Natural Variations in the Composition of Groundwater

It is commonly believed that the composition of groundwater does not change naturally. So if a change is recognized through taste and/or odor, many may think that something bad has happened. Perhaps the aquifer has become contaminated, or something is wrong with the well. However, a common cause of changes in water quality is through natural variations within the aquifer. What gives groundwater its taste is the amount and type of dissolved minerals within it. Pure water, i.e., having no dissolved minerals, doesn’t occur naturally. Factors that control the dissolved minerals in groundwater include [1] the types of minerals that make up the aquifer, [2] the length of time that the water is in contact with the minerals, and [3] the chemical state of the groundwater. Virtually all groundwater comes from precipitation that soaks into the soil and passes down to the aquifer. Rainwater has a slightly acidic pH, therefore it tends to dissolve solid minerals in the soil and in the aquifer. Different rocks, e.g., sandstone, limestone and basalt all have different minerals and therefore, groundwater in contact with these materials will have different compositions (factor [1] above). Some of the constituents found in groundwater, however, are not abundant in common rocks and minerals, for example, chloride (Cl) and sulfate (SO ). The 4 probable source for these chemicals is the infiltrating precipitation. Factor [2] above is called the residence time of the groundwater in the aquifer. The longer the groundwater is in contact with the minerals, the greater the extent of its reaction with those minerals and the higher will be the content of dissolved minerals. The chemical state of groundwater (factor [3]) is usually defined in terms of pH, temperature and oxidation-reduction potential. These parameters control and are influenced by chemical reactions. As temperatures change seasonally, as the water table rises and falls, or as recharge rates vary, the chemical state will change and, as a result, so will the composition of groundwater.