Freshwaters around the world have higher concentrations of calcium when they are located closer to calcium-rich soils and rocks. Typical calcium concentrations worldwide are less than 15 mg/L, but waters close to calcium-rich carbonate rocks often have calcium concentrations exceeding 30 mg/L. Over 75% of Florida lakes have calcium concentrations that are less than 16 mg/L. Waterbodies in the Florida LAKEWATCH database have average calcium concentrations ranging from 0.2 to 94 mg/L.

Calcium enters aquatic environments primarily through the weathering of rocks such as limestone, which is largely composed of calcium compounds. In some circumstances, calcium can also be deposited in waterbodies as a result of human activities, often because of the extensive use of calcium-containing chemicals in agriculture and industry.

Having calcium in your water supply may cause inconveniences. For example, you may have experienced difficulties in getting soap to lather in what is called "hard" water. In hard water, calcium combines with soap molecules and inhibits their foaming ability. High concentrations of calcium can also cause a crusty accumulation called "scale" to form in pipes and hot water heaters. To prevent problems with hard water and scale formation, many cities "soften" water by removing calcium from their drinking water supply.

Health Concerns:
Calcium in waterbodies is not known to have any serious effects on human health. Drinking large amounts of hard water when the body is accustomed to soft water, however, may have a laxative effect.

The Role of Calcium in Waterbodies:
Calcium has been shown to influence the growth of freshwater plants and animals. It is a necessary structural component of plant tissues, animal bones, and animal shells. Calcium is involved in many chemical cycles that occur in waterbodies, often in rather complex ways. For example, adding calcium carbonate to a waterbody can either stimulate aquatic plant growth or inhibit it.

Example 1 - when calcium carbonate, in the form of lime, is added to an acidic waterbody:

• the lime will cause the water to become less acidic (the pH will become higher);
• as the water becomes less acidic, phosphorus, a necessary nutrient, will be converted to forms that then can be used by aquatic plants, potentially stimulating their growth.

Example 2 - when a waterbody is not acidic (with a pH above 8.2) and contains an abundance of calcium carbonate:


• calcium carbonate will combine with the phosphorus in the water to form a solid called a "precipitate", that can sink to the bottom or be attached to aquatic plants;
• plants and algae will be deprived of phosphorus and their growth can be slowed.