Groundwater represents the terrestrial subsurface component of the hydrologic cycle. As such, groundwater is generally in motion, moving from elevated areas of recharge to lower areas of discharge. Groundwater usually moves in accordance with Darcy’s law (Dalmont, Paris: Les Fontaines Publiques de la Ville de Dijon, 1856). Groundwater residence times can be under a day in small upland catchments to over a million years in subcontinental-sized desert basins. The broadest definition of groundwater includes water in the unsaturated zone, considered briefly here. Water chemically bound to minerals, as in gypsum (CaSO4 • 2H2O) or hydrated clays, cannot flow in response to gradients in total hydraulic head (pressure head plus elevation head); such water is thus usually excluded from consideration as groundwater. In 1940, M. King Hubbert showed Darcy’s law to be a special case of thermodynamically based potential field equations governing fluid motion, thereby establishing groundwater hydraulics as a rigorous engineering science (Journal of Geology 48, pp. 785–944). The development of computer-enabled numerical methods for solving the field equations with real-world approximating geometries and boundary conditions in the mid-1960s ushered in the era of digital groundwater modeling. An estimated 30 percent of global fresh water is groundwater, compared to 0.3 percent that is surface water, 0.04 percent atmospheric water, and 70 percent that exists as ice, including permafrost (Shiklomanov and Rodda 2004, cited under Groundwater Occurrence). Groundwater thus constitutes the vast majority—over 98 percent—of the unfrozen fresh-water resources of the planet, excluding surface-water reservoirs. Environmental dimensions of groundwater are equally large, receiving attention on multiple disciplinary fronts. Riparian, streambed, and spring-pool habitats can be sensitively dependent on the amount and quality of groundwater inputs that modulate temperature and solutes, including nutrients and dissolved oxygen. Groundwater withdrawals can negatively impact riparian habitats by depriving ecosystems of adequate fresh water and fragmenting communities when streams go dry. Biochemical reactions in shallow groundwater can remove anthropogenically elevated nitrogen compounds and reduce—but only to a point—the greening of waterways and shorelines with periphyton and harmful algal blooms. Groundwater extraction for beneficial use is increasingly limited by water-quality constraints imposed by naturally occurring and introduced substances. Overdrafting can cause land-surface subsidence, damaging buildings and roads and disrupting canals, sewers, and other gravity-flow conveyances. Increases in groundwater levels can cause soil salinization in dry regions and erosive sapping and flooding in wet regions. Coastal saltwater intrusion, groundwater flooding, salinization associated with groundwater-irrigated agriculture, induced seismicity from injected wastes, and the detrimental impacts of groundwater depletion are among the major environmental challenges of our time.
Introductory textbooks on hydrology (Hornberger, et al. 2014) and groundwater (Freeze and Cherry 1979; Anderson, et al. 2015; and Todd and Mays 2005, all cited under Textbooks) provide general overviews of the topic. Hydrogeology (also called “geohydrology” in early literature) considers groundwater in the context of geologic settings and processes. Ingebritsen, et al. 2006 presents a topically broad treatment of groundwater from the perspective of shallow-to-deepest crustal fluids and shows that understanding groundwater is essential for understanding ore formation, plate tectonics, seismicity, volcanoes, and other environmentally relevant phenomena. The mathematics of groundwater flow, solute dispersion, and geologic heterogeneity has long been the focus of research; Bear 1979 provides an early entry point and still-useful reference; Yeh, et al. 2015 (cited under Textbooks) an up-to-date one. An influential overview of challenges and opportunities in the hydrologic sciences argues for groundwater to be studied as part of a hydrosphere that interacts with the biosphere and atmosphere, requiring development of a “hydrospheric science” on par with atmospheric and biologic sciences (National Research Council 1991). This overview and a subsequent 2012 report floated the idea of and then later expounded upon the need for large-scale observational hydrologic networks such as GEWEX and CZO (Lin, et al. 2011), which are deepening in scope to incorporate groundwater flow systems as research moves forward. The field of ecohydrology considers groundwater in the context of community dynamics and landscape evolution; Rodríguez-Iturbe and Porporato 2004 provides an early overview of this interdisciplinary field in which groundwater (especially as soil moisture) plays a key role in land-surface–atmosphere interactions. The issue of model complexity is central to progress in environmental groundwater science; case studies by Hunt, et al. 2007 and Clement 2011 provide opposing perspectives (see also Doherty, et al. 2011, cited under Groundwater Modeling and Management: Groundwater Modeling).
Bear, Jacob. 1979. Hydraulics of groundwater. New York: McGraw-Hill.
Mathematically rigorous and comprehensive treatment of groundwater flow, representing the distillation of core material from his 1972 treatise Transport of fluids in porous media (New York: American Elsevier). Includes chapters on saltwater–fresh-water interfaces, hydrodynamic dispersion, flow in the unsaturated zone, and linear programming in aquifer management.
Clement, T. Prabhakar. 2011. Complexities in hindcasting models: When should we say enough is enough? 2010. Groundwater 49:620–629.
An instructive contaminated-groundwater case study that provides a retrospective overview of the debate on the limitations of numerical modeling.
Hornberger, G. M., P. L. Wiberg, J. P. Raffensperger, and P. D’Odorico. 2014. Elements of physical hydrology. 2d ed. Baltimore: John Hopkins Univ. Press.
Well-rounded clear and compact introductory undergraduate-level textbook that introduces groundwater in the broad context of hydrologic science.
Hunt, Randall J., John Doherty, and Matthew J. Tonkin. 2007. Are models too simple? Arguments for increased parameterization. Groundwater 45:254–262
A contrasting contaminated-groundwater study that provides additional overview on appropriate complexity in numerical modeling.
Ingebritsen, Steven E., Ward E. Sanford, and Christopher E. Neuzil. 2006. Groundwater in geologic processes. 2d ed. Cambridge, UK: Cambridge Univ. Press.
Thorough overview of groundwater in the context of geologic processes, including ore body formation; hydrocarbon transformation, transport, and accumulation; sediment diagenesis; metamorphism; suboceanic groundwater; geothermal systems; and poroelastic and inelastic deformation.
Lin, Henry, Jan W. Hopmans, and Daniel D. Richter. 2011. Interdisciplinary sciences in a global network of critical zone observatories. In Special issue: Interdisciplinary sciences in critical zone observatories (CZOs). Vadose Zone Journal 10.3: 781–785.
Introduction to a special issue dedicated to results from Critical Zone Observatory studies, with attention to weathering inputs to groundwater.
National Research Council. 1991. Opportunities in the hydrologic sciences. Washington, DC: National Academy Press.
Visionary overview of groundwater within the context of hydrology as transitioning from a problem-solving engineering discipline to a question-driven scientific discipline.
Rodríguez-Iturbe, Ignacio, and Amilcare Porporato. 2004. Ecohydrology of water-controlled ecosystems: Soil moisture and plant dynamics. Cambridge, UK: Cambridge Univ. Press.
Thorough overview of groundwater in the context of soil moisture as a key feedback variable on vegetation patterns.
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