to minimize or mitigate the undesirable consequences of human activities.

Perhaps the most fundamental underlying scientific theme for resource and environmental issues is the behavior of fluids in the soils, sediments, and rocks that constitute Earth’s uppermost crust. Water is the most common fluid of concern. Water in the ground generally comes from water at the surface, and the behavior of surface water and ultimately, precipitation, is an important aspect of environmental geology. In addition to water, various gases, organic liquids, and both gaseous and liquid carbon dioxide are important geological fluids. Mixtures of fluids—immiscible liquids like water and hydrocarbons, gas-liquid mixtures (two-phase fluids), and mixtures of a gas phase plus two immiscible liquids (multiphase fluids)—can be particularly challenging materials to understand in natural underground settings. Some of the scientific issues associated with fluids in shallow crustal environments also apply to deeper-Earth processes, and many of them also overlap with issues of earthquake prediction, climate prediction, the evolution of continents, the behavior of volcanoes, the formation of ore deposits, and the properties of Earth materials.

Since water, as the best example, is a commodity of critical importance to humankind, and also an agent for so many important geological, chemical, physical, and biological processes, there is a continuing desire to better understand how it works—especially underground where we cannot see it directly, but also as an agent of erosion and sediment transport at the surface. Ultimately, it is desirable to be able to manipulate water and other fluids in the environment. Such manipulation has been done for millennia in the case of surface water and is also done in the subsurface, although still with modest efficiency, in petroleum extraction and subsurface remediation of contaminants. To improve our ability to control, or at least predict, the effects of subsurface fluids, and to better manage surface water and sediment, will require major advances in our understanding of how fluids transport materials and modify their environment by chemical and physical interactions.

How Do Fluids Flow in Geological Media?

The flow of fluids through soils and rocks is easily understood in the abstract but continues to present roadblocks to understanding in natural settings. We have a general understanding of how fluid moves through a granular solid (i.e., the mineral grains or rock fragments are packed together but separated by pore space), based on models of fluid flow through a medium of homogeneous grain size and pore structure. Natural materials are not homogeneous, however, especially on the 100- to 100,000-m scale of groundwater systems, but even on scales of microns to meters. The rate of flow through porous materials varies exponentially with porosity and grain size, so predicting the spatial pattern of fluid flow even in a relatively simple, but heterogeneous, porous material can be difficult. At the pore scale of individual mineral grains, surface tension also affects flow; the liquid phase present at the boundaries of multiple grains has different properties than a bulk liquid and can effectively be held in place by capillary forces. At larger scales, Earth’s subsurface is composed of a variety of rock types, with greatly varying porosity and permeability, that are further complicated by faults and fractures.

When a rock medium is not granular but crystalline, the pore space is typically not visible to the naked eye and its distribution within the rock is exceedingly variable. Most of the pore space in crystalline rock is attributable to fractures, so the flow of fluid can be almost entirely limited to a few fractures that happen to be connected. Many geological media, especially volcanic rocks, are both porous and fractured. In these cases much of the fluid flow may be confined to fractures, but there is also chemical and heat exchange by diffusion (and slow flow) between the fractures and the porous rock between the fractures.

Given this battery of uncertainties, geologists have developed a number of strategies to predict fluid flow patterns in rocks, including some that are largely empirical. A more promising approach is to treat the structural variability with statistical methods, based on observations of analogous rocks that can be studied at the surface. But the flow of fluids through rocks underground remains exceptionally difficult to predict. Generally the best results are mere estimates, and even these are obtainable only from direct observations, usually by drilling into the subsurface and making measurements of returned fluids and rock cores. Still, there is cause for optimism because increasingly powerful measuring tools are being developed—using approaches such as isotope geochemistry and geophysics—and more effec-

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