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Ground Water Quality Chemistry
Ground water quality chemistry is one way to study what minerals, compounds and other elements found in water contribute to water quality and what are part of water pollution. Clean water, that is water with no inclusions at all, may not taste and delicious as water with some minerals and other "ingredients".
The highest water quality is widely believed to be pure water: H20 and nothing else. But in fact, good-tasting, natural water is full of chemicals. Water is one of the most powerful solvents found in nature, capable of dissolving and containing a wide variety of elements, minerals, and other compounds. Ground water quality chemistry is the study of the chemicals and particles found in ground water. Some of the materials found in naturally occurring water - called solutes - are classified as pollutants because they can harm human health. But other solutes are essential to health, and even improve the taste of water.
"Water chemistry" is the profile of all the chemicals dissolved in a given sample of ground water - water which has fallen as rain and flowed through soil. By analyzing the kinds and proportions of chemicals found in ground water, hydrologists can estimate where the ground water sample came from; the path it took from the moment it hit the earth until the sample was collected; and even how long it took the ground water sample to make the trip.
Most ground water chemistry activity occurs in the soil zone, the layer of dirt, clay, and rocks that lies atop the water table (or aquifer). From the moment ground water falls as rain and begins to filter through soil, it dissolves carbon dioxide created by decomposition of organic materials in the soil. A weak acid is formed which helps dissolve other minerals in the ground water. Evaporation and plants draw off some of the ground water, increasing the concentrations of chemicals. Ground water acquires most of its solutes by the time it reaches the aquifer beneath the soil zone. The aquifer is a sort of liquid highway that may carry ground water vast distances before it reaches the surface again.
Only seven solutes make up 95 percent of all ground water chemistry. They include calcium (CA); magnesium (Mg); sodium (Na); potassium (K); chloride (Cl); sulfate (SO4); and bicarbonate (HCO3). In addition to these dominant solutes, small quantities of trace elements and isotopes of solutes dissolve in ground water. The concentrations of these trace elements and isotopes compared to known deposits of minerals and pollutants tell hydrologists much about the path taken by ground water.
The mineralogy of aquifers can be broadly classified as reactive or non-reactive. Ground water chemistry changes greatly when water passes through an aquifer containing highly reactive minerals such as limestone, gypsum, halite (common salt), etc. Non-reactive materials such as sand and gravel leave ground water chemistry relatively unchanged.
Ground water chemistry is affected when water enters an aquifer through precipitation, seepage from surface waters such as rivers, or as ground water underflow from neighboring aquifers deep underground. Ground water chemistry differs depending on the source of water; the degree to which it has evaporated; the types or rock and minerals it has encountered; and the length of time it has been in contact with reactive minerals.
Ground water quality chemistry, combined with many other types of geological data, help hydrologists estimate the extent of aquifers; the flow of water through aquifers; the degree to which waters from different sources mix within an aquifer; and the sources of pollutants as well as beneficial solutes.
