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Geologic Framework (continued) 3

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Geology and Water Quality

The quality of surface and ground water, in the absence of human influences, is largely determined by the physical and chemical nature of the rock minerals they contact during flow. Some minerals, such as quartz, interact hardly at all with ground water so the chemistry of ground water is likely to be little changed by flowing through quartz-rich rocks. Most of the Proterozoic rocks fall into this category and ground waters flowing through them tend to be neither strongly acidic nor strongly alkaline, i.e., they are near-neutral fluids. These types of ground waters usually contain low to moderate amounts of such elements as sodium and calcium, but very low concentrations of potentially harmful metals.

The Paleozoic sedimentary rocks also fall into this category, especially the sandstones. However, ground waters flowing through the carbonate rocks generally tend to be more alkaline, bicarbonate-rich fluids. Ground waters that dissolve evaporites will also have distinctive chemistry, especially increased salt content. However, even these fluids will carry very low concentrations of heavy metals.

The presence of ore deposits may, however, lead to serious degradation of water quality. In south-central Arizona, the exposure of Proterozoic rocks is insufficient to estimate the presence of mineral deposits of this age. The known occurrence of ore deposits in similar rocks of this age elsewhere implies that there is a high probability that concealed ore deposits occur in these basement rocks. In this case, the assumption that the ground water quality should be good may be incorrect.

The most important rocks with regard to water quality are those of Late Cretaceous to Early Tertiary age which include a large number of economic ore deposits, and large areas of mildly mineralized and strongly altered ground. An important point is that the present day ground waters use the same fracture systems in moving into the aquifers as was used by the ancient fluids that produced large areas of alteration and mineralization. Thus, the ground waters have ample opportunity to react with the large accumulations of metallic elements that form the ore deposits. These chemical interactions are quite different from those associated with fluid movement through metal-poor rocks. Most of the Cretaceous-Tertiary ore deposits are very rich in sulfide minerals, particularly the iron sulfide pyrite that can react with oxygen-bearing ground water to form very acidic fluids. Such fluids can leach large quantities of potentially toxic elements associated with the sulfide minerals. Movement of these fluids is called "acid drainage". In addition, the presence of large regions of altered rocks which are chemically inert means that the fluids can maintain their acidity for long periods of time and over long distances. It is these types of fluids that have the potential to degrade the quality of ground water as it moves toward the aquifer system.

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