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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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