the problem—this site was recommended by the secretary of the Department of Energy (DoE) and approved for detailed study. DoE subsequently prepared a site characterization plan, in accordance with the requirements of the Nuclear Waste Policy Act, to summarize existing information about geological conditions at the site, to describe the conceptual design, and to present plans for acquiring the necessary geological information. To date, detailed geological, seismic, and regional geophysical mapping has been accomplished, and 182 boreholes and 23 exploration trenches have been excavated within a radius of 9.5 km around the prospective site. A 1992 NRC study, Ground Water at Yucca Mountain, How High Can It Rise?, assessed the long-term outlook for this site and is a good example of the need to engage scientists with a broad range of geological and geophysical expertise. The report found no likelihood of a postulated environmental risk from future tectonic and hydrologic changes.

Contaminated Water, Air Pollution, and Acid Rain

Complex changes in water chemistry take place throughout the hydrologic cycle, from the time rain falls on the Earth to the time when it flows into the ocean. Some water moves quickly to streams, providing flood flows and moving large loads of sediment. Some moves through the soil and the shallow saturated groundwater system, sustaining the base flow of streams. Some water moves deeper into the crust, circulating for long periods. The deeper water becomes a major transportation system for mass and heat in the Earth and influences the complex chemical changes taking place within the crust.

Since the first irrigation projects, humans have affected the flow and chemistry of water as it circulated along the surface. During this century, such influence has greatly increased. Dams have been built on many of the streams of the world. Water quantity and quality are now seriously at risk, and societally generated contamination may be the most serious of water problems. The consequences of agricultural and resource-extraction practices threaten humans by contaminating the water supply and posing a threat to the natural environment by changing ecosystems.

Wastes also are released into the atmosphere, increasing particulate matter and adding greenhouse gases that prevent heat from escaping the atmosphere. Modifying the greenhouse effect has the potential to seriously disturb climate, which means a change in rainfall. Much of hydrology, as it has been practiced, and many statistics are based on the assumption that the hydrologic cycle does not change within spans of decades to centuries, in contrast with geological time. A precipitously changing climate alters the basic assumption, making a large proportion of analyses suspect. A vacillating hydrologic cycle makes deeper scientific understanding of the basic phenomena much more important. Hydrologic instability, with more intense extremes such as floods and droughts, appears to be more likely during a period of change. Predicting future climate and the associated rainfall and runoff is a great challenge to science.

The atmosphere is composed principally of nitrogen and oxygen, with minor amounts of carbon dioxide and water vapor and traces of many other gaseous materials. The airborne waste products of civilization include gases such as sulfur dioxide and sulfur trioxide, nitrogen oxides, carbon dioxide, and ozone. Rainfall dissolves these gases, from the atmosphere, transporting them to the ocean. The sulfur and nitrogen species and the carbon dioxide are hydrolyzed to yield weakly acidic solutions that may still be substantially more acidic than ordinary surface waters. Eventually, these solutions will increase the acidity of the surface waters, especially in those lakes and rivers where the water acidity is poorly buffered by rocks and soils. The enhanced solubility of many major and trace constituents of ordinary rocks in acid waters releases unfamiliar elements and compounds into the water. These increasing amounts of rock-derived chemical constituents can threaten biological systems as well as the increasing acidity of the water.

Much sulfurous gas comes from electricity-generating plants that burn sulfur-bearing coal and from smelters processing sulfide ores. Coal contains sulfur both as sulfur-bearing organic compounds and as iron sulfides. Although it is possible to remove some of the iron sulfide prior to burning, the organic sulfur is an integral part of the coal. Corrective approaches for coal have relied on using the lowest-sulfur coals and on scrubbing the effluent gases. Corrections for smelter emissions have principally taken the form of prohibitory regulations, which have put the smelters out of business and led to transferral of the mining-smelting industry to other countries. Limited studies of alternatives to thermal smelting—such as dissolution mining or solvent extraction of finely crushed ores—have not yet yielded widely acceptable technologies, but they offer significant scientific and technical challenges as well as long-term economic, environmental, and national security incentives.

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