Sulfate and water quality

Sulfate is second to bicarbonate as the major anion in hard water reservoirs. Sulfates (SO₄^2-) can be naturally occurring or the result of municipal or industrial discharges. When naturally occurring, they are often the result of the breakdown of leaves that fall into a stream, of water passing through rock or soil containing gypsum and other common minerals, or of atmospheric deposition. Point sources include sewage treatment plants and industrial discharges such as tanneries, pulp mills, and textile mills. Runoff from fertilized agricultural lands also contributes sulfates to water bodies.

Methodology: Sulfate can be analyzed by colorimetric method or Ion Chromatography.

Environmental Impact: Sulfur is an essential plant nutrient. Aquatic organisms utilize sulfur, and reduced concentrations have a detrimental effect on algal growth. The most common form of sulfur in well-oxygenated waters is sulfate. When sulfate is less than 0.5 mg/L, algal growth will not occur. On the other hand, sulfate salts can be major contaminants in natural waters.

A sulfur cycle exists which includes atmospheric sulfur dioxide (SO₂), sulfate ions (SO₂^2-) and sulfides (S^1-). Sulfides, especially hydrogen sulfide (H₂S), are quite soluble in water and are toxic to both humans and fish. They are produced under conditions where there is a lack of oxygen (anaerobic). Because of their foul "rotten egg" smell they are avoided by both fish and humans. Sulfides formed as a result of acid mine runoff from coal or other mineral extraction and from industrial sources may be oxidized to form sulfates, which are less toxic.

Sulfates are not considered toxic to plants or animals at normal concentrations. In humans, concentrations of 500 - 750 mg/L cause a temporary laxative effect. However, doses of several thousand mg/L do not cause any long-term ill effects. At very high concentrations sulfates are toxic to cattle. Problems caused by sulfates are most often related to their ability to form strong acids, which changes the pH of the water. Sulfate ions also are involved in complexing and precipitation reactions which affect solubility of metals and other substances.

Sulfates in water to be used for certain industrial processes such as sugar production and concrete manufacturing must be reduced below 20 mg/L.

Criteria: Recommended limits for water used as a domestic water supply are below 250 mg/L.