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Hydrogeology
William Cunningham, Norman G. Grannemann, and Paul M. Barlow

What is the state of the science of hydrogeology? An introspective paper by Schwartz and Ibaraki (Ground Water, v. 39, n. 4, p. 492-498) addressed this question in 2001 by using journal citations as a measure of the scientific impact of research. They hypothesized that current ground-water research is inefficient, with much produced for little scientific gain, and that there is cause for concern about the future of our science. The article challenged the community of ground-water scientists to develop innovative research ideas and technologies to "propel the field upward." Schwartz and Ibaraki's hypothesis has generated much discussion among ground-water scientists. Historically, hydrogeologic advances have been evolutionary rather than innovative. Whether evolutionary or innovative, advances in the year 2001-particularly in the areas of the freshwater/saltwater interface, ground-water recharge, and hydrologic landscapes-should help to improve our overall understanding of hydrogeologic processes.

Freshwater/saltwater interface

In ocean coastal zones, one of the most difficult aquifer management challenges is saltwater intrusion of freshwater aquifers. In April 2001, the First International Conference on Saltwater Intrusion and Coastal Aquifers was held in Essaouira, Morocco, to review state-of-the-art knowledge and exchange technological advancements for monitoring, modeling, and managing saltwater intrusion (www.olemiss.edu/sciencenet/saltnet and Transport in Porous Media Journal, v. 43, n. 1). The conference highlighted recent advancements that have been made in the development and application of three-dimensional ground-water flow and solute-transport models such as SEAWAT, SUTRA3D, and MOCDENS3D for simulation of saltwater intrusion into coastal aquifers and ground-water discharge to coastal waters.

The role of ground-water discharge in delivering freshwater flow and dissolved constituents to the coastal zone is a relatively new area that has received much interest recently. Methods for determining ground-water discharge to coastal waters include modeling, direct physical measurements, and geochemical tracers (Burnett and others, Eos, v. 83, n. 11, p. 117-123). New applications of geochemical tracers were made by Top and others (Journal of Coastal Research, v. 17, n. 4, p. 859-868,) using dissolved helium and radon anomalies to quantify ground-water input to Florida Bay waters. Swarzenski and others (Chemical Geology, v. 179, p. 187-202) used radium and strontium isotopes, methane, and radon gas to better understand the sources of freshwater, ground-water flowpaths, and ground-water travel times to the submarine Crescent Beach Spring off the northeast coast of Florida.

Although much of the research on ground-water discharge to coastal waters has been concerned with the delivery of nitrogen to coastal ecosystems, Basu and others (Science, v. 293, p. 1470-1473) demonstrated that ground-water discharge from the Ganges-Brahmaputra Delta in the Bengal Basin may be a potentially significant source of strontium to the oceans.

Recharge rate estimates

After many years of being overshadowed by water-quality concerns, the issue of ground-water availability has emerged as one of the more critical elements of water-resources studies. Recharge rates are an important factor in assessment of ground-water resources. Scanlon and others (Hydrogeology Journal, v. 10, n. 1, p. 18-39) presented an excellent summary of the techniques available to quantify recharge, listed the spatial and temporal scales for each of the techniques, and provided examples of their use. These examples cited the uncertainty associated with individual methods and emphasized the need to apply multiple techniques to increase the reliability of recharge estimates. Additional work has shown that types and abundance of vegetation are linked to rates of recharge and zones of ground-water discharge, thus relating riparian zone issues to ground-water systems.

Climatic variations also are a complicating factor for assessing rates of ground-water recharge. The relation between climate and ground-water resources is most widely studied in arid regions, but ground-water-level monitoring of drought conditions in humid areas is becoming more commonplace as demand for ground-water resources increases (Taylor and Alley, U.S. Geological Survey Circular 1217). The U. S. Geological Survey, for instance, is developing a climate response ground-water level network to help address a national need for ground-water-level data, particularly in times of drought. Work is also under way to more comprehensively evaluate the effects of human activities on rates of ground-water recharge (Lerner, Hydrogeology Journal, v. 10, n. 1, p. 143-152).

Hydrologic landscapes

The importance of assessing ground water and surface water as a single resource has been clearly demonstrated by many authors (for example, Winter and colleagues, U.S. Geological Survey Circular 113). Most evaluations of ground-water systems remain centered on aquifers, however, while evaluations of surface-water systems remain centered on watersheds. A search for a framework that considers the complete hydrologic system-the interaction of ground water and surface water and how these waters are affected by climate-led to the concept of hydrologic landscapes (Winter, AWRA Journal, v. 37, n. 2, p. 335-349). By describing the landscape in terms of land-surface slope, hydraulic properties of rocks and soils, and the balance between precipitation and evapotranspiration, hydrologic landscapes can be conceptualized in a uniform way. This framework can then be the foundation for design of research and data networks, syntheses of information from local to national scales, and comparisons of process-oriented research across small study units in a variety of settings.

Water-resource issues in the United States and elsewhere are unlikely to diminish in the coming decades; in fact, hydrologic research is likely to be more important in the future than in the present. Advances in hydrogeologic and related research in 2001 may propel our field forward to meet these societal challenges.

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Cunningham is assistant chief of the Office of Ground Water at the U.S. Geological Survey (USGS). E-mail. Grannemann is coordinator of the Ground Water Resources Program at USGS, and Barlow is a hydrologist in the USGS Office of Ground Water.

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