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Water-quality constituents and their significance

This page describes water-quality constituents commonly found in ground and surface water, their most likely sources, and their significance for different uses. If you need to review drinking water, stock water, and irrigation water standards for these constituents, click here to go to a page containing that information. Note: The information presented here has been gathered from many sources and is intended to provide general descriptions of the impacts of various water-quality constituents. Application to specific situations may require evaluation of additional, more specific information.

Constituent or Physical Property Source or Cause Significance
Calcium (Ca) and Magnesium (Mg) Dissolved from most soils and rocks, especially limestone, dolomite and gypsum. Ca and Mg are found in some brines. Cause hardness and most of the scale- forming properties of water; soap consuming (See hardness). Usually have no effect on suitability of water for irrigation or stock water.
Sodium (Na) and Potassium (K) Dissolved from most rocks and soils. Also found in brines and sewage. High concentrations give a salty taste when combined with chloride. For most purposes moderate levels have little effect on the use of water. Sodium salts may cause foaming in boilers and high sodium adsorption ratio may limit use of water for irrigation.(See Sodium Adsorption Ratio).
Iron (Fe) Dissolved from most rocks and soils. May also be derived from iron pipes, pumps, and other equipment. On exposure to air, iron in ground water oxidizes to reddish brown sediment. More than about 0.3 mg/L stains laundry and utensils reddish brown. Iron and manganese together should not exceed 0.3 mg/L. Greater concentrations cause unpleasant taste and favor growth of iron bacteria but do not endanger health. Excessive iron may also interfere with the efficient operation of exchange-silicate water softeners.
Manganese (Mn) Dissolved from some rocks and soils. High concentrations often associated with high iron content and with acid waters. Same objectionable features as iron. Causes dark brown or black stain. For taste and aesthetic reasons iron and manganese together should not exceed 0.3 mg/L.
Silica (SiO2) Dissolved from most rocks and soils, usually at low concentrations (5 to 30 mg/L). Forms hard scale in pipes and boilers.
Bicarbonate (HCO3) and Carbonate (CO3) Dissolved from carbonate rocks such as limestone and dolomite; oxidation of organic carbon. Bicarbonate and carbonate produce alkalinity. Bicarbonates of calcium and magnesium in boilers and hot water heaters form scale and release carbon dioxide gas.
Chloride (Cl) Dissolved from rocks and soils. Present in sewage and found in natural and industrial brines. Chloride salts in excess of 100 mg/L give a salty taste to water. When combined with calcium and magnesium, chloride may increase the corrosive activity of water.
Sulfate (SO4) Dissolved from rocks and soils containing gypsum, iron sulfides, and other sulfur compounds. Often present in some industrial wastes. Sulfate in water containing calcium forms hard scale in boilers. In high concentrations, sulfate in combination with other ions gives a bitter taste to water. Concentrations above 250 mg/L may have a laxative effect. Domestic water supplies containing more than 1000 mg/L sulfate can be used for drinking if a less mineralized water supply is not available.
Nitrate (NO3) Decaying organic matter, sewage, nitrate in soil and in fertilizers. Concentrations much greater than the local average may suggest pollution. High concentrations are generally a characteristic of individual wells and not of entire aquifers. Nitrate encourages growth of algae and other organisms which produce undesirable tastes and odors. There is evidence that more than about 10 mg/L may cause methemoglobinemia ("blue baby syndrome") in infants, which may be fatal. Interference Syndrome is likely in cattle if stock water exceeds 50 to 100 mg/L of nitrate, especially for long periods of time. At more than 100 mg/L of nitrate there is the possibility of acute losses to Interference Syndrome and secondary disease.
Fluoride (F) Dissolved in low concentrations from most rocks and soils. Most hot and warm springs contain more than the recommended concentration of fluoride. When consumed during the period of enamel calcification fluoride in drinking water reduces the incidence of tooth decay in children. But fluoride may cause mottling of the teeth, depending on the concentration of fluoride, the age of the child, the amount of drinking water consumed, and the susceptibility of the individual. 0.8 to 1.7 mg/L is optimum, depending on the air temperature.
Total dissolved solids (TDS) Chiefly mineral constituents dissolved from rocks and soils. Includes almost all of the material that is in solution in the water. Older analytical methods determined dissolved solids by evaporation of the sample and the weight of the residue. During evaporation, however, some of the bicarbonate (HCO3) was lost, causing under-reporting of dissolved solids. Modern analytical methods retain all of the bicarbonate, but the calculation for total dissolved solids includes only the percentage of the bicarbonate that would have been retained under conditions of evaporation. Water with more than 1000 mg/L of dissolved solids may contain minerals which impart a distinctive taste. Water with more than 2000 mg/L dissolved solids is generally too salty to drink.

Total dissolved solids concentrations are useful for comparison to established water-quality standards.

Sum of dissolved constituents Chiefly mineral constituents dissolved from rocks and soils. Includes all material that is in solution in the water. The calculation includes all of the bicarbonate measured in the sample. The Sum of Dissolved Constituents more accurately reflects the actual amount of dissolved mineral matter in the water than does total dissolved solids (TDS). However, most standards are written for TDS and the Sum of Dissolved Constituents should not be compared to those standards.
Specific conductance Dissolved minerals in the water. Specific conductance is a measurement of the water's capacity to conduct an electric current. Conductance varies with the concentration of dissolved solids in the water and their degree of ionization. When measured in micromhos/cm it is generally 1 to 1.5 times the total dissolved solids content.
pH (Hydrogen-ion activity) Acids, acid-generating salts, and free carbon dioxide lower pH. Carbonate, bicarbonate, hydroxide, phosphate, silicate, and borate raise the pH. The pH is a measure of acidity. A pH of 7.0 indicates neutrality of a solution. Values higher than 7.0 denote increasing alkalinity; values lower than 7.0 indicate increasing acidity. Corrosiveness of water generally increases with decreasing pH, but excessively alkaline waters may also attack metals. A pH range between 6.0 and 8.5 is acceptable and normal for most waters in Montana.
Hardness as CaCO3 In most waters nearly all the hardness is because of calcium and magnesium. Hard water consumes soap before a lather will form, deposits soap on bathtubs, and forms scale in boilers, water heaters, and pipes. Waters of hardness 0 to 60 mg/L are termed soft; 61 to 120 mg/L moderately hard; 121 to 180 mg/L hard; and more than 180 mg/L very hard.
Alkalinity Formed by the presence of certain anions, predominantly HCO3 and CO3. These anions are formed by the action of carbon dioxide in water on carbonate rocks such as limestone and dolomite. Certain organic materials may also produce alkalinity. Alkalinity is an indicator of the relative amounts of carbonate (CO3), bicarbonate (HCO3), and hydroxide ions.
Sodium Adsorption Ratio (SAR) Indicates the relative abundance of sodium as compared to calcium and magnesium. Greater SAR values indicate a greater relative abundance of sodium. A high sodium concentration in irrigation water combined with low calcium and magnesium concentrations usually reduces soil tilth and impairs plant growth.
Trace metals Dissolved from rocks and soils. Some metals may be released from plumbing, pipes, etc. Limits are usually recommended for health reasons. Limits for drinking water generally are conservative and higher concentrations may be permitted if the water is the best available supply.
Bromide Present in high concentrations in some brines. The presence of low concentrations in fresh water is not known to endanger health.
Strontium Dissolved from igneous and sedimentary rocks. The presence of low concentrations in fresh water is not known to endanger health.
Boron Dissolved from igneous and sedimentary rocks. Boron is essential to plant growth, but exceedingly toxic to plants at concentrations only slightly above optimum. The optimum concentration varies with plant type and ranges from about 300 micrograms/L to 4,000 micrograms/L. (What is optimum for one plant type may be toxic to another type).

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