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Hardness is a measure of polyvalent cations in water. Metals' toxicity is also a function of water hardness. Chemically, hardness is defined as the sum of calcium and magnesium ions present in the water, converted to milligrams per liter (mg/L) of calcium carbonate (CaCO3).
Running water with high relative hardness (>200 mg/L) is most often associated with high primary productivity – the process through which plants convert light energy into sugar. Plant material supports life cycle functions by becoming food for invertebrates, which in turn support fish species. More species of fresh water insects and fish are therefore associated with hard water streams than with soft water streams. The presence of dissolved calcium and magnesium can also protect aquatic life in streams that contain
high concentrations of metals. Calcium and magnesium ions out-compete metal ions for uptake sites on fish gills.
Hard water has some disadvantages as well. It can create coatings in pipes, faucet heads, and household implements such as teakettles. Hardness also can affect the taste of water although it does not introduce toxic components into our drinking water supply.
The level of hardness usually increases from upstream to downstream because of increased exposure of water to minerals in rocks and soils. The hardness of rivers and streams generally ranges from 1-500 mg/L, from 500-1000 mg/L in lakes, and from thousands to tens of thousands of mg/L in groundwater. Hardness values do not tend to change daily, although values may decrease due to dilution by snowmelt and large rainstorms. There is no state standard for hardness but hardness values are used to calculate many metals standards. |
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