The chemical character of groundwater is influenced by the minerals and gases reacting with the water in its relatively slow passage through the rocks and sediments of the Earth’s crust. Many variables cause extensive variation in the quality of groundwater, even in local areas. Generally, groundwater increases in mineral content as it moves along through the pores and fracture openings in rocks. This is why deeper, older waters can be highly mineralized. At some point, the water reaches an equilibrium or balance, which prevents it from dissolving additional substances.
About 50 properties are subject to determination, but only certain ones need be known to determine its usefulness. Dissolved minerals are reported in terms of several differing units of measurement. The most common practice is to report dissolved minerals in parts per million (ppm) by weight. One ppm is equivalent to the weight of one part of the dissolved mineral contained in one million parts by weight of the solution. Some agencies report analyses in units of milligrams per liter, which is equivalent to ppm. Hardness has been commonly expressed in grains per gallon. One grain per U.S. gallon equals 17.12 ppm.
The following characteristics of groundwater give it certain advantages over surface water.
1. Groundwater usually contains no suspended matter.
2. Groundwater, very rarely contains pathogenic bacteria; generally it contains microbes native to the formation, unless contaminated by human activity (Michael J. Schieders, Water Systems Engineering Inc., via personal communication.)
3. Groundwater is clear and colorless unless tainted with humic material.
4. The temperature of groundwater is relatively constant and is equivalent to, or greater than, the mean air temperature above the land surface. Temperatures can be altered by human influence.
The data presented is derived from work published by J.H. Criner, E.M. Cushing, and E.H. Boswell of the USGS (1961, Source and significance of dissolved mineral constituents and physical properties of natural waters, USGS Training Aid No. 1).
Water that attacks metal is said to be corrosive. It frequently results in "red water" caused by solution of iron; however, not all red water is the result of corrosion. Water from some formations contains considerable iron in solution, which, on being exposed to the air, precipitates readily and gives a red-water effect. Acids and strong bases are capable of causing corrosion, and, together with an extreme pH, they support electrochemical processes that cause deterioration of water pipes, steam boilers, and water-heating equipment. Preventive measures involve control of these active agents or minimizing their effects and include maintaining proper pH stability in the treated water. Electrolysis control inside steel reservoirs and protective coating on metal surfaces also are used for protection against corrosion. Free carbon dioxide and other gases normally are removed by aeration and, if necessary, neutralized by the addition of either lime or soda ash.
In the photo at right, the red iron coloring and metals enrichment in this Colorado spring are caused by groundwater coming in contact with naturally occurring minerals present as a result of ancient volcanic activity in the area. Photo courtesy USGS.
Dissolved from some rocks and soils, and not so common as iron, manganese (chemical symbol Mn) has many of the same objectionable features as iron. The oxidized form of manganese causes dark brown or black stains. Large quantities of manganese are commonly associated with high iron content and acid water.
Calcium and magnesium
Dissolved from practically all solids and rocks, but especially from limestone, dolomite, and gypsum, calcium (Ca) and magnesium (Mg) are found in large quantities in some brines. Magnesium is present in large quantities in sea water. It causes most of the hardness and scale-forming properties of water. Water low in calcium and magnesium is desired in electroplating, tanning, dyeing, and in textile manufacturing. Calcium and magnesium are the principal cause of the formation of scale in boilers, water heaters, and pipes, and to the objectionable curd in the presence of soap. These mineral constituents and hardness greatly affect the value of water for public and industrial uses.
Sodium and potassium
Dissolved from practically all rocks and soils, sodium (Na) and potassium (K) are also found in ancient brines, sea water, some industrial brines, and sewage. Large amounts (500 ppm or more) in combination with chloride give a salty taste. High sodium content commonly limits use of water for irrigation. Sodium salts (50 ppm or more) may cause foaming in steam boilers. Compounds of sodium and potassium are abundant in nature and highly soluble in water. Some groundwater that contains moderate amounts of dissolved material may, in passing through sodium- and potassium-containing rock formations, undergo base exchange and become soft at greater depths.
Bicarbonate and carbonate
Generated by the action of carbon dioxide in water on carbonate rocks such as limestone and dolomite, bicarbonate (HCO3-) and carbonate (CO3-2) produce an alkaline environment. Bicarbonates of calcium and magnesium decompose in steam boilers and hot-water facilities to form scale and release corrosive carbonic acid gas. In combination with calcium and magnesium, they cause carbonate hardness. Bicarbonate is of little significance in public supplies except in large amounts, where taste is affected or where the alkalinity affects the corrosiveness of the water.
The Earth’s temperature or chemical reaction affects the usefulness of water for many purposes. Most users desire water of uniformly low temperature. In general, temperatures of shallow groundwater show some seasonal fluctuation whereas temperatures of groundwater from moderate depths remain near or slightly above the mean annual air temperature of the area. In deep wells, the water temperature generally increases 1 °F for each 60 feet to 100 feet of depth.
Sulfates (SO4-2)are dissolved from rocks containing gypsum, iron sulfides, and other sulfur compounds. Commonly present in mine water and in some industrial wastes, large amounts have a laxative effect on some people and, in combination with other ions, give a bitter taste. Sulfate in water containing calcium forms a hard scale in steam boilers.
Chlorides (Cl-) are dissolved from rocks and soils. Present in sewage and found in large amounts in ancient brines, sea water, and industrial brines, large quantities increase the corrosiveness of water and, in combination with sodium, give a "salty" taste. The chlorides of calcium, magnesium, sodium, and potassium are readily soluble. Drainage from salt springs and sewage, oil fields, and other industrial wastes may add large amounts of chloride to streams and groundwater reservoirs. Small quantities of chloride have little effect on the use of water. Sodium chloride imparts a salty taste, which may be detectable when the chloride exceeds 100 ppm, although in some water ,500 ppm may not be noticeable. Chlorides in high concentrations present a health hazard to children and other young mammals.
Aluminum (Al) is derived from bauxite and other clays. Although present in many rocks, aluminum is not highly soluble and precipitates readily. There is no evidence that it affects use of water for most purposes. Acid water (low pH) often contains greater amounts of aluminum. Such water is troublesome for boiler feed because of the formation of scale.
Dissolved from practically all rocks and soils, silica (SiO2) is generally found in small amounts from 1 ppm to 30 ppm. Higher concentrations generally occur in highly alkaline water. Silicas form a hard scale in pipes and boilers. Carried over in steam of high-pressure boilers, silicas form damaging deposits on the delicately balanced blades of steam turbines. Silica also inhibits the deterioration of zeolite-type water softeners, but does not affect water for domestic purposes. Groundwater generally contains more silica than surface water.
Extremely common, iron (Fe) is dissolved from practically all rocks and soils. Water having a low pH tends to be corrosive and may dissolve iron in objectionable quantities from pipe, pumps, and other equipment. More than 1 ppm to 2 ppm of soluble iron in surface water generally indicates the presence of acid wastes from mine drainage or other sources. More than about 0.3 ppm stains laundry and utensils reddish-brown. Objectionable for food processing, beverages, dyeing, bleaching, ice manufacturing, brewing, and other processes, moderately large quantities cause unpleasant taste and favor the growth of iron bacteria under slight oxidizing conditions and typical groundwater temperatures. On exposure to air, iron in groundwater is readily oxidized and forms a reddish-brown precipitate. Iron can be removed by oxidation, sedimentation, and fine filtration, or by precipitation during removal of hardness by ion exchange (not a recommended practice).
Sources of nitrate (NO3-) are decaying organic matter, legume plants, sewage, nitrate fertilizers, and nitrates in soil. Nitrate encourages growth of algae and other organisms that cause undesirable tastes and odors. Concentrations much greater than the local average may suggest pollution. Nitrate in water may indicate sewage or other organic matter. In amounts less than 5 ppm, nitrate has no effect on the value of water for ordinary uses.
Chiefly, "dissolved solids" is the total quality of mineral constituents dissolved from rocks and soils, including any organic matter and some water of crystallization. Water containing more than 1,000 ppm of dissolved solids is unsuitable for many purposes. The amount and character of dissolved solids depend on the solubility and type of rocks with which the water has been in contact. The taste of the water often is affected by the amount of dissolved solids.
Hardness as magnesium and calcium carbonates
In most water, nearly all the hardness is due to calcium and magnesium carbonates. All of the metallic cations other than the alkali metals deposit soap curd on bathtubs. Hard water forms scale in boilers, water heaters, and pipes. Hardness equivalent to the bicarbonate and carbonate is called carbonate or "temporary" hardness because it can be removed by boiling. Any hardness in excess of this is called noncarbonate or "permanent" hardness. Noncarbonate hardness is caused by the combination of calcium and magnesium with sulfate, chloride, and nitrate. Scale caused by carbonate hardness usually is porous and easily removed, but that caused by noncarbonate hardness is hard and difficult to remove. Hardness is usually recognized in water by the increased quantity of soap or detergent required to make a permanent lather. As hardness increases, soap consumption rises sharply, and an objectionable curd is formed. In the development of a water supply, hardness is one of the most important factors to be considered. In general, water of hardness up to 60 ppm is considered soft; 61 to 120 ppm moderately hard, 121 to 180 ppm hard, and more than 180 ppm very hard.
Water turbidity is attributable to suspended matter such as clay, silt, fine fragments of organic matter, and similar material. It shows up as a cloudy effect in water and for this reason alone is objectionable in domestic and many industrial water supplies. Filtered water is free from noticeable turbidity. Unfiltered supplies, including those that contain enough iron for appreciable precipitation on exposure to air, may show turbidity. In surface water supplies, turbidity is usually a more variable quantity than dissolved solids. Photo courtesy NCDFR.
Color refers to the appearance of water that is free of suspended matter. It results almost entirely from extraction of coloring matter and decaying organic materials such as roots and leaves in bodies of surface water or in the ground. Natural color of 10 units or less usually goes unnoticed and even in larger amounts is harmless in drinking water. Color is objectionable in the use of water for many industrial purposes, however. It may be removed from water by coagulation, sedimentation, and activated carbon filtration.
Dissolved in small to minute quantities from most rocks and soils such as fluorspar and cryolite, fluoride (Fl) in drinking water has been shown to reduce the incidence of tooth decay when the water is consumed during a child’s period of tooth enamel calcification. However, it 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.
Reactions with formation minerals
A small number of minerals comprise nearly the entire mass of sandstone aquifers. The average sandstone, as determined by F.W. Clarke (1924, The data of geochemistry, fifth ed., USGS Bulletin 770), consists of 66.8 percent silica (mostly quartz), 11.5 percent feldspars, 11.1 percent carbonate minerals, 6.6 percent micas and clays, 1.8 percent iron oxides, and 2.2 percent other minerals. Limestone and dolomite aquifers are primarily calcium carbonate and calcium magnesium carbonate, respectively, but impure ones may contain as much as 50 percent noncarbonate constituents such as silica and clay minerals.
Quartz, the main constituent of sandstones, is the least reactive of the common minerals and, for all practical purposes, can be considered nonreactive except in highly alkaline solutions (Roedder, E., 1959, Physics and Chemistry of the Earth 3). Clays have been demonstrated to react with highly basic or highly acidic solutions.
Clay minerals are common constituents of sedimentary rocks. Roedder (1959) stated that sandstones containing less than 0.1 percent clay minerals might not exist anywhere in the United States, except possibly in small deposits of exceedingly pure glass sand. Clay minerals are known to reduce the permeability of sandstone to water as compared with its permeability to air (Johnston, N., and C.M. Beeson, 1945, Water permeability of reservoir sands, Petroleum Development and Technology, in Transactions of the American Institute of Mining and Metallurgical Engineers 160: 43-55; Baptist, O.C., and S.A. Sweeney, 1955, Effect of clays on the permeability of reservoir sands to various saline water, Bureau of Mines Report of Investigations 5180; Land, C.S., and A. Baptist, 1965, Effect of hydration of montmorillonite on the permeability of water-sensitive reservoir rocks, Journal of Petroleum Technology October). The degree of permeability reduction to water as compared with air is termed the water sensitivity of a sandstone by Baptist and Sweeney.
The above information is excerpted in large part from Chapter 23 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors.