of the water (its ability to resist pH changes). This means that if the carbonate hardness is high, then the pH will be highly stable, or if the carbonate hardness is low, the water's pH will fluctuate quickly. The term general hardness should be replaced with a simpler term: Hardness.
Water hardness is the measurement of the number of ions that have lost two electrons (divalent cations) dissolved in the tested water and is, therefore, related to total dissolved solids.
The more divalent cations dissolved in the water, the "harder" the water. Generally, the most common divalent cations are calcium and magnesium. However, other divalent cations may contribute, including iron, strontium, aluminum, and manganese. Typically the other divalent cations contribute little to no appreciable additions to the water hardness measurement. A stream or river's hardness reflects the geology of the catchment's area and sometimes provides a measure of the influence of human activity in a watershed. For example, sites that have active or abandoned mines nearby often have higher concentrations of iron ions in the water resulting in a very high hardness degree.