Alkalinity and Hardness - Understanding Lake Data

EQUATION 5. Hardness and Alkalinity.

TABLE 8. Categorization of hardness by mg/l of calcium carbonate (CaCO3).

	Level of hardness	Total hardness as mg/l CaCO3
	soft                      0-60  mg/l
	moderately hard	         61-120 mg/l
	hard                    121-180 mg/l
	very hard	          >180  mg/l

High levels of hardness (greater than 150 mg/l) and alkalinity can cause marl (CaCO3) to precipitate out of the water. Hard water lakes tend to produce more fish and aquatic plants than soft water lakes. Such lakes are usually located in watersheds with fertile soils that add phosphorus to the lake. As a balancing mechanism, however, phosphorus precipitates with marl, thereby controlling algae blooms.

If the soils are sandy and composed of quartz or other insoluble minerals, or if direct rainfall is a major source of lake water, hardness and alkalinity will be low. This is the case in much of northern Wisconsin, where glacial deposits contain little limestone or other soluble minerals. Lakes with low amounts of alkalinity are more susceptible to acidification by acid rain and are generally unproductive.