Natural Contamination of Water Supplies

Adapted from “Ecolands,” an unpublished manuscript by Global Lab teacher Dr. Shirley Griffin.

Pollution in public or private wells is usually caused by:
• Natural phenomena
• Human activities
• Over-pumping

Natural Causes of Well Pollution

Various natural processes cause rocks to mineralize and these minerals can mix with groundwater, and, finally, enter wells. This mineralization may increase water toxicity, but in most cases causes water discoloration, odors, and/or poor taste. The interaction of water and rock can cause specific problems, including hardness, acidity, and high concentrations of magnesium, iron, and arsenic.

Hardness

Hardness reflects the amount of calcium carbonate present in the water. Calcium carbonate can occur in the water because of human activity, but is most often the result of natural calcium and magnesium within underlying rock deposits. If water contains more than 60 mg/L of calcium and magnesium ions, it is “hard.” If water contains less than 60 mg/L of calcium and magnesium ions, it is “soft.”

Hard water interacts with soap to form residues in sinks and on clothing. Soft water soaps easily and makes suds. The hardness of water is important for industry, as it can cause problems with machines and other equipment used to transport water for industrial purposes.

Acidity

The acid condition of water is linked to the number of hydrogen ions in solution. Acidity causes metals in pipes, soils, and rocks to be released into the water. Water supplies should have a pH of 7 (neutral) or be within the 6.5 to 8.5 range. Levels of pH below 6.5 (acidic) or above 8.5 (alkaline) can cause dangerous leaching of metals in soils, pipes, and rock.

The following metals that can oxidized and then slightly dissolved in water include:

Manganese

Manganese appears naturally in water from local rock. When manganese is exposed to oxygen in water, it forms a dark brown precipitate that can color cooking vegetables or stain laundry. Manganese also affects the taste of water. Generally less than .05 mg/L of manganese is preferred in drinking water.

Iron

Iron found in rocks and soil can easily enter the water. The more acid the groundwater, the more likely the iron will enter the water. When iron comes in contact with oxygen it oxidizes. Iron, like manganese, will cause water to taste poorly, and it will discolor laundry. Drinking water should have less than .3 mg/L of iron.

Arsenic

Recent water quality tests conducted on deep wells have reported levels of arsenic. Arsenic is found naturally in many parts of the United States. In some areas, naturally occurring amounts of arsenic exceed drinking water standards. Water levels with more than .05 mg/L of arsenic should not be used for drinking.