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Hydrogeology in North America: past and future

T. N. Narasimhan

Abstract This paper is a retrospective on the evolution of hydrogeology in North America over the past two cen-turies, and a brief speculation of its future. The history of hydrogeology is marked by developments in many dif-ferent fields such as groundwater hydrology, soil me-chanics, soil science, economic geology, petroleum en-gineering, structural geology, geochemistry, geophysics,marine geology, and more recently, ecology. The field has

been enriched by the contributions of distinguished re-searchers from all these fields. At present, hydrogeologyis in transition from a state of discovering new resourcesand exploiting them efficiently for maximum benefit, toone of judicious management of finite, interconnectedresources that are vital for the sustenance of humans andother living things. The future of hydrogeology is likely tobe dictated by the subtle balance with which the hydro-logical, erosional, and nutritional cycles function, and thedecision of a technological society to either adapt to theconstraints imposed by the balance, or to continue toexploit hydrogeological systems for maximum benefit.Although there is now a trend towards ecological and

environmental awareness, human attitudes could changeshould large parts of the populated world be subjected tothe stresses of droughts that last for many decades.

Rsum Cet article est une rtrospective de l’volutionde l’hydrogologie en Amrique du Nord sur les deuxderniers sicles, et une brve valuation de son futur.L’histoire de l’hydrogologie est marque par le dve-loppement de plusieurs techniques de terrain telles, l’hy-drologie des eaux souterraines, la mcanique des sols, lessciences du sol, la gologie conomique, l’ ingnierieptrolire, la gologie structurale, la gochimie, la go-

physique, la gologie marine et plus rcemment l’colo-gie. La science a t enrichie par la contribution de plu-sieurs chercheurs distingus, provenant de toutes cesbranches. A prsent, l’hydrogologie est la transitionentre la volont de dcouvrir de nouvelles ressources et l’exploitation la plus bnfique au possible, et un mana-gement judicieux des ressources finies, interconnectes,qui sont vitales pour l’ approvisionnement des hommes et

autres formes de vie. Le futur de l’ hydrogologie seradict par la balance subtile dans laquelle intervient lescycles de l’hydrologie, de l’rosion, de la nutrition, et ladcision d’une socit technologique qui s’adapterait auxcontraintes de la balance, ou qui continuerait d’exploiterles systmes hydrologiques pour un bnfice maximum.Par ailleurs il y a une nette tendance inclure les aspectscologiques, les aspects environnementaux, et les chan-gements humains qui pourraient Þtre influencs par lesmodifications hydrogologiques observes depuis unedizaine d’annes.

Resumen Este articulo es una retrospectiva sobre la

evolucin de la hidrogeologa en Norte Amrica en lospasados dos siglos, y una breve especulacin de su futuro.La historia de la hidrogeologa est marcada por desarro-llos en muchos campos diferentes tal como hidrologa deaguas subterrneas, mecnica de suelos, ciencia del suelo,geologa econmica, ingeniera del petrleo, geologa es-tructural, geoqumica, geofsica, geologa marina, y msrecientemente, ecologa. El campo se ha enriquecido porlas contribuciones de investigadores distinguidos en todosesos campos. Actualmente, la hidrogeologa se encuentraen transicin de un estado de descubrir nuevos recursos yexplotarlos eficientemente para un beneficio mximo, a unestado de gestin juiciosa de recursos finitos, interconec-tados, que son vitales para el sustento de humanos y otrascosas vivientes. El futuro de la hidrogeologa posiblementeest determinado por el balance sutil con el cual funcionanlos ciclos nutricionales, erosionales e hidrolgicos, y ladecisin de una sociedad tecnolgica para ya sea adaptarsea las restricciones impuestas por el balance o para conti-nuar con la explotacin de los sistemas hidrogeolgicospara un beneficio mximo. Aunque existe actualmente unatendencia hacia la conciencia ambiental y ecolgica, lasactitudes humanas podran cambiar en caso de que grandespartes del mundo poblado estn sujetas a las presiones desequas que duran por muchas dcadas.

Received: 1 March 2004 / Accepted: 25 November 2004Published online: 25 February 2005

Springer-Verlag 2005

T. N. Narasimhan ())Department of Materials Science and Engineering,Department of Environmental Science, Policy and Management,University of California,210 Hearst Memorial Mining Building, Berkeley, CA,94720-1760, USAe-mail: [email protected]

Hydrogeol J (2005) 13:7–24 DOI 10.1007/s10040-004-0422-5



Keywords General hydrogeology · History of hydrogeology · Future of hydrogeology

“The waters which are from heaven, and which flow after being dug, and even those that spring by themselves, thebright pure waters which lead to the sea, may those divinewaters protect me here”

Hymn from the Rg Veda, VII.49.2

Introduction

Lucas (1877) defined hydrogeology as “ Hydrogeology ...takes up the history of rainwater from the time it leavesthe domain of the meteorologist, and investigates theconditions under which it exists in passing through thevarious rock types it percolates after leaving the surface”(Davis 1989; Mather 2001). Essentially, hydrogeologydeals with geological processes influenced by water. Theearly history of hydrogeology before the 19th century was

much influenced by events in Europe. From the late 19thcentury, the United States witnessed hydrogeologicaldevelopments that rivaled those of Europe. The presentwork is restricted to these developments. No effort hasbeen made to survey the substantial hydrogeologicalcontributions from Europe, Russia, and elsewhere overthe past century and a half. Table 1 summarizes some of the important events in the history of hydrogeology.

This work is unabashedly a personal statement, basedon the author’s experiences and his interactions with peersfrom groundwater hydrology, soil mechanics, petroleumengineering, geomorphology, economic geology, soilscience, geochemistry, agronomy, civil engineering,

structural geology, geophysics, marine geology, history,and law. It is unwise to believe that a single person canhave an in-depth understanding of all these fields. Im-perfections of thought and detail are inevitable in thisattempt.

Early history

Meinzer (1934) and Hall (1954) gave informative surveysof hydrogeology prior to the 20th century. Until well intothe 16th century, it was believed that springs originatedfrom subterranean evaporation and condensation of seawater, and that rain water was not sufficient to sustain theobserved flows. Bernard Palissy (1509–1589) of Francewas the first to argue that springs were sustained byrainfall. During the late 17th century, physicists PierrePerrault (1608–1680), and Edm Marriott (1620–1684)pioneered quantitative hydrogeology. Based on rainfallmeasurements, Perrault (1674) estimated total precipita-tion over the Seine river basin and concluded that rainfallwas adequate to maintain observed river discharge.Marriott (1686) hypothesized that rainwater infiltratedvertically down until it reached impermeable rock, thenmoved laterally to replenish aquifers, springs and wells.

In the early 19th century, the French pioneered well-drilling technology, encouraged by spectacular successeswith artesian wells in the Paris Basin. The French ScienceAcademy showed keen interest in data from the artesianwells. One of the best studied wells at this time was atGrenelle in the greater Paris area (Davis 1999). Do-minique-Francois Jean Arago (1786–1853), noted physi-cist, studied temperatures of deep mines and water tem-

peratures from artesian wells, and estimated the geother-mal gradient in the Paris area to be 29.4 m/C (53.6 ft./ F). In 1835, he used Grenelle well data and calculatedthat a 500-m deep well would provide water at a tem-perature that would be 18.8C above the ambient tem-perature at the land surface and well-suited for space-heating during winter.

Alexander von Humboldt (1769–1859) of Germany,who led a 5-year scientific expedition to South America,observed in a cave in north-eastern Venezuela that thewater from a spring was colder than the ambient meanannual temperature. He concluded that the spring waterwas recharging at a higher elevation, and that the water

had not yet equilibrated with the host rocks (Davis 1999).In Venezuela, he also measured temperatures of many hotsprings with near-boiling temperatures. Noting that thenearest volcanic manifestations were far away, Humboldtconcluded that the high temperatures were due to deepcirculation of groundwater. Using European estimates of geothermal gradient, Humboldt (1844) estimated thesource depth to be 2,176 m.

The modern conception of hydrogeological systems asconstituting the lower part of the hydrological cycle wasestablished by Marriott, Arago, and Humboldt. Theyrecognized deep infiltration of groundwater, and its returnto land surface because of permeability variations, and

geothermal heat.

Mathematization of hydrogeology

The 19th century witnessed the birth of quantitative hy-drogeology. Fourier’s (1822) heat conduction model wasextended to electricity by Ohm, to flow in capillaries byPoiseuille and Hagen, and to molecular diffusion by Fick.In this atmosphere, Henri Darcy (1803–1858), found thatthe flux of water filtering through a sand column wasdirectly proportional to the gradient of hydraulic head,now referred to as Darcy’s Law (Darcy 1856). Darcy wasthe first to extend Fourier’s law to flow of water in naturalporous materials, and to explicitly incorporate gravity indefining hydraulic head. Much credit for bringing thesteady flow of groundwater within potential theory goesto Jules-Juvenal Dupuit (1804–1866), who idealized apermeable medium such as sand to be a collection of small diameter tubes, and showed that Darcy’s Law was aspecial case of de Prony’s equation, with inertial effectsneglected (Dupuit 1857). In a work of great insight, Du-puit (1863) portrayed an artesian aquifer within theframework of a groundwater basin, confined by claylayers and connected to the water table at higher eleva-

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Table 1 Some events of his-torical interest in hydrogeology

1580 Palissy argues that springs are fed by rain1674 Perrault estimates that rainfall is adequate to maintain stream flows in the Seine Basin1686 Marriott suggests that infiltrating rainwater moves laterally, sustains springs, aquifers,

and wells1822 Publication of Fourier’s Analytic Theory of Heat1828 Du Commun explains the statics of freshwater–salt water interface in a coastal aquifer in

New Jersey1836 Arago provides estimate of geothermal gradient and discusses implications to artesian well

at Grenelle in the Paris area1844 Humboldt presents field evidence from Venezuela to assert that springs are manifestations

of deep groundwater circulation1856 Darcy describes the law relating water flux to hydraulic gradient in a porous medium1857 Dupuit idealizes sand as a complex of capillary tubes and describes Darcy’s Law as a special

case of de Prony’s equation1863 Dupuit portrays an artesian basin and artesian wells in terms of hydraulic head profiles,

and solves the radial flow equation to steady flow in a aquifer with a free surface1877 Lucas defines hydrogeology1879 U.S. Geological Survey created on a recommendation by the National Academy of Sciences1896 Darton describes the regional flow system of the Dakota sandstone, including vertical leakage

through very low permeability shale layers1907 Invention of the deep-well turbine pump in California, inspired by irrigation demands1907 Buckingham defines a capillary potential for an unsaturated soil, and relates it to moisture

content and hydraulic conductivity1912 Lee prepares water budget for the Owens Valley groundwater basin, integrating surface water,

groundwater, evaporation, and consumptive needs of plants1922 Gardner invents the tensiometer to measure capillary potential, and presents moisture

characteristic curves measured for the first time1923 Terzaghi defines effective stress, and solves the one dimensional transient groundwaterflow equation to analyze consolidation of a clay column

1923 Meinzer’s monograph on groundwater and its occurrence in the United States. Groundwaterhydrology takes formal form, as part of the hydrological cycle

1926 First reported land subsidence at the Goose Creek oil field in Texas caused by subsurface fluid(oil) production

1928 Meinzer’s intuitive description of elastic deformation of aquifer material and expansionof water during transient water production

1931 Richards formally describes transient unsaturated flow as a non-linear partial differentialequation

1933 First reported occurrence of land subsidence due to groundwater withdrawal in San Jose,California

1935 Theis publishes the non-equilibrium formula for non-steady radial flow to a well1940 Jacob gives physical meaning to Theis’ storage coefficient in terms of the vertical

compressibility of the porous medium, the compressibility of water, and the porosity

of the aquifer1940 Hubbert defines a fluid potential for groundwater systems, describes nature of hydraulicconductivity, and suggests that the interface between freshwater and sea water will be inclinedunder dynamic conditions of groundwater flow

1941 Biot generalizes Terzaghi’s one-dimensional consolidation theory to three dimensions,incorporating coupling between fluid flow and matrix deformation through effectivestress

1944 Introduction of Piper diagram for interpretation of groundwater geochemistry1946 Jacob opens up groundwater hydraulics to leaky multiple aquifer systems1952 First successful attempt to solve the Richards equation for transient unsaturated flow1953 Hubbert extends theory of groundwater motion to oil–water contact in petroleum reservoirs

and significantly influences oil exploration1954 Scheidegger applies Brownian Motion idea to formulate the advection–dispersion equation

and defines the parameter dispersivity1954 Skempton defines the coefficients governing undrained response of water-saturated soils1955 Chebotarev’s notion of metamorphism of natural waters along groundwater flow path

1956 Bullard introduces heat flow probe for measurement of heat flux from ocean floor1957 Hubbert and Willis analyze hydraulic fracturing, and launch a new technique for measurementof in situ rock stresses

1959 Hubbert and Rubey propose a model for overthrust faulting drawing upon Terzaghi’s effectivestress

1959 Hem and Cropper use the Eh–pH diagram for ferrous–ferric iron stability fields1960 Garrels’ monograph on mineral equilibria inspires a new generation of geochemical models1960 Clough introduces the finite element method1960 Bear introduces the dispersion tensor for macroscopic description of velocity-dependent

spreading of solutes in porous media1960 Barenblatt, and Warren and Root propose the double porosity model for naturally fractured

reservoirs1960 Back introduces the concept of hydrogeochemical facies in groundwater systems1962 Identification of interbasin groundwater movement in the Great Basin of Nevada

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tions. Dupuit also pioneered mathematical analysis of steady flow to wells in confined and unconfined aquifers.

The mathematical descriptions of Darcy and Dupuitinspired investigations of seepage through soils, engi-neering geology, and water supply. Philipp Forchheimer(1852–1933) of Austria perfected the application of flownets to analyze steady state seepage in engineered earthsystems. In Germany, Adolf Thiem and his son GntherThiem introduced the use of tracer tests and well tests todesign water supply systems.

Birth of hydrogeology in the United States

During early 19th century, natural springs and spas werepopular in the United States for their medicinal benefits.The earliest chemical analyses of waters from springswere reported by physicians as far back as 1827 (Davisand Davis 1997). In California, the discovery of largeborax crystals in the muds of Borax Lake, and the dis-covery of travertine deposits by William Blake in theSalton depression of the Imperial valley during the early1850s were attributed to geochemical mixing of waters of volcanic origin with waters of arid inland lakes. Another

insightful early American contribution was that of DuCommun (1828). In 1824, an artesian well had beendrilled at New Brunswick, New Jersey by America’s firstwell-driller Levi Disbrow (Carlston 1963). By 1827 ob-servations from this 67-m (220-foot) well showed that thewater level rose from 2.4 to 4.3 m (8–14 feet) above thesurface of the local stream, and that the well dischargerose and fell exactly and continually with the ocean tides.Joseph Du Commun, a French instructor of the WestPoint Military Academy, explained the observed phe-nomenon with the simple device of a U-tube in which twoimmiscible liquids (saltwater and freshwater) were inhydrostatic equilibrium. He extended the idea to the

coastal interface between freshwater and saltwater, andpresented quantitative estimates based upon the densitycontrasts of saltwater and freshwater. Du Commun’s es-timates preceded, by several decades, the commonly ac-cepted contributions of Ghyben (1888) and Herzberg(1901) on the static interface between freshwater andsaltwater.

Prior to 1870, science in the United States was largelyin the domain of the military. This situation changed in1879 when the Congress created the U.S. GeologicalSurvey on a recommendation by the National Academy of

Sciences. The nation’s boundaries were rapidly expandingto unexplored territories in the west, and there was anurgent need for classification of public lands, and theexamination of geological structure and mineral resourcesof the national domain. The motivation was to educateand assist the settlers in a new land towards a wise use of natural resources. With the inception of the U.S. Geo-logical Survey, significant amounts of public funds cameto be invested in the exploration of water resources forpublic benefit. Ever since, the U.S. Geological Survey hasplayed a major role in the growth of hydrogeology in theU.S. and in other parts of the world.

The early hydrogeological investigations were dis-posed towards recognizing hitherto unknown phenomena,and to understand the role of groundwater within thehydrological cycle. As early as 1888, Franklin H. Kinghad correlated diurnal water table fluctuations with dis-charge of water by vegetation (transpiration). King (1892)had also discovered that in an observation well situatednear a railroad, the water level rose whenever a train wentby, but fell again as soon as the train had passed. An earlytheoretician in groundwater hydrology was C.S. Slichter(1864–1946), a mathematician who used complex vari-able theory to analyze two-dimensional steady state flowsystems (Slichter 1899).

Grove K. Gilbert (1843–1918), and Israel Russell(1852–1906) ushered in paleohydrology with their studiesin the Great Basin. Gilbert (1890) deciphered the qua-ternary history of Lake Bonneville in Utah, shaped by acombination of glaciation, tectonics, and erosion. Inspiredby Gilbert’s work, Russell (1885) recognized a similarhistory of ancient Lake Lahontan in northwestern Nevadaduring the Quaternary.

A bulk of the systematic hydrogeological workthroughout the nation was carried out by a band of USGSgeologists. Notable among these were Nelson Darton(1865–1948), Walter Mendenhall (1871–1957), andCharles Lee (1883–1967). Darton (1896), and coworkersdeciphered the artesian aquifer system of the DakotaSandstone, recharged over the Black Hills on the west,and having areas of discharge 300–500 km to the east inSouth Dakota (Bredehoeft et al. 1982). Darton intuitivelyexplained the existing regional hydraulic gradient to theeast, and upward leakage through a few hundred meters of overlying clay formations. Mendenhall, who mappedgroundwater systems in the San Joaquin Valley and in theSan Bernadino Valley of California recognized that theaquifer systems of these valleys, directly connected to

Table 1 (continued) 1962 Meyboom describes a prairee profile describing regional groundwater movement inSaskatchewan, Canada

1962 Toth describes the anatomy of regional groundwater flow systems1963 Failure of the Baldwin Hills reservoir near Los Angeles due to differential land subsidence1964 Tyson and Weber use computer models for groundwater basin evaluation1966 First report of earthquake swarms induced by deep-well injection at the Rocky Mountain

Arsenal well near Denver, Colorado1971 Failure of the Lower San Fernando Dam near Los Angeles due to soil liquefaction during

a major earthquake1976 First experimental demonstrating that earthquakes could be triggered by water injection

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surface water, were not only large storage reservoirs, butwere also well suited for regulating flow to aid irrigationand other needs. Between 1908 and 1913, when the LosAngeles Aqueduct was built to bring Owens River waterto Los Angeles, Lee (1912) carried out a detailed waterbudget of the groundwater basin, considering surfacewater flows, evaporation, transpiration by plants, andgroundwater recharge and discharge.

The most noteworthy among these hydrogeologistswas Oscar Meinzer (1876–1948), who had the vision andthe scholarship to appreciate groundwater in its totality,including its connections to surface water and the plantkingdom. His elucidation of elastic deformation of aqui-fers, and its role in the transient response of aquifers areindicative of a field geologist with admirable intuition.His 1923 publication on the general principles of groundwater occurrence and movement, and his 1927publication on plants as groundwater indicators are re-markably relevant today as our awareness of ecosystemsincreases.

Soil physics and the vadose zone

By late 19th century, the U.S. Department of Agriculturewas investing substantial resources into improving irri-gated agriculture through scientific research. At that time,agricultural scientists were puzzled by an observationalparadox: soils of arid regions, at depths a little below thesurface, were found to be generally wetter and held theirmoisture for much longer periods than soils of humidareas in the dry season. Edgar Buckingham (1867–1940)of the Bureau of Soils, a physicist, helped clear thisparadox. Applying the concept of capillarity, and usingFourier’s model of thermal conduction, Buckingham(1907) defined a capillary water potential for a soil, anddemonstrated that water movement in soils is driven by acombination of gravity and the gradient of capillary po-tential, and that soil hydraulic conductivity was a functionof capillary potential.

However, Buckingham did not show how capillarypotential could be measured. He simply inferred it fromcapillary rise in hydrostatic columns. Willard Gardner(1883–1964) overcame this shortcoming by inventing theporous-cup “capillary potentiometer”, now known as thetensiometer (Gardner et al. 1922). For the first time, itbecame possible to quantitatively express soil moisturecontent and hydraulic conductivity in terms of a mea-surable quantity, the capillary potential. A decade later,Richards (1931) formally wrote down the non-linearpartial differential equation for three-dimensional mois-ture movement in soils.

Transient groundwater flow

It became obvious in the early 20th century that the flowof water in hydrogeological systems was time-dependent,and that the steady-state assumption of potential theory

was too limiting. Handling such transient systems re-quired a new hydraulic property, analogous to specificheat in the heat equation. The identification of such aproperty, the hydraulic capacitance, evolved over threedecades from parallel developments soil mechanics, soilphysics, and groundwater hydrology (Narasimhan 1986).

The first person to successfully apply the transient heatequation to fluid flow in a porous medium was Karl

Terzaghi (1883–1963), who solved the one-dimensionalclay consolidation problem of water expulsion caused byan imposed external load. In this work, Terzaghi (1923)elucidated analogies among temperature and pore pres-sure, heat content and water content, thermal conductivityand clay permeability, and specific heat and clay com-pressibility.

In the artesian basin of South Dakota, Meinzer (1928)found that the time taken for stabilizing artesian headsafter opening or capping a well was longer in the case of relatively more compressible fine-grained formations thanin more stiff coarse-grained materials. He attributed thisto the differences in the ability of the different formations

to take water into storage. He also reasoned that theability to take water into storage involved a combinationof change in porosity and expansion of water. Soonthereafter, Charles Theis (1900–1987), treated change instorage due to change in hydraulic head as analogous tospecific heat, and applied the heat equation to transientflow of water to a well. In this work, Theis (1935) washelped by mathematician Clarence Lubin of the Univer-sity of Cincinnati (Freeze 1985). In the illustrative ap-plication, Theis used data from a water-table aquifer, andestimated its specific yield. Jacob (1940) provided a rig-orous interpretation of Theis’ specific storage coefficientby combining vertical compressibility of the porous me-

dium and water compressibility. Jacob’s analysis showedthat Theis’ formulation was appropriate for confinedaquifers, rather than water table aquifers. Theis launchedgroundwater hydrology in the new direction of well hy-draulics, which dominated hydrogeology over the nexthalf a century. Jacob (1946) and Mahdi Hantush (1921–1984) played major roles in this by extending transientflow analysis to multiple aquifer systems.

Transient flow in unconfined aquifers demanded amore involved mathematical analysis because of time-dependent drainage from the unsaturated zone. To keepmathematics tractable, Boulton (1954) assumed thatdrainage from above the water table was exponentiallyrelated to time, leading to a mathematical form in whichhydraulic capacitance (specific yield) amounted to a sumof an instantaneous constant value and an exponentialdependence on time. Among other developments in well-hydraulics, mention may be made of the transient analysisof water level fluctuations in response to seismic waves ina well piercing confined aquifer by Cooper et al (1965).This analysis showed that under certain conditions, seis-mic signals can be amplified by wells. The first attemptsto solve the non-linear partial differential equation fortransient flow in unsaturated soils were made by Klute

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(1952) and Philip (1956), who pioneered modern theoryof infiltration in soils.

By the 1970s, the transient equation was being used intwo forms, one for fully saturated media, and the other forunsaturated media. In the former, the hydraulic capaci-tance (specific storage) included deformation propertiesof the porous matrix and water. In the latter, hydrauliccapacitance was entirely accounted for by the rate of

change of moisture content. A single transient flowequation uniting the saturated and unsaturated regionswas lacking because of the differences in the storagemechanisms of the two regimes. Drawing upon Bishopand Blight’s (1963) extension of Terzaghi’s effectivestress principle to unsaturated materials, Narasimhan(1975) defined a generalized hydraulic capacitance whichaccounted for compressibilities of the porous medium andwater, and desaturation of the soil, integrating transientflow in the saturated and the unsaturated domains.

Deformation of water-saturated porous media

From about the 1880s, civil engineers were studyingground settlement in relation to embankments, dams, andother structures built on water-saturated clays. However, acredible quantitative theory was lacking to explain thedeformation of water-saturated geological formations.During the 1920s, Terzaghi revolutionized the study of soil deformation through insightful experiments on water-saturated clays, and founded the field of soil mechanics.Central to his contribution was the hypothesis that volu-metric strain in a porous medium was a function of thedifference between the external stresses acting on the soiland the water pressure, defined as the effective stress(Terzaghi 1923). Going beyond volume change, Terzaghialso established that the frictional strength of water-filledshear planes in soil masses was dependent on effectivestress acting on the plane. This finding soon became thebasis for explaining landslides, earthquakes, and the for-mation of folded mountains such as the Alps and theHimalayas. Maurice Biot (1905–1985) extended Terza-ghi’s one-dimensional theory to general three dimensionaldeformation., accounting for the response of a water-saturated soil to loads imposed on the boundaries and thethree dimensional stress–strain responses associated withthe drainage of water (Biot 1941).

A water-saturated soil subjected to a change in externalstress (e.g., ocean tides, passing trains, earth tides), re-sponds in such a way that part, if not all, of the imposedstress is instantaneously borne by the water, resulting in astep-wise change in water pressure. Subsequently, theexcess stress is gradually transferred to the solid skeletonas the water drains with time, until the excess waterpressure is bled off. An analysis of undrained loading,combining the effects of shear stresses and normalstresses was presented by Skempton (1954).

Terzaghi’s theory, embellished by the contributions of Biot, Skempton, and Bishop, largely forms the modern

basis for quantitatively analyzing passive and catastrophicdeformation of hydrogeological systems.

The 1950s saw two papers spearheaded by MarionKing Hubbert (1903–1989). About this time, oil-welldrillers had discovered that reservoir rocks could befractured by injecting water under high pressure, and thatthe artificial fractures enhanced reservoir productivity. Byanalyzing stress distribution along the perimeter of the

well-bore during water injection, Hubbert and Willis(1957) showed that a system of two vertical fractures willinitiate and propagate perpendicular to the direction of theleast horizontal principal stress. In areas of regionalcompression where the least of the three principal stressesis vertical, the vertical fracture would rotate and becomehorizontal. In addition to placing the technology of res-ervoir stimulation on rational foundations, the Hubbert–Willis contribution opened up a new way of measuring insitu tectonic stresses in rocks.

The hydraulic fracturing work led Hubbert to useTerzaghi’s effective stress as a means of logically ex-plaining the puzzling paradox of the transport of huge

blocks of the earth’s crust over long distances along lowangle thrust faults in orogenic belts. Hubbert and Rubey(1959) hypothesized that the frictional strength of faultplanes was directly related to effective stress acting nor-mal to the fault plane, and that if fluid pressures withinthe plane became sufficiently high, the strength coulddramatically decrease, enabling the transport of enormousrock blocks over long distances. Today, the effectivestress concept, in conjunction with Mohr–Coulomb fail-ure criterion constitutes the standard model to explainsuch phenomena as landslides, natural and triggeredearthquakes, and orogeny.

The turn of the 20th century witnessed an explosion in

oil production in many parts of the United States. Inparallel, the growth of irrigated agriculture in Californiaspawned the invention, around 1907, of the deep-wellturbine pump (Freeman 1968), which enabled water to belifted from great depths. Within a decade, the effects of oil production and groundwater overdraft led to notice-able environmental effects. The first observed land sub-sidence due to subsurface fluid withdrawal was reportedby Pratt and Johnson (1926) from the Goose Creek oilfield, located at the head of Galveston Bay, Texas. Inaddition to a 1-m deep subsidence bowl, earth-fissures,presumably caused by differential subsidence, were alsoobserved. The first recorded land subsidence due togroundwater withdrawal was fortuitously discoveredaround San Jose, California in 1932 by the U.S. Coast andGeodetic Survey, during repeat precision-leveling (Rap-pleye 1933). Meinzer (1937), based on his experience inSouth Dakota, explained the observed more than 1.2 m(4 feet) of land subsidence as due to the compaction of fine-grained sediments, caused by declines in fluid pres-sure associated with groundwater overdraft. A catas-trophic consequence of land subsidence was the failure of the Baldwin Hills reservoir near Los Angeles in 1963.Excessive pumping of oil had led to differences in themagnitude of land subsidence on either side of a normal

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fault, which triggered vertical displacement, leading tofailure of the dam.

Evans (1966) reported, for the first time, a strongcorrelation between deep-well injection of water in theRocky Mountain Arsenal Well near Denver, Colorado,and a swarm of over 700 earthquakes during a 2-yearperiod. Following Hubbert and Rubey (1959), the quakeswere attributed to gravity sliding along relatively steep

faults whose frictional strength had been decreased byincrease in water pressure caused by injection. Thecredibility of the Terzaghi–Hubbert–Rubey hypothesiswas experimentally established by controlled fluid injec-tion in an oil field at Rangely, Colorado by the U.S.Geological Survey (Raleigh et al. 1976). These ideas werelater extended to understand earthquakes triggered byman-made reservoirs beneath which large water pressuresmay be generated either due to undrained response, or dueto transient groundwater motion (Simpson and Nar-asimhan 1990).

An interesting manifestation of undrained response isthe subtle, but measurable systematic response of fluid

pressure in confined aquifers due to variations in earthtides, caused by the gravitational pull of the sun and themoon. Making the simplifying assumption that stressesassociated with earth tides act horizontally, and that theearth is free to deform in the vertical direction, Brede-hoeft (1967) provided a method of estimating the storagecoefficient of the aquifer from the response of a well toearth tides.

A more dramatic manifestation of response to un-drained loading of a water-saturated granular soil came tolight with the failure of the Lower San Fernando Valleydam near Los Angeles following an earthquake onFebruary 9, 1971. This hydraulic-fill earthen dam failed

due to the liquefaction of coarse granular material on theupstream side of the dam, caused by excessive porepressures generated by the shaking motion and the con-sequent densification of the soil without adequate time forwater to drain (Seed et al. 1973).

Regional groundwater motion

In parallel with developments in well-hydraulics, Hubbert(1940) invigorated interest in looking at groundwatersystems as a whole. He interpreted groundwater motiondynamically in terms of impelling and resistive forces,and an energy potential, with the systems bounded bygroundwater divides and impermeable barriers. Within,isopotential surfaces and flow lines that refracted at ma-terial interfaces characterized the flow pattern. Analyzingforce balance at the immiscible interface between fresh-water and saltwater, Hubbert showed that it will be in-clined under conditions of groundwater flow. Hubbert(1953) extended the freshwater–saltwater interface idea tothe oil–water interface in petroleum reservoirs, and elu-cidated the segregation, migration, and entrapment of oiland gas under the influence of regional groundwatermotion.

Meanwhile, geochemists studying groundwater sys-tems began recognizing connections between variations ingroundwater quality and groundwater movement. Cheb-otarev (1955) noted that the anionic content of ground-water revealed much about the chemical processes towhich the moving groundwater is subject. Based upon alarge number of observations, he suggested that ground-water starts off by being rich in bicarbonate in areas of

recharge, and successively acquires sulfates, followed bychloride. Thus, at discharge locations far removed fromrecharge, water is enriched in chloride. Back (1960),based on the disposition of chemical analysis data showedthat the outcrops of Cretaceous and Eocene sediments inthe Atlantic Coastal Plain constitute areas of discharge.He suggested that the outcrop area of an artesian aquiferneed not necessarily be an area of recharge.

Following the detonation of underground nuclear de-vices in 1957, the U.S. Geological Survey began sys-tematic groundwater studies of the Nevada Test site andits vicinity, with the objective of assessing the impacts of the tests on local groundwater resources. Aided by water

level data from shallow and deep wells, and chemical andisotopic data, these studies soon led to the identificationof intra basin and interbasin regional flow systems in thisarid region, over several intermontane valleys extendingover thousands of square kilometers. Winograd (1962)presented some of the early evidence of deep interbasinflows.

These ideas came together in the Hydrology Sympo-sium held at the University of Alberta, Canada, late in1962. Toth (1962) presented results of hypothetical stea-dy-state calculations for a rectangular, homogeneous flowdomain subject to sinusoidal boundary conditions imi-tating a fluctuating water table. By varying the dimen-

sions of the domain and the parameters of the sinusoidalwave, he showed that groundwater flow patterns can beorganized into shallow, intermediate and deep flow sys-tems, and that such a recognition can help in decipheringthe relationships among groundwater circulation, waterquality, distribution of mineral phases, and spatial varia-tions of plant communities. In a companion paper Mey-boom (1962), presented field data to illustrate regionalgroundwater flow patterns associated with a prairie pro-file in Saskatchewan, governed by the extensive presenceof a permeable horizon at depth, connecting the area of recharge with an area of discharge located far away. Thiswork used data from nested piezometers, designed todiscriminate vertical components of flow. The prairieprofile suggested the possibility of groundwater flowsystems on a continental scale. Using the digital com-puter, Freeze (1966) illustrated the practical utility of theconcept through extensive parametric studies on two-di-mensional steady-state systems.

Some examples Abnormally high fluid pressures in some sedimentarybasins constitute unusual hydrogeological environments.Among the possible causes of such systems, one is therapid build-up of sediment over-burden which causes

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undrained pore pressure generation that may dissipategradually over geological time. These manifestationssuggest that large hydrogeological systems need to betreated as transient on appropriate time scales. Toth andMillar (1983) suggested, based on analysis of pore pres-sure data from depths in excess of about 1,220 m(4,000 feet) in the Red Earth Region of Alberta, Canada,that the observed fluid pressures are still responding to

erosional changes to topography initiated 1–12 millionyears before present.During the early 1970s, the Arab oil embargo moti-

vated serious study of geothermal systems in the UnitedStates. Geothermal systems fall into two categories: liq-uid-dominated and vapor-dominated. In the ImperialValley of California, which is part of an incipientspreading center covered by rapid sedimentation by theColorado River, hydrothermal convective systems withtemperatures of 340C (640F) are known at depths of 2,130–4,570 m (about 7,000–15,000 feet). Exploratorydrilling during the 1970s revealed that the hot brines, 8–10 times as saline as sea water, and charged with metals

such as gold, silver, copper, and zinc were manifestationsof active ore-forming fluids.More remarkable than the liquid-dominated systems is

the vapor-dominated geothermal system at the Geysers,some 110 km (about 70 miles) north-east of San Fran-cisco. The largest of its kind in the world, the Geysersrepresents an abnormally under-pressured natural system.Here, no water table has been encountered down to morethan 3,660 m (about 12,000 feet). Before active produc-tion, steam pressure and temperature were, respectively,243C (470F), and about 3.45 mPa (500 pounds persquare inch), even in wells over 2,440 m (8,000 feet)deep. In contrast, the expected hydrostatic pressure at

2,440 m (about 8,000 feet) would be in excess of 20.7 mPa (3,000 pounds per square inch). The geologicalconditions that give rise to this type of under-pressuring isas yet not understood. Based on mathematical modeling,Pruess and Narasimhan (1982) postulated that vaporproduction from wells was the consequence of heat andmass transport in a double-porosity fractured porousmedium.

The Mississippi Valley-type massive, stratiform lead–zinc deposits found distributed around the peripheries of many sedimentary basins (e.g., Michigan Basin, IllinoisBasin, Anadarko Basin) in central North America con-stitute hydrogeological systems of continental propor-tions. They are epigenetic deposits, formed at tempera-tures of 100–150C, at relatively shallow depths due tothe action of chloride rich brines, presumed to haveoriginated at great depths within sedimentary basins. Akey question concerns the mechanisms by which thebrines are transported on a continental scale from depthsin the center of the basin to the near-surface around theperiphery to cause ore deposition. Cathles and Smith(1983) suggested, based on computer models, that thesedeposits formed due to episodic expulsion of ore-formingfluids, separated by long quiescent periods of fluid pres-sure build-up and rupture.

Geochemistry of hydrogeological systems:aqueous–solid chemical interactions

A discussion of 20th century geochemical developmentsin hydrogeology can be found in Back and Freeze (1983).An early interpretive view of groundwater chemicalanalysis was made by Palmer (1911), who postulated thatsalts dissolved in natural waters constitute a chemical

system of balanced values. The next four decades wit-nessed a recognition that water quality data indicatepredictable interactions among rain water, soil, and therock types in the path of groundwater. Rogers (1917)compared the chemistry of shallow waters of the SanJoaquin Valley, California with deeper oil field watersand concluded that due to the reducing action of oil andgas, sulfate, a dominant anion of the shallow waters, hadbeen reduced to hydrogen sulfide. He also speculated onthe role of sulfate reducing bacteria. Renick (1924)studied groundwater in Tertiary formations of east-centralMontana and found that shallow waters, rich in calciumand magnesium, progressively became enriched with

depth in sodium at the expense of calcium and magne-sium due to base ion exchange facilitated by the presenceof certain silicate minerals. These observations motivatedthe development of systematic procedures for the classi-fication and graphical presentation of chemical data tofacilitate interpretation. A pioneering work is that of Piper(1944), who showed how tri-linear diagrams could beused to screen a large number of water-analyses forcritical study with respect to sources of the dissolvedconstituents, modifications in chemical character as waterpasses through an area, and related geochemical prob-lems.

During the late 1940s geochemistry became estab-

lished as a separate discipline, and rapidly asserted itsinfluence in the earth sciences. Accumulating data fromoil-field brines, and associated zones of anomalously highwater pressures stimulated considerable interest. Berryand Hanshaw (1960) studied abnormally high pore pres-sures from three widely separated areas in North Americaand argued that pressure-salinity anomalies in deep sed-imentary basins can be accounted for by compacted shaleacting as selectively permeable membranes. White (1965)surveyed brines from several sedimentary basins in NorthAmerica, and concluded that the early diagenetic changesin these waters were significantly influenced by organiccontent and bacterial activity, as well as ion exchange. Hesuggested that many characteristics of saline waters arebest explained by the hypothesis that fine-grained sedi-ments act as semi-permeable membranes permitting se-lective escape of water, carbon dioxide, sodium, boronand sulfur, relative to chloride and calcium.

John Hem (1916–1994) made two influential contri-butions to groundwater chemistry. In the first, Hem(1959) summarized the knowledge and experience of theU.S. Geological Survey in sampling, analyzing and in-terpreting groundwater quality. In the second, Hem andCropper (1959), introduced the use of Eh–pH diagrams tointerpret the aqueous–solid stability relations of redox

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species in groundwater systems. Contemporaneously,Garrels (1960) introduced the Eh-pH diagram based onthermodynamic considerations, and demonstrated itspower in understanding diagenesis, low-temperature oredeposits, and the genesis of soils. Garrel’s work soon ledto rapid developments in computer-based mathematicalmodels. Helgeson (1968) was among the earliest totranslate the thermodynamic equilibrium interactions into

a set of partial differential equations amenable to beprogrammed on a digital computer. Geochemical analysisof aqueous–solid interactions require consideration of other processes as well. These include adsorption onmineral surfaces, and ion-exchange between water andminerals such as zeolites.

Micro-organisms play important roles in earth pro-cesses as widely varying as weathering and soil genesis,formation of rocks and minerals, and genesis of oil de-posits. It is now believed that they thrive in aqueous en-vironments at depths of 3 km or more, and at temperaturesof more than 66C (150F). Although observationalknowledge of these organisms is accumulating, little is

known about the way they function. Microorganismsextract energy for their sustenance from specific chemicalreactions that would otherwise release energy slowly.

The inventions of the atomic mass spectrometer andthe electron microprobe during the 1940s opened up newways of understanding the functioning of hydrogeologicalsystems. The former helped precise measurement of sta-ble and unstable isotope abundances, while the latterhelped high resolution profiling of chemical concentra-tions within mineral grains. Over the past six decades,these instruments have seen spectacular improvements.Besides providing clues about the age and source of thewaters, these instruments have helped greatly in under-

standing the evolution of the liquid phase as well as themineral phases in hydrogeological systems.It is fitting to conclude a discussion of geochemistry

with the grandest of all hydrogeological problems, theorigin of oceanic and continental crusts. Deming (2002)pointed out that plate tectonics was established in theEarth’s crust only some 2.5 billion years ago, at the be-ginning of the Proterozoic, and that the nature of the crustduring the previous 2 billion years of the Earth’s history islargely unknown. The continental crust is believed tohave come into existence through hydrolysis of theprimitive crust. The large volumes of water required, it isspeculated, came about slowly from extra-terrestrial ac-cretion.

Chemical transport Within a hydrogeological system, aqueous–solid interac-tions occur at a given location. For these reactions toinfluence the regional system, the reaction products haveto be transported. Among groundwater hydrologists, theimpetus to consider chemical transport emerged duringthe 1950s with concerns about water quality problems of saltwater intrusion. Migration of dissolved substances ingroundwater involve a combination of bulk movementwith the flowing water at average water velocity, referred

to as advection, and concomitant diffusive spreading.Scheidegger (1954) looked at Darcy’s Law from theperspective of Brownian Motion, and by superposingrandom motion of particles over average bulk motion(referred to as drift) showed that the random motion lo-cally introduces a dispersion, which can be quantified bya new parameter, dispersivity. For homogeneous media,dispersivity can become a constant under some statistical

assumptions. This idea was extended to describe the mi-gration and spreading of dissolved contaminants in geo-logical materials by Bear (1960), who showed that ve-locity controlled spreading in a homogeneous isotropicgroundwater body can be represented with the help of asecond-rank dispersion tensor with longitudinal andtransverse components. The sum of this dispersion andmolecular diffusion is referred to as hydrodynamic dis-persion. The velocity-dependent dispersion coefficientbecomes a strong function of scale in heterogeneousmedia. To gain insights into scale-dependence, Gelhar etal. (1979) applied stochastic calculus to the steady flow of water in a stratified aquifer and found that for large time

periods, the dispersion coefficient approaches a constantvalue that depends on the statistical properties of themedium.

Contamination of shallow and deep groundwater sys-tems occur due to a variety of human activities includingmining, agriculture, landfills, disposal of toxic wastes,septic tanks and animal wastes, and industrial effluents.Abandoned mines, mine wastes, and solid wastes fromthermal power plants generate highly acidic effluents thattransport toxic elements such as arsenic, boron, lead,chromium, copper, molybdenum, zinc, and so on. Duringthe 1980s, contamination of groundwater by organicchemicals came to be recognized as a very serious envi-

ronmental concern. The sources of these contaminantsinclude leaky gasoline tanks, organic solvents and othereffluents from manufacturing plants, and electricaltransformer fluids. Some of these chemicals are known tobe carcenogenous at concentrations as low as a few partsper billion. Although most of these organic compoundswill ultimately decay to carbon dioxide or methane, verylittle quantitative data are available about the decay pathsof most of these contaminants. Compounding the problemis the fact that new chemicals are introduced into thehuman habitat by the tens of thousands each year.

Discontinuous media

Geological formations with fractures, joints, solutioncavities, and large openings formed due to various causesmay be collectively referred to as discontinuous media.The nature of occurrence of water in these media, andexploitation of water residing in them continue to intrigueand challenge hydrogeologists. Unlike sediments such asclay or sand that can be visualized as “continuous” or“homogeneous” in small samples, discontinuous mediadefy such visualization even in very large volumes. Thus,these systems are not amenable to mathematical analysis

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with methods that have otherwise proved to be successfulin analyzing physical systems. The discontinuous naturealso poses challenges to exploration for water.

Discontinuous media may be divided into two groups.Karst and Karst-like systems are understood on a scaleranging from hundreds of meters to kilometers. Fracturedmedia and fractured porous media are understood on ascale of centimeters to tens of meters. Karst-like land-

scapes are characterized by closed depressions integratedwith underground drainage, with disappearing surfacestreams and caves (White 1988, p. 347). The large un-derground drainage channels may have originated throughsolution of carbonate rocks (true Karst), solution of evaporite deposits and salt domes, or may have formeddue to rapid cooling of thick lava flows (lava tubes).Whereas fractured media do not generally constitutehighly productive aquifers, Karst-like formations mayform highly productive aquifers and may sustain largesprings. In Nevada and California, highly productive,deep carbonate aquifers are known to transfer ground-water across several water divides (Eakin 1966).

Somewhat more amenable to mathematical analysisare systems in which consolidated, brittle rock formationshave been mechanically broken by fractures or joints thatusually occur in sets. Members of each set have similarattributes of orientation and size. Despite this broad reg-ularity, individual fractures pinch out along their lengthand breadth, and their apertures vary spatially. Thesepinch-outs and aperture changes are difficult to charac-terize and predict, and herein lie the hydrogeological at-tributes peculiar to fractured rock systems. The fractureporosity of rocks is usually less than one percent, andbecause the compressibility of fractures is very small (of the same order as compressibility of water), the hydraulic

capacitance of these rocks is very small. However, indi-vidual fractures can transmit water with great ease.Hence, fractured rocks tend to possess relatively highhydraulic conductivity. The high hydraulic conductivityand the low porosity and compressibility of the rockscombine to bestow these rocks with very high hydraulicdiffusivity. Consequently pressure transients can migratevery rapidly through fractured rocks.

The bulk hydraulic conductivity of fractured rocksstem from a combination of fracture apertures, fracturelengths, their number and connectivity. Idealizing afracture as a special case of a water-filled rectangulartube, Boussinesq (1868) showed that the flux through afracture is proportional to the third power of its aperture.This observation is often referred to as the cubic law.Despite the difficulties inherent in assigning a single ap-erture magnitude to a rough-walled fracture, the cubic lawis still used as an approximation. Because fractures canconduct contaminants rapidly, the study of fractured rockshas attracted much attention since the 1960s. Snow (1965)revived interest in the parallel plate model in applying itto problems of engineering geology and groundwaterhydrology. Snow (1969) also showed that an idealizedfracture set, with continuous fractures of uniform aperturecan be effectively treated as an anisotropic medium. At

present, statistical methods are widely used to understandthe connections between statistically described fracturenetworks and scale-dependent hydraulic conductivity of bulk rock.

Materials such as sandstone or shale that have primaryporosity, when fractured, give rise to fractured porousmedia. In such media, a bulk of the stored fluid (water,oil, or gas) resides in the porous matrix, but the trans-

mission of the fluids over long distances takes placethrough the interconnected fractures. The low-diffusivitymatrix blocks occur as islands in the high diffusivityfracture network. Barenblatt et al. (1960) and Warren andRoot (1963) proposed the double-porosity model to de-scribe transient fluid flow in such media, which helpedpreserve the effects of fine-scale structure on large scalepressure transients.

The hydraulic attributes of a fracture dramaticallychange when it changes from a state of full water satu-ration to one of partial saturation. In this case, fracturesbecome strong hydraulic resistors in comparison with therock blocks. Under such conditions, water tends to move

very slowly through the rock blocks and along films onthe walls of the fractures. This remarkable attribute hasplayed a role in the selection of the Yucca Mountain sitein Nevada by the U.S. Congress in 1987 as the mostdesirable site to be investigated for the disposal of highlevel commercial radioactive wastes. At Yucca Mountain,the water table lies at a depth of 457 m (about 1,500 feet),in a desert area with very low rainfall. The mountain itself comprises extremely fine-grained fractured ash-fall tuffs.Both the unsaturated fractures and the porous matrix arecharacterized by extremely low hydraulic conductivities.Additionally, one of the ash-fall layers below the pro-posed repository depth is rich in zeolites, capable of ad-

sorbing significant quantities of toxic cations. The prob-lem of radioactive waste disposal at Yucca Mountain aswell as at other places within and outside the U.S. hasfostered substantial research to understand the movementof water and contaminants in fractured rock systems. Themovement of water and contaminants in these systemscontinues to elude precise mathematical description.Nonetheless, the licensing process for authorizing thecommissioning of the site requires assurance that toxiccontaminants will not reach the accessible environmentfor 10,000 years or more. On this time scale, the diffi-culties of fracture flow characterization are compoundedby uncertainties of future climatic changes.

Marine hydrogeology

By the end of the World War II, geologists came to rec-ognize that the large scale structure of the Earth was muchsimpler beneath the oceans than on the continents. Thismotivated active scientific exploration of the ocean bot-tom during the 1950s. The exploration of the ocean bot-tom is so intimately tied with understanding fluid circu-lation in the oceanic crust that a significant part of marinegeology and marine geophysics comes within the scope of

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hydrogeology. A milestone in the history of marine geo-physics was the invention of a heat-flow probe by EdwardBullard (1907–1980), which enabled the measurement of natural heat flux from the oceanic crust (Bullard et al.1956). During the 1960s, measurements made with thisprobe and its variants from the Atlantic, the Pacific, andthe Indian oceans helped greatly in the establishment of the theory of plate tectonics.

Modern exploration of the ocean bottom includesdrilling of boreholes into the oceanic crust, supported bydetailed studies of the ocean floor in the vicinity of theborehole. The boreholes are subjected to detailed geo-physical and geochemical logging, and hydraulic packertests where possible, to estimate in situ hydraulic con-ductivity. Frequently, detailed heat flow measurementsare made in the vicinity of the boreholes to prepare heatflow contour maps. These data constitute the infrastruc-ture to understand circulation of water in the oceaniccrust.

Marine hydrogeological systems can be divided intothose that are associated with crustal extension (with

plates moving away from a rift), and those associated withregions of subduction. In regions of crustal extension, aprincipal goal is to understand the attributes of the con-vective cells, as one moves progressively away from theridge axis. The zones of subduction are regions wheremeteoric water begins its migration towards the mantle.One interesting hydrogeological feature of subduction isthat the gradual displacements along the plane of under-thrusting, in conjunction with the very low permeabilityof the sediments, can lead to the generation of abnormallyhigh pore pressures, leading to the occurrence of mudvolcanoes under the weight of several thousand meters of sea water.

Because of the difficulty of accessing the ocean bot-tom, marine hydrogeology has inspired the invention of novel tools and technologies. As an example, one maycite certain special boreholes of the Ocean Drilling Pro-

ject. Drilled below water depths in excess of 3,660 m(about 12,000 feet), these boreholes can be periodicallyreentered after intervals of many months to years todeepen the borehole, run repeat logs, or carry out varioustypes of tests. Hydrogeologists used to studying conti-nental systems, have much fascinating knowledge to gainfrom developments in marine hydrogeology.

Numerical methods

The mathematical techniques which Fourier pioneered tosolve the diffusion equation are practically useful in thosehydrogeological systems with known symmetry and het-erogeneity. The solution techniques are inadequate forregional systems characterized by arbitrary geometry,heterogeneity, and boundary conditions. Prior to the1970s, efforts were made to overcome this deficiencywith the help of physical models and analog models.However, the rapid growth of digital computers startingfrom the late 1960s helped establish numerical methods as

the most powerful approach for modeling hydrogeologi-cal systems. The following discussion is restricted tonumerical methods.

The use of the digital computer to solve the transientdiffusion equation was pioneered during the World War IIby von Neumann using the finite difference approach, inwhich spatial gradients are approximated along principaldirections by finite differences. However, it was too

limiting to restrict grid points along the principal direc-tions of the coordinate axes, especially when the flowdomain had a complex shape. This limitation was over-come by taking a physical, intuitive view of the governingequation as a mass conservation statement over an ele-mental volume of known geometry, whose surface is di-vided into a finite number of segments. The earliestproponent of this approach was MacNeal (1953). Thisintuitive approach was first applied to groundwater basinsby Tyson and Weber (1964), who used polygonal volumeelements. The shortcoming of the method is that it cannotconveniently evaluate fluxes in anisotropic media. Thefinite element method, developed by Clough (1960) for

solving stress–strain problems in solid structures, pro-vided a convenient way of handling anisotropic media.The central theme of this novel approach was to use asmall triangular region defined by three non-collinearpoints as the basis of evaluating spatial gradients in anydirection, rather than using two points to evaluate a gra-dient in a fixed direction. This method was extended in-tuitively to solve problems of heat conduction in massconcrete structures by Wilson (1968). Since the 1960s, theFinite Difference approach, the approach of usingpolygonal elements, and the finite element approach havebeen employed to solve for steady and non-steady flow inmultidimensional systems involving saturated and unsat-

urated groundwater flow.In general, hydrogeological systems are idealized asinteractions among: flow of multiple fluid phases, defor-mation of the porous medium, transport of heat, transportof dissolved and suspended chemical compounds, andaqueous–solid chemical interactions. Mathematically,these interactions are represented by a set of partial dif-ferential equations, with each process being representedby a diffusion-type equation, supplemented by advectionwhere appropriate. Chemical interactions are commonlyhandled through equilibrium thermodynamic considera-tions, which include the Law of Mass Action, mass con-servation, electrical neutrality, electron conservation, andGibb’s phase rule. Additionally, surface processes such asadsorption and ion exchange are handled through empir-ical relationships obtained from batch experiments. De-parture from equilibrium are handled with kinetic coef-ficients. The interactions referred to above are quitecomplex, and the various parameters relevant to the pro-cesses are not available a priori. Even now, most modelsaddress only a few of these couplings at a time, as forexample, flow and deformation, multi-phase flow andheat, and fluid flow with heat flow and chemical reac-tions.

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The standard mode of application the numerical modelis to assume that the geometry, material properties, initialconditions, and boundary conditions are known, and tosolve for the dependent variable, the hydraulic head. Thisis the forward problem. In general, this problem ismathematically well-posed, and unambiguous solutionscan invariably be found. However, one is often confrontedwith the situation where the hydraulic conductivity and

the hydraulic capacitance are not a priori known. In thissituation, one seeks to use field measurement of the hy-draulic head at a finite number of points to estimate themagnitude of the hydraulic parameters, with the numeri-cal model. This is referred to as the inverse problem. Thedifficulty with the inverse problem is that its solutioninherently non-unique, even if data on hydraulic head,system geometry, and boundary conditions are all knownin detail. Among the earliest to examine a groundwaterbasin from the perspective of the inverse were Vemuriand Karplus (1969). They considered transient flow in anunconfined aquifer in the Eastern portion of the SanFernando Basin in California. Field data on water table

elevations were inverted using a hybrid computer to arriveat spatial variation of aquifer transmissivity. Using aheuristic approach, estimates were also made of storagecoefficient, and the boundary of the aquifer.

Emsellem and de Marsily (1971) addressed the inverseproblem for steady state flow in an aquifer using thedigital computer. The thrust of their automatic solutionmethod was that the inverse problem, which is normallypoorly-posed, becomes amenable to solution if availablephysical information about the structure of the system isincorporated into the solution process. At present, inversemodels are mainly used in groundwater hydrology, di-rected at estimating the spatial distribution of hydraulic

conductivity. The inversion is carried out assuming theflow system to be under steady-state. To overcome thedifficulty stemming from paucity of data, one may useany available geological or hydrological knowledge of thefield system to precondition the inversion process. Thismethodology, referred to as the Bayesian approach, wasintroduced during the 1970s by Delhomme (1979), Neu-man and Yakowitz (1979), and others.

A related problem concerns the dependence of themagnitude of the parameter, hydraulic conductivity, onscale. This parameter is reasonably understood physicallyin terms of impelling and resistive forces on the scale of laboratory samples of sand, clay, silt, and the like.However, when one wishes to use the parameter for largeheterogeneous masses of earth materials, its physicalmeaning becomes less clear. Yet, the parameter is prac-tically needed on the large scale to mathematically sim-ulate very large groundwater systems. The simple way tohandle this is to use various types of mean values(arithmetic, harmonic, or geometric means) as appropri-ate. The phrase “upscaling” is sometimes used to expressthe methods used to describe the generation of parametersfor large-scale models from small-scale information. Suchupscaling of hydraulic conductivity can be useful in thecase of steady-state systems, but may only be of limited

values in transient systems. The serious challenge insimulating the flow behavior of large heterogeneoussystems is to define a set of large-scale average parame-ters that will somehow preserve important small-scaleeffects.

The inverse problem, as well as the problem of up-scaling are treated as problems in statistics, or in sto-chastic processes, as applied to steady-state, heterogene-

ous systems. Because of the mathematical complexity andabstract nature of statistics and stochastic processes, themathematical symbolism can often be overwhelming andmask a physical grasp of the results generated by thesemodels.

Another important problem in numerical modeling isthe uncertainty associated with predictive modeling. Thisis because of the errors and ignorance inherent in thegeometrical description, the hydraulic parameters, and theinitial conditions that are used in the calculations. Intraditional engineering practice, such uncertainties aretaken into account through factors of safety incorporatedinto the design, or through sensitivity analysis. A more

mathematical approach to handle uncertainty in predic-tion is to quantify the magnitude uncertainty throughMonte Carlo simulations, by treating the governing of groundwater flow as a diffusion process with parametersthat are random variables, whose magnitudes are ex-pressed in terms of a mean and a variance.

During the 1960s, when the first numerical modelsappeared, there was great anticipation about their abilityto solve a large number of practical problems in hydro-geology. This excitement has since undergone modera-tion. It is now recognized that numerical models can onlybe valuable in providing insights into the potential be-havior of complex hydrogeological systems, and to test

alternate hypotheses to better understand observed phe-nomena. Considering the inaccessibility of the Earth’ssubsurface, the pervasive heterogeneity on many spatialscales, the strong interactions among fluid flow, defor-mation, heat flow, and chemical interactions, and lack of knowledge of future forcing functions, it will not beprudent to assume that numerical models will predict thefuture with confidence, even with the availability of themost powerful computing machines.

Near-surface hydrogeological processes

At the turn of the 21st century, processes of the Earth’snear-surface and the human habitat are increasingly en-gaging the attention of earth scientists. Limnologists,aquatic ecologists, hydrologists, geomorphologists, cli-matologists, and landscape managers are coming together,reflecting emerging concerns about human impacts on theenvironment. New disciplines such as biogeochemistry,ecohydrology, and bioclimatology are emerging, all of which have strong connections to hydrogeology. All sharea common goal of understanding the interconnected hy-drological, erosional and nutrient cycles which sustain alllife. The Earth’s near-surface, where these disciplines

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intersect, includes the soil, the vadose zone, riparian andhyporheic habitats, wetlands and agricultural lands.

The vadose zone, topped by the soil, mediates betweenthe atmosphere and the water table. The hydraulic-dividebetween the upward-migration of moisture due to evap-otranspiration, and gravity-driven downward flow fluc-tuates in the vadose zone. Chemical weathering, break-down of organic matter, fixation of nitrogen, and other

chemical reactions so vital for the nutrient cycle largelyoccur in the vadose zone, strongly influenced by thepresence of water, air, and microorganisms. Finally, waterthat recharges the groundwater reservoir, and the con-taminants released at the land surface have first to passthrough the vadose zone. The physical, chemical and bi-ological processes that occur here are of fundamentalgeological interest. Here pedology and geology converge.

Lakes, streams, and wetlands, constitute aquaticecosystems. The riparian and the hyporheic zone repres-ent, respectively, the transition between the aquatic en-vironment of a stream and the adjoining land, the localgroundwater. Microbial organisms and invertebrates of

the aquatic environment demonstrate adaptation to thephysical and chemical attributes of pore water in thesezones. Here, biogeochemistry, ecology and hydrogeologycome together. Likens and Bormann (1977), in presentingtheir findings on a forested ecosystem point out that dataon hydrology is of paramount importance in understand-ing the biogeochemistry of an ecosystem because water isa chemical solvent, a catalyst, and a transporting agent.Euliss et al. (2004) advance the concept of a wetlandcontinuum, which allows wetland managers, scientists,and ecologists to simultaneously consider the influence of climate and hydrologic setting on wetland biologicalcommunities.

Irrigated agriculture around the world has profoundlyimpacted the human habitat. One major consequence of prolonged irrigation is the degradation of soil fertility andgroundwater quality due to water logging. For more thana century, irrigation engineers have focused attention onovercoming this problem on a local scale, with the pri-mary objective of maintaining productivity through effi-cient water use, export of water through tile drains, andsoil amendments. Since export of accumulated salt is notonly expensive and is subject to serious environmentalconsequences, the management of irrigated agriculture isan important issue that needs to be addressed on the scaleof a regional hydrogeological system. On this scale,pedology, soil geomorphology, agriculture, ecosystems,and water quality are all rationally integrated.

Groundwater and society

Water occupies a central role in the history of most cul-tures. Not surprisingly, attitudes towards water manage-ment around the world are influenced by local traditions.Social dimensions of water, therefore, constitute an in-valuable adjunct to a scientific understanding of hydro-geological systems. Through a study of the history of the

native peoples of the western hemisphere, Back (1981)provided perspectives on how water resources played asignificant role in the growth as well as the decline of theearly civilizations of North and South America.

Water is often an object of contention between thosewho seek its control, and the larger community that de-pends on it for sustenance. Understandably, litigation forwater has been part of the American society over the past

two centuries. During the nineteenth century, Americancourts believed that groundwater was a mysterious phe-nomenon. For example, the Supreme Court of Ohio(1861) held that groundwater is “so secret, occult, andconcealed that an attempt to administer any set of legalrules in respect to [it] would be involved in hopelessuncertainty”. This perception of mystery began to changeat the beginning of the 20th century with the systematicgathering of scientific data from around the country bythe U.S. Geological Survey, and with common citizenstaking note of the effects of groundwater overdraft on theenvironment. Over the past few decades, the courts inAmerica have recognized that groundwater systems can

be scientifically understood in terms of cause–effect re-lationships, and that such scientific knowledge shouldform a rational basis of adjudication.

In the American west it was believed during the late19th century that the overlying landowner had unlimitedright to all groundwater below one’s land and that therewas no limit to the quantity of water that may be extractedfrom a well. However, this view became unsustainablewith marked declines in well productivity during periodsof drought. In an influential decision, the Supreme Courtof California (1903) held that burdens due to declinedgroundwater yields should be proportionately borne byoverlying land owners. This principle is known as cor-

relative rights.With the invention of the deep-well turbine pump inCalifornia around 1907, and the availability of electricpower, groundwater became an inexpensive alternative tosurface water for irrigation throughout the Americanwest. Encouraged by the potential for economic pros-perity, and poorly defined ownership rights, many west-ern states witnessed over-production of groundwater.Within decades, this led to large declines in water levels,disappearance of artesian flowing wells, land subsidence,earth fissuring, and saltwater intrusion in coastal aquifers.Clearly, groundwater withdrawal had to be controlled.But, the early laws were primarily intended to fostereconomic growth, and did not show awareness of thefiniteness of the resource. Planned groundwater devel-opment required enactment of new laws.

In the United States, water management is a statesubject, except in regard to water rights of NativeAmericans. The states own all water within their bound-aries in trust for the people. In regard to Native Ameri-cans, the Federal government acts as the guardian of theirwater rights. That the states own all water in trust for thepeople is based on historic Roman law traditions, and isreferred to as the doctrine of public trust (Narasimhan2003). This doctrine is now being used as a basis by many

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states to mandate integrated management of surface waterand groundwater on a basin-wide scale. Individuals, cor-porations, and municipalities are required to obtain per-mits for specific beneficial use of water, with the under-standing that water will not be wasted. The permit isusufructuary in the sense that the resource itself will notbe unduly damaged in the process of the beneficial use.State and federal environmental protection laws have

been set in place to protect groundwater bodies fromchemical degradation due to human activities. Although,during the 1980s, some courts extended the public trustdoctrine to ecological values, its application to ground-water is still being contested in courts by those who ac-quired riparian and appropriation rights during the 19thand early 20th century.

In the wake of the strong environmental movement of the 1960s, there exist two opposing perceptions aboutgroundwater. Those with acquired water rights would likehaving groundwater treated as private property. This isespecially true in California, where some farmers ownvery large tracts of land and associated water rights. On

the other hand, many hydrogeologists, ecologists, andenvironmentalists would like to see groundwater andsurface water treated as a single resource, and subjected tointegrated management on a basinal scale. On a scientificlevel, it is becoming clear that the groundwater reservoirmust be so managed as to play the role of a buffer that canbe relied upon to tide over water deficiency during peri-ods of drought. Many western states, such as Arizona,Nebraska, New Mexico, and Texas have already moved inthe direction of asserting the State’s responsibility tomanage surface water and groundwater as a single inte-grated resource.

An interesting recent development is that institutions

and agencies involved with water have made public ed-ucation a part of their activities. As a result, the commoncitizen is becoming increasingly aware of the nature of the hydrological cycle, and within it, the groundwatersystem. This public literacy, it appears, will eventuallylead to a wise management of surface water andgroundwater, duly considering the needs of humans andof ecosystems.

Future: a speculation

There are many facets to hydrogeology: the science itself,its applied benefits, the forces that drive its growth, andits relation to society. Speculation about its future,therefore, must be based on expectations about how thesemay change with time. Hydrogeology constitutes thestudy of geological processes influenced by water. Themotivation to pursue it may be mere intellectual curiosity,or may stem from a desire to solve problems of interest tosociety, or both. In our contemporary society, hydroge-ology research is driven by governmental funding, and toa lesser extent, industrial support. Society is increasinglybecoming aware of groundwater, as freshwater is ren-dered scarce by the needs of a technological society.

The observational base of hydrogeology is expandingat a phenomenal pace. We now observe the Earth’s his-tory, in real time, at very high spatial and temporal res-olutions. There is fervent expectation that these data canhelp predict and control the behavior of hydrogeologicalsystems for much benefit to society. How realistic is thisexpectation?

The science of hydrogeology is based on principles of

the physical sciences. Physical sciences strive for precisemeasurement, detailed description, and reliable predictionof the behavior of the physical world. Inspired by this,modern technology pursues the goal of controlling thephysical world for societal benefit. Nonetheless, the in-accessibility of the Earth’s subsurface, pervasive hetero-geneity on many spatial and temporal scales, and thelimitations inherent in mathematical tools that seek toquantify hydrogeological processes cast serious doubt onthe ability of the physical sciences to precisely predict andcontrol the behavior of these systems. Moreover, the be-havior of living things, stemming from their ability toadapt to their changing environment complicates human

aspirations of prediction and control of hydrogeologicalsystems.Hydrogeology is a historical science. It is concerned

with comprehending why the earth is what it is todaybecause of what happened in the past. In this role, hy-drogeology is enjoying enormous success. Experience hasshown that, despite this success, predicting the long-termfuture of hydrogeological systems is very difficult, oreven impossible, because of limitations inherent in ourmodels, and the unpredictability of the forces that drivethe earth.

The ability to collect data of unprecedented resolutionon the one hand, and the severe limitations that confront

prediction and control on the other, raise a serious issue.The future of hydrogeology will clearly depend on theexpectations with which financial support will be forth-coming for research. Although a small proportion of totalfunding may be designated for uncommitted scientificinquiry, a major portion will be allotted with expectationsof applied benefit. Some important questions arise.Should society continue emphasis on gathering more andmore data, or should resources be diverted to bettermanagement of hydrogeological systems based on whathas already been established? What should be the moti-vation for collecting data from hydrogeological systems?Should it be the timely detection of unacceptable conse-quences of human actions, or should it aim at more effi-cient control of such systems to serve human needs?Funding agencies face a dilemma: Should it be assumedthat evolving technology will continue to overcome Na-ture’s constraints and enhance human ability to predictand control hydrogeological systems, or should it be as-sumed that these systems cannot be precisely predicted orcontrolled. Each of these assumptions will demand adifferent type of research output. The former coursewould take a path of increased exploitation of hydroge-ological systems, while the latter will lead to adaptivemanagement, constraining the use of such systems within

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definite bounds. The former would anticipate new dis-coveries and inventions. The latter, careful monitoring of existing systems for timely detection of potentially un-acceptable impacts.

Over 50 years ago, Hubbert studied the growth anddecline of a number of non-renewable resources such asminerals, coal, and hydrocarbons, and vigorously arguedthat there are not enough natural resources in the Earth to

sustain significant economic growth indefinitely into thefuture. His view was that there already exists enoughscientific know-how to pursue a more realistic long-termgoal of steady or very slight economic growth over longperiods of time (Hubbert 1973). It is of value to extendHubbert’s analysis to hydrogeological systems, and un-derstand how they degrade with time. This task is morecomplicated because many hydrogeological systems arepartly renewable, and the notion of degradation has toaddress not only water itself, but also the soil and othercomponents of the ecosystem that sustain flora and fauna.

An emerging trend in the earth sciences is the recog-nition that the hydrological, the erosional the geochemical

and the nutritional cycle are intimately interlinked. Thesecycles sustain life, and are, in turn, influenced by it. It isin this context that the future of hydrogeology may berationally speculated upon. Just as hydrogeology found itslinks to geochemistry, tectonics, and petroleum geologyduring the 1950s, so also, hydrogeology is currently dis-covering new connections to biogeochemistry, ecohy-drology, bioclimatology, and other emerging areas of study. In so far as water is the principal agent thattransports energy and matter in the lithosphere, hydro-geology has the potential to bring all these disciplinestogether through the framework of regional groundwaterflow systems. As attention shifts to understanding the life-

sustaining cycles, it is likely that distinctions among fieldssuch as hydrogeology, biogeochemistry, ecohydrology,may become less important.

Substantial knowledge now exists about the physicaland chemical processes that govern hydrogeological sys-tems, including fluid flow, deformation, energy transfer,and chemical reactions. One would like to believe that theconceptual mathematical knowledge that exists in theseareas probably exceeds our current abilities to make fieldmeasurements. At the same time, experience with themodeling of hydrogeological systems have shown that theequations of mass and energy conservation, transport, andthermodynamics on which the models are based, arethemselves only approximations to the observable system.As a result, considerable difficulties exist in making theobserved system fit even the best available mathematicalmodels, and adequately characterize the modeled system.The maximum potential for new knowledge is likely to bein two areas; the role of microbes in hydrogeologicalsystems, and marine hydrogeology.

Over the past few decades, substantial evidence hasaccumulated showing that microbial organisms play aprofound role in many hydrogeological processes fromchemical weathering, soil formation and petroleum gen-esis to nutrient cycling. Yet, the physical and chemical

mechanisms associated with their actions is largely un-known. Additionally, microbes possess life, and they“respond”, “adapt”, and exhibit a “will to survive” withintheir changing environment. These attributes are notamenable to description by physical laws that constitutethe basis for describing the behavior of inanimate things.

Marine hydrogeology is a young field, with impressiveinternational cooperation, and driven largely by questions

aimed at understanding how the Earth functions. In thisfield, research questions are posed in a manner that theanswers must account for physical, chemical, and bio-logical observations on a variety of space and time scales.Compared with continental hydrogeology, the marinecounterpart is more resource intensive, and larger in scale.As a consequence, the future of marine hydrogeology willvery much depend on the ability of funding agencies toinvest in what is heavily basic science, with less emphasison immediate applied benefit.

In applied hydrogeology, a topic of great interest issustainable management of groundwater resources. Here,sustainable management implies the maintenance of sta-

ble water supplies for society, and assuring the longevityof the resources for future generations. Longevity, in thiscontext, may significantly exceed traditional time scalesof engineering decision-making, and extend to thousandsof years. There are many components to this task: coor-dinated use of surface water and groundwater in such away that the latter stores excess supplies during wet years,and acts as a buffer during droughts; maintenance of ri-parian, hyporheic, and other habitats, minimization of chemical contamination of fresh groundwater bodies, andrestoration of aquifers and habitats that have been im-paired by prior human actions. Clearly, managing waterinvolves more than just water. Vigorous industrial pro-

duction is inevitably accompanied by stressing the pro-ductivity of hydrogeological systems to their limits, andcontaminating them chemically. For a long time, theimpacts of these degradations were simply ignored, infavor of the purported economic benefits. More recently,efforts have been made to look at resource degradation interms of monetary benefit–cost analysis. However, thereare concerns that these analyses are far from adequate inaccounting for the value of the lost resource to futuregenerations. Judicious management of groundwater sys-tems will, in the future, entail far more than mere science.Many challenges lie ahead in combining human valueswith applied hydrogeology.

Speculation about the future of hydrogeology cannotignore the role of computers and mathematical models.During the 1960s, when computer models for analyzinggroundwater systems made their appearance, there wasgreat excitement about their potential ability to predict thefuture behavior of groundwater systems, and thus tomanage them for great benefit and profit. Despite explo-sive developments in computing power since then, currentexpectations about the ability of the computers to predictthe future are quite subdued. It is now recognized thatmathematical models are valuable tools for looking atdifferent scenarios, and testing alternative hypotheses,

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rather than being predictive tools. Modern computingmachines provide phenomenal possibilities for storingand retrieving large amounts of data, and portraying themin useful ways.

One inherent limitation of mathematical models of hydrogeological systems is that boundary conditions thatforce the systems to change are external to the models,and have to be made available independently. Common

practice is to prescribe these boundary conditions basedon limited past experience. However, emerging knowl-edge about paleohydrology has added a new dimension tothe difficulty of modeling hydrogeological systems. Re-cent developments based on the study of ice cores fromAntarctica and elsewhere (Alley et al. 2003) suggest thatdrastic climatic changes from glacial to inter-glacial cli-mates can occur over a period of just a few decades, andthat droughts on a continental scale can occasionallypersist for a century or more. The forces that cause suchchanges are as yet unknown. Suppose large tracts of theUnited States or Europe are subjected to droughts span-ning many decades. What would be the impact on the

hydrogeological systems that sustain these regions? Howwill the technological societies adjust themselves? It isimpossible to answer these questions because human be-havior under conditions of extreme stress cannot be pre-dicted. What can be stated, though, is that the worldwideresponse of societies under such contingencies could ei-ther be peaceful adjustment, or violent destruction. Watercould conceivably play a global role that will rival therole of energy. Should society choose to adapt itself to theconstraints of a finite nature and the unpredictability of forcing functions, then monitoring of sustained hydroge-ological systems will become an integral part of manag-ing such systems. The purpose of monitoring would be to

foresee, in a timely way any potential degradation of thesystem in unacceptable ways so that adequate preventiveactions can be initiated. Here monitoring is to be inter-preted broadly to include physical, chemical, and bio-logical attributes.

Two centuries after the industrial revolution, humansare beginning to comprehend the importance of the hy-drological, nutritional, geochemical, and erosional cyclesthat are vital for the sustenance of all life. The future of hydrogeology depends on how humans choose to livewithin the constraints of these cycles. Philosophically,hydrogeology has the potential to provide a unifyingframework to bring together a number of disciplineswithin the earth sciences, biological sciences, and socialsciences. Much will depend on how hydrogeologistsperceive their own identity, and how the various fundingagencies perceive the existence of a technological societywithin a finite and bounded Earth.

Acknowledgments I am grateful to Stanley N. Davis for manyinformative exchanges over the years on the history of hydroge-ology. I thank Ghislain de Marsily, David E. Prudic, and Clifford I.Voss for a critical reading of the manuscript and many constructivecriticisms. This work was partly supported by funds from theAgricultural Extension Service, through the Division of NaturalResources, University of California

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