Although civilization's contribution to atmospheric acidity is a serious problem, our role can seem puny when compared with that of nature. Even modest volcanic eruptions can inject vast amounts of sulfur into the atmosphere very quickly, and they may remain airborne for several years. The sulfur disperses globally as sulfur dioxide in stratospheric aerosols and returns to the surface gradually. Scientific experience in observing such events has lately taken a leap forward with observations of the plume from the 1991 volcanic eruption of Mount Pinatubo in the Philippines. Understanding the global consequences of a major eruption is likely to be important for volcanology, for understanding waste emission, and for global change studies.

Atmospheric emissions and their consequences have been reviewed repeatedly by the NRC over the past decade, and with the passage of the Clean Air Act in 1990 it became clear that the national perspective on these matters was in the process of change. Hydrologic research was the topic of a recent full-scale assessment by the NRC. Solid-earth scientists are closely involved in hydrology, and their research dominates both surface drainage and underground water. Underground hydrology plays an important role because of the need to ensure the isolation of radioactive waste and untreatable toxic fluids, both of which are considered for deep disposal only. In the past, shallow groundwater was often studied as a separate resource, assumed to be isolated from deeper waters. Modern theory recognizes the importance of shallow and deeply circulating groundwater in the hydrologic cycle and thus in environmental pollution scenarios. For example, fluid contaminants introduced into a deep formation by a waste-injection well might someday reach the biosphere through shallow aquifers, unless precautions are taken in the design of such facilities and enough research is invested to understand the hydrogeological setting.

Much current research in groundwater hydrology and environmental engineering is centered on the study of contaminant transport in shallow aquifers, with respect to both fluid flow and chemical reactions. Significant progress has been made with computational and laboratory simulation of transport processes, although the problems of accurately predicting contaminant transport over human or geological time scales remain largely unsolved. In this light, studies of hydrothermal processes such as sediment diagenesis and ore mineralization may provide insight and natural analogs for verifying models of chemical transport.

There are problems of cleanup and prevention of contamination in both surface water and groundwater. These involve the transport of chemical constituents by water, often with simultaneous chemical reactions. It is essential that more robust and accurate analytical models be developed for predicting the chemical fate and transport of contaminants in groundwater. Certain contaminants can be chemically remedied by means of absorption or caging, and many of the chemical reactions are controlled by microorganisms. Understanding the kinetics of complex chemical reactions in a heterogeneous medium populated by microorganisms poses an exciting, though daunting, scientific challenge. That challenge must be met.

GLOBAL CHANGE

Recorded history affords but a brief glimpse of environmental change, one that fails to shed light on events and conditions that may confront us in the decades ahead. Global climates may soon be warmer than at any time during the past 1 million years, and sea level may stand higher than at any time during the past 100,000 years. Within decades, we may find ourselves in the midst of a biotic crisis that rivals the most severe mass extinctions of the past 600 million years. Nature is a vast laboratory that we can never manipulate or duplicate artificially on the scale necessary to test theories of global change. In developing plans for the future, we must therefore rely heavily on observations of nature itself.

Change is universal throughout the earth system, and has been since the origin of the Earth, but the term ''global change" has come to be used much more narrowly to refer to human-induced changes affecting the atmosphere, hydrosphere, and biosphere. These changes are beginning to be recognized as likely to modify our environment in unprecedented ways within the next century. While many of the processes causing the changes are not unusual, it is an accelerated rate of change that human activities have introduced into the earth system. That accelerated rate of change may prevent normal adaptation mechanisms in the atmosphere, hydrosphere, lithosphere, and biosphere from working without major consequences—at least to humans. Public interest has focused mainly on global climatic change, but the scientific community recognizes that expected changes are much broader, extending to changes in such phenomena as sea level, groundwater quality, pollution, and biodiversity. Exactly what phenomena are included as elements of global change varies, depending on the breadth of the



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