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Geochemistry

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Geochemistry in Mineral Environmental Studies

Inorganic chemistry plays a vital role in geology. Geochemistry attempts to understand the underlying chemical principles involved in Earth processes by applying the laws of inorganic chemistry, and the methodologies of analytical chemistry, to rocks, minerals and fluids. Geochemical studies frequently require precise chemical tests on Earth materials, including major and trace element analysis, and radiogenic and stable isotope determinations. These empirical data may then be used to test process models that approximate a wide variety of geologic processes, adding a predictive aspect to geochemistry. Environmental geochemistry focuses on the role, and application, of chemistry to environmental geologic problems.

Learn about the chemical elements

In this study, geochemistry is being applied to an understanding and evaluation of the effects of acid drainage, on the watershed scale, in a heavily mineralized, ecologically sensitive, semi-arid environment. Little is known about acid drainage in this type of climate. A knowledge of the operative geochemical processes, which involve complex mass exchanges between the minerals of rocks, transient fluids and the atmosphere, is an essential part of this task.

Three fundamental considerations guide such an investigation. These are:

1. the chemical, mineralogic, and physical nature of the source regions for acid drainage;
2. the mechanisms of chemical transport by fluids and the chemical processes occurring along the fluid pathways;
3. the identification of, and chemical processes occurring within, sinks.

Large volumes of rock within the Upper Santa Cruz River Watershed are mineralized and highly altered. The mineralization/alteration may be dispersed, or focused in specific ore deposit districts. Sulfides are ubiquitous, particularly pyrite (the common precursor mineral to acid drainage), in almost all types of mineralization. Pristine sulfides often contain high concentrations of many elements considered toxic in relatively small concentrations, e.g. As, Se, Cd, Tl. Thus, the potential exists for transfer of toxic elements from minerals to surface and ground waters.

The extent to which this potential is realized depends, in large part, on:

1. the availability and volume of fluids, and to what degree fluids are reactive with available minerals;
2. the crystallographic siting of toxic elements within specific minerals;
3. and the mineral parageneses.

If toxic metals and compounds are solubilized, the transient nature of most fluids results in transport of metals and compounds away from their source, potentially into ecosystems where they can cause environmental harm.

The geochemical processes that occur during transport determine:

1. the residence time of specific elements within the fluid phase, and
2. the chemical composition of compounds dissolved within the fluid and precipitated from it. The latter determines the potential for dissolution of a precipitated phase by a different fluid at a later time, and
3. the degree to which the toxicity of dissolved and precipitated compounds is intensified or mitigated.

During fluid movement on the surface and within the subsurface there may be many sinks (permanent or temporary storage regions) for toxic metals and compounds. Sinks represent environments of chemical change that cause simple and complex ions to become unstable in solution. The result is precipitation of solid phases, which may or may not be, immobilized. Chemical changes in fluid chemistry that cause precipitation may be sudden, e.g. through the mixing of two or more fluids of different composition and/or temperature, or they may be gradual, e.g. through the progressive loss of acidity along a mineral-buffered fluid flow path.

Fluid flow within the Upper Santa Cruz River Watershed is slowed, but not immobilized, within the relatively flat lying sedimentary fill of the Tucson Basin and the shallower sub-basins to the south. Sedimentary layers with low permeability, and others with higher permeability and porosity provide aquitards and aquifers; some of the latter provide potable water for the major metropolitan areas. The aquifers are charged by both surface and ground water.

The quality, i.e., chemical composition, of water within the aquifer/aquitard system is determined by:

1. the compositions of surface and ground water entering the system;
2. the relative volume contributions of surface and ground water;
3. where ground water enters the sedimentary basins; and fluid-mineral chemical reactions within aquifers and aquitards.

In the Upper Santa Cruz River watershed, only two of these factors are known with any degree of confidence. These are:

1. the chemistry of the larger surface water courses entering the sedimentary basins, and;
2. the chemistry of potable water extracted from the basins.

In the Tucson basin water, withdrawal has caused a lowering of the water table and thus the development of a substantial vadose zone. The quality of ground water percolating downward into the aquifers will be improved during passage though the vadose zone. However, the chemistry of ground water added laterally below the vadose zone will remain unaltered.

More Environmental Projects in the Mineral Resources Program at: http://minerals.cr.usgs.gov/team/amlstudies.html

More Geochemistry

For more information contact: Pat Shanks (isotope geochemistry) or Rich Wanty (trace element geochemistry).