Hardness.....

What is is? | Why is it important? | What influences it? | What happens downstream? | Photograph | Homepage and Map

 WHAT IS IT?

     Hardness measures the concentration of ions (chemical elements) with a charge of greater than +1 in river water. Calcium (Ca+2) and Magnessium (Mg+2) are the most common of these ions in fresh water, so hardness is generally a measurement of how much Ca+ and Mg+2 are in the water.

 WHY IS IT IMPORTANT?

     1. Hardwater streams (streams with lots of Mg+2 and Ca+2) generally have high levels of animal and plant productivity. Soft water has lower primary productivity (growth of algae and plants) than hard water, and that can affect other parts of the ecosystem. "Brown trout, Salmo trutta, tend to reach a larger body size in hard water than in soft water." (Windell, 1992) Hardwater streams are often noted for high levels of productivity because Mg+2 and Ca+2 are usually associated with other important nutrients and minerals.

     2. Hardness helps limit the damaging effects of toxic or dangerous metals. Elevated levels of metals can clog up the gills of aquatic animals. In theory, scientists believe that if there are a lot of Ca+2 and Mg+2 ions in the river (higher hardness) these harmless atoms end up on the gill, and not the damaging metals. Therefore, a harder water river could handle higher levels of toxic metals and still support healthy organisms.

Levels of water hardness (mg/L CaCO3)  

 Soft

0-75

 Moderate

75-150

 Hard

150-300

 Very hard

300+
SOURCE: Environmental Protection Agency, 1976

 WHAT INFLUENCES IT?

     1. Geology and Soils: As water flows over land, some of the chemicals in the rocks and soils seep into the water. For example, calcium and magnessium leach out of rocks such as limestone and gypsum. The longer a river flows over these rocks, the more calcium and magnesium the water accumulates. In general, hardness increases as you go downstream.

     2. Seasonal Fluctuations: In Colorado, hardness decreases with spring runoff as mountain snows melt and fill rivers. Hardness increases during the winter. By increasing river volume, runoff decreases the concentration of Mg+2 and Ca+2.

      3. Irrigation: Irrigation can increase levels of Mg+2 and Ca+2 because as the water travels over land, it can collect more minerals.

WHAT HAPPENS AS WE TRAVEL DOWNSTREAM?

Click here for the author's interpretation of the data.

Bathers enjoy a cool dip in South Platte River water as the river moves through Chatfield Reservoir near Denver, CO.

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