Scientific or 

Rule-of-Thumb Techniques 

of Ground-Water ManagementWhich Will Prevail? 

Scientific or 

Rule-of-Thumb Techniques 

of Ground-Water ManagementWhich Will Prevail? 

By C. L. McGuinness 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 608 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

W. A. Radlinski, Acting Director 

First printing 1969 

Second printing 1m 

Free on application to Branch of Distribution, U.S. Geological Survey 

1200 South Eads Street, Arlington, Va. 22202 
CONTENTS 

Page 

Abstract 1 

Introduction -------------------------------------------------------------- 1 

The hydrogeologist's job 2 

The ground-water system ------------------------------------------------- 3 

The model ---------------------------------------------------------------- 3 

The moment of truth ----------------------------------------------------- 4 

What else do we need to know? ------------------------------------------- 5 

Artificial recharge ---------------------------------------------------- 6 

Permeability distribution -------------------------------------------- 6 

Vertical permeability ___________ ------------------------------ ________ 7 

Storage coefficient 7 

lJnsaturated zone ---------------------------------------------------- 7 

Geochemical problems ------------------------------------------------ 7 

Geophysics ----------------------------------------------------------- 8 

Conclusion --------------------------------------------------------------- 8 

References cited ---------------------------------------------------------- 8 

III 

Scientific or Rule-of-Thumb Techniques of Ground-Water 

Management-Which Will Prevail?! 

By C. L. McGuinness 

ABSTRACT 

Emphasis in ground-water development, once directed 

largely to quantitatively minor (but sociologically vital) 

service of human and stock needs, is shifting: aquifers 

are treated as possible regulating reservoirs managed 

conjunctively with surface water. Too, emphasis on 

reducing stream pollution is stimulating interest in 

aquifers as possible waste-storage media. 

Such management of aquifers requires vast amounts 

of data plus a much better understanding of aquifersystem behavior than now exists. Implicit in this 

deficiency of knowledge is a need for much new research, lest aquifers be managed according to ineffective rule-of-thumb standards, or even abandoned as 

unmanageable. 

The geohydrologist's task is to define both internal 

and boundary characteristics of aquifer systems. Stratigraphy is a primary determinant of these characteristics, but stratigraphically minor features may make 

aquifers transcend stratigraphic boundaries. For example, a structurally insignificant fracture may carry 

more water than a major fault; a minor stratigraphic 

discontinuity may be a major hydrologic boundary. 

Hence, there is a need for ways of defining aquifer 

boundaries and quantifying aquifer and confiningbed characteristics that are very different from ordinary stratigraphic techniques. Among critical needs 

are techniques for measuring cross bed permeability; 

for extrapolating and interpolating point data on 

direction and magnitude of permeability in defining 

aquifer geometry; and for accurately measuring geochemical properties of water and aquifer material, 

and interpreting those measurements in terms of 

source of water, rate of movement, and waste-sorbing 

capacities of aquifers and of confining beds-in general, 

techniques adequate for predicting aquifer response to 

imposed forces whether static, hydraulic, thermal, or 

chemical. Only when such predictions can be made 

routinely can aquifer characteristics be inserted into a 

master model that incorporates both the hydrologic and 

1 Prepared for presentation at the 23d International Geological 

Congress, Prague, August 1968. 

1 

the socioeconomic facts necessary to intelligent social 

actions involving water. 

INTRODUCTION 

A quiet revolution-but a revolution nevertheless-is underway in the use of aquifers to 

meet human needs. Ground water has always 

been a high-quality resource-that is, it has 

commonly been used to meet high-value, highpriority needs such as for domestic and municipal water supply. Thus, even though in most 

highly developed countries ground wr.ter accounts for only a quantitatively mino':" share 

of the total withdrawal of water, its vdue has 

always been recognized as high-even incalculable-because in many places and situations 

there has been no feasible alternative source. 

Now ground-water reservoirs are being 

asked to assume a larger and partly different 

role. As sources of naturally replenishei water 

they are expected-and in many pla~es are 

able-to yield larger amounts of fresh v·ater to 

meet growing needs. But three additional and 

potentially important roles are now foreseen 

for them. One is the use of their vast storage 

capacity to accommodate large quantities of 

surface floodwater, incidentally to redu~e flood 

damage but mainly to hold this water over 

from times of surplus to times of need. Another 

is the use of aquifers for the controlled storage 

of some waste products, as an alternative to 

other uses of these aquifers and t1 other 

methods of waste disposal. A third is withdrawal of saline water for desalination, in areas 

where fresh-water supplies are scarce. 
At the outset, let me make clear that I believe 

firmly in the ability of ground-water reservoirs 

to meet the demands proposed to be put on 

them. The question is not whether they can do 

it, but whether they can do it at a cost which is 

less than that of alternatives and low enough 

that it will not create an undesirable drag on 

the economy. 

Make no mistake about it. The economy of 

the United States, like that of other highly 

developed countries, is built in substantial part 

on inexpensive, readily available water. The 

people expect water to continue to be an abundant, low-cost commodity. Although they realize 

it will cost more than it used to, they are not 

willing for its price to rise high enough to 

cause a real reduction in their ability to buy 

other desired commodities. 

Now I believe intuitively that ground-water 

reservoirs not only can meet the demands proposed to be put on them but can do so at reasonable cost and in many places at lower cost than 

that of alternative measures. But this is just an 

opinion. Between the assumption and the proof 

there is a substantial obstacle. It is that the 

need for information on the characteristics and 

capabilities of an aquifer goes up exponentially 

as the use of the aquifer increases from the 

moderate level associated with laissez-faire development of a part of its naturally replenishable supply to the higher (though, perhaps, not 

inordinately higher) level associated with nearmaximum exploitation of its capacity. 

To pull an example out of the air: It might 

be possible, in a year's time and at a total cost 

of $25,000, to study, and publish a description 

of, the basic ground-water capabilities of an 

area of a thousand square kilometers. The information might be adequate to form a sound 

basis for relatively casual, unplanned development of a total of 1,000 liters per second from 

100 scattered wells, with only local difficulties 

due to inadequate spacing between wells. 

But now, say that it becomes desirable to 

develop all the ground water that the area c~n 

comfortably yield on a long-term basis to a 

particular pattern of wells without artificial 

replenishment. This quantity might prove to 

be, say, 4,000 liters per second. Can this maximum-scale development be planned and managed effectively on the basis of additional 

2 

studies bringing the total cost of ground-water 

investigations to $100,000? No indeed. Very 

likely a total of one to several n1illion dollars 

would have to be spent over a per~od of several 

years to provide a sound factual basis for the 

water-management operations reouired. 

The obstacle is not only financial. It involves 

time than can ill be spared. It involves the 

availability of trained ground-water hydrologists, of whom there is a shortage. And it 

involves hydrologic unknowns which may or 

may not prove to be amenable tc clarification 

at a feasible cost in money, manpower, and 

time. 

Small wonder, then, that there is widespread 

ignorance or uncertainty as to the role of 

ground-water reservoirs in tomorrow's water 

picture. Not enough hydrologic facts are available, and grave deficiencies still exist in the 

ability of hydrogeologists and wzter planners 

to communicate meaningfully witJ· each other. 

THE HYDROGEOLOGIST'S JOB 

What does the hydrogeologist see as the 

deficiencies in knowledge it is his r~sponsibility 

to remedy? At one time he had a rather simple 

job. He mapped the geology of his area, or 

refined mapping done earlier by others. He 

gathered as much information as he could on 

the depth and productivity of wells, the kinds 

of rocks they penetrated, and the kinds of water 

they yielded. He interpreted th~se data in 

terms of the subsurface stratigrapl'Y and structure and of the water-bearing IJ roperties of 

the different geologic units. Ultimately, he 

prepared a report, maps, and chG.rts showing 

where and at what depths water could be obtained, and, in a general way, how much and 

of what quality. For rocks not penetrated by 

enough wells in his area to yield reliable information, he extrapolated infornation from 

other areas where similar rocks were better 

known, according to his experience and his 

familiarity with the literature. 

This procedure was fine for a start. As 

ground-water development progrer~ed and demands became larger, he found it necessary to 

think in quantitative terms about the permeability and storage coefficients of the aquifers and 

the effects on ground-water levels of pumping 

increasing amounts of water from more closely 

spaced wells. 
He tried to keep track of the effects of the 

withdrawals, not only on water levels but in 

diminishing natural discharge, perhaps increas~ 

ing natural recharge, inducing the inflow of 

salt water or other water of undesirable quality, 

and so on. He discovered that it was not only 

water levels that were affected. In certain areas 

he found the withdrawal actually causing the 

land surface to subside. 

Gradually, he began to realize that he was 

dealing with a systern-a geologic-hydrologicchemical-biologic system-in which substances 

and events of nature and the actions of man 

interreacted so complexly that only by understanding how the system operated could he 

predict its response to future events of nature 

and actions of man and, thus, provide a basis 

for controlling the response in desired ways. 

This is where we are today. We are trying 

to define subsurface hydrologic systems in 

terms of their internal characteristics and their 

external boundaries, and to devise models of 

them that will simulate their response to various planned or otherwise anticipated events. 

Repeated tests of the models will reveal the 

fidelity of their simulation of the prototype, 

and will reveal also their sensitivity to data 

inputs of various types and thus serve as a 

guide to data-collection programs. In addition, 

the models will form the physical basis of the 

planner's overall model of the hydrologicsocioeconomic system. 

THE GROUND-WATER SYSTEM 

What is this ground-water system whose 

operation the hydrogeologist must understand? 

It is first of all a geologic system. It consists 

of rocks, consolidated and unconsolidated, 

formed and placed where they are by geologic 

processes. It is a hydrologic system because 

the rocks contain openings that can store and 

transmit water. It is a chemical system because 

the rock skeleton, far from being inert, reacts 

chemically with the water and its dissolved and 

entrained constituents, and in this reaction 

changes are produced in both the rocks and the 

water. It is a biologic system because living 

organisms play an important part in the chemical reactions that take place in it, or at least 

in determining the chemistry of the water that 

enters it. 

The principal aquifers are sedimentary, and 

3 

therefore the controls on their size, shape, and 

position and on their hydraulic properties are 

primarily sedimentologic and stratigraphicprimarily, but not exclusively. It is most important to recognize that it is the water and not 

the stratigraphy that makes the rules so far 

as definition of aquifer-system boundaries is 

concerned. Porosity and permeability may extend right across major stratigraphic l''lundaries. Thu~, an aquifer conceivably could include, for example, Quaternary, Paleozoic, and 

Precambrian rocks and still be a hydrologic 

unit in spite of its stratigraphic heterogeneity. 

On the other hand, a stratigraphically or 

structurally insignificant feature might constitute an important hydraulic boundary or 

conduit. A bed of clay so thin that it could 

hardly be seen in outcrop and would never be 

recognized in drill cuttings might divide a mass 

of permeable sand and gravel into two aquifers. 

A joint, only one of many, might carry more 

water than a major fault, clogged by its own 

clayey gouge. 

The hydrogeologist therefore must "see it 

the way it is," as the currently popular saying 

goes. He must learn to identify the rock 

features that actually control the location and 

movement of subsurface waters in a given 

body of rock, and to define his hydrologic system on their basis rather than in the usual 

stratigraphic terms. 

THE MODEL 

To understand his system the hydroge')logist 

must make a model of it. If the "system" is an 

extremely simple situation, then the model too 

is simple, just a mental picture. The decision 

maker considers two or three alternative actions, visualizing their effects on the system, 

and makes his choice of actions according to 

the desired result and the costs and benefitsall mentally. Such a situation might involve the 

choice among a well, a cistern, and a sohtr still 

to obtain a domestic water supply. The information at hand might be adequate to enable an 

easy choice of one as much less costly tr an the 

others, or feasible where the others are not. 

If the system or situation is a little more 

complicated, involving calculations thz.t cannot be made mentally, it is possible that a 

simple mathematical model will do. For example, the problem might be to predict the 
drawdown to be anticipated if a well is located 

so as to draw, eventually, its water from a 

stream cutting an alluvial aquifer, yet be 

located at a certain minimum distance from 

the stream to achieve adequate filtration of 

the water to remove bacteria. The work of 

C. V. Theis (1941, p. 734), perhaps supplemented by that of J. G. Ferris (Ferris and 

others, 1962, p. 146-151) if the aquifer is 

narrow, forms an adequate basis if the transmissivity and storage coefficient of the aquifer 

can be determined or estimated. 

Few situations are so simple. Generally, 

aquifer systems are complex, replenishment 

and natural discharge rates are variable and 

difficult to determine, there are existing groundwater developments that must be considered, 

there are possible quality complications resulting from waste-disposal practices or the presence of natural bodies of inferior water, there 

are economic or legal ramifications, and there 

is not enough information. Always, there is 

not enough information. 

The problem now is to develop, at feasible 

cost and in feasible time, a model of the system 

-generally, a passive electrical analog or a 

digital computer, or a combination of thesethat will operate like its prototype, and that 

can be tested for its response to the effects of 

various planned actions and for its sensitivity 

to the various kinds of data that must be 

gathered to refine it once its basic character is 

established. Such a model of the hydrologic 

system, supplemented by economic and social 

facts or assumptions, becomes the water 

planner's tool for scientifically correct-or at 

least scientifically generated-decisions as to 

how to attack problems (Skibitzke, 1967). 

THE MOMENT OF TRUTH 

For ground water the moment of truth 

arrives early. In many cases the question is 

not whether ground water can be used, but 

whether it use can even be considered, in view 

of the expense and time required to obtain 

enough information for a reasonably accurate 

assessment of costs, including the cost of side 

effects such as interference with other supplies 

or encroachment of inferior water; and of benefits, including benefits resulting from decisions 

such as that to leave a stream in its natural 

state instead of damming it up. 

4 

For the plain fact is that for too many areas 

we are not able to give the decirion maker the 

facts he needs, in the form in which he must 

have them, for rational decisions and choices 

affecting ground water. In the press to make 

decisions now, he may choose ground water for 

its anticipated-or hoped-for-benefits, later 

to learn that his choice was a co~tly one. After 

one or two such experiences, he n1ay be inclined 

in a new situation to avoid ground waterlater, perhaps, to reach the saine conclusion, 

that his choice was a costly one 1: ~cause ground 

water turns out to have been the preferable 

source after all. 

Another plain fact is that ground-water 

studies cost money, and the mor~ detailed the 

information needed, the higher tl'.e cost. Working with a "marginal" commodity such as 

water, one whose cost is low and is expected 

to continue to be low, it is not at all impossible 

that in a particular situation the cost of a 

study required to prove that gr1und water is 

the most economical source of water for a given 

use might raise the total cost of the development-cost of study plus cost of wells and 

pumps-to or above the total cost of developing 

the next most economical source. 

But let us not downgrade ground water just 

because it is a difficult subject. There are 

situations where its potentialities are very 

clear and very great. Take Long Island, N.Y., 

for example. New York City is an area where 

desalination has been considered ~.s one possible 

means of supplementing the fre~h-water supply in the future. The lowest estiinated production cost of desalted water I can recall is about 

22 cents per thousand gallons ( 5.8 cents per 

m13 ) for the 570,000 m3/day plant to be built in 

southern California, and this ex:Jected cost is 

far less than any that have actually been 

achieved to date. Long Island has ground water 

by the millions of cubic ~eters per day. The 

recharge and natural discharg~~ amount to 

perhaps more than 1 million gallons per day 

per square mile ( 1,460 m3 I day /km2 ) , or well 

over 1 billion gallons (3.8 million m3 ) per day 

for the 3,600-km2 island (Jacob, 1945, p. 938). 

A large proportion of the natural discharge 

must be allowed to continue, to r0ld back salt 

water. Also, more than a million cubic meters 

per day is already being pumped. On the other 
hand, some of the water is or can be used n1ore 

than once-that is, used, returned to the 

ground, and pumped and used again. 

Without presuming to suggest that large 

amounts of Long Island water are available for 

the public supply of New York City (for the 

island's water by and large will be developed 

to meet island needs) , it can be pointed out 

that under carefully controlled conditions 

another million cubic meters per day or more 

of additional water could be developed, much of 

it at a well-head cost of a· cent or two per 

cubic meter. Here, then, is a place where 

existing knowledge is sufficient to point to 

ground water as currently a cheaper source 

than at least one proposed alternative. The 

additional knowledge required to make these 

additional volumes of water become real will 

cost several million dollars to obtain-but the 

difference between 22 and, say, 5 cents per 

thousand gallons (6 and 1.3 cents per m3 ) 

amounts to more than a million dollars per 

year for each hundred thousand cubic meters 

per day. 

The title of this paper asks a question, but 

one that seems rhetorical. Of course, scientific 

decisions are to be preferred to rule-of-thumb 

decisions. But the Long Island example just 

described is a rule-of-thumb judgment that 

large quantities of ground water are available 

at a cost low enough to justify easily the expenditure for studies needed to provide a scientific basis for the management actions involved 

in obtaining the additional water. Other similar 

situations could be cited, many of them in 

the water-rich Coastal Plain of which Long 

Island is one of the northern outliers. Example : 

Florida, our most permeable State where a 

high proportion of 1,350 mm of rainfall goes 

underground each year. Example: Mississippi, 

where 991/2 percent of the public water-supply 

systems use wells but where the remaining 

ground-water potentialities greatly outweigh 

the existing developments. Example: Southeastern Louisiana north of Lake Pontchartrain ' where millions of cubic meters per day of soft, 

good-quality ground water awaits development. 

There is even water to be had in our driest 

State, Nevada, where ground water . in only 

four of more than 100 major alluvial vaileys can 

be considered fully developed or overdeveloped 

5 

(McGuinness, 1963, New York, Florida. Mississippi, ~ouisiana, Nevada sections). By 

choosing the best areas, we could undoubtedly 

develop several times the current total of about 

60 billion gallons per day (about 225 nillion 

m3 /day) now withdrawn in the United ftates 

and do it with only modera.te effect on eYistin~ wells and on streams. 

WHAT ELSE DO WE NEED TO KNOW? 

It appears that rule-of-thumb evaluations 

such as the above have their place. At least, 

they tell us that large quantities of ~~round 

water are available, and where. But they are 

only the beginning. It is not enough to know 

that water is available. To produce it for use 

costs money-tax money, or business or personal income-and those spending the money 

have both the desire and the obligation to see 

that it is spent wisely. So, even in ar~as of 

k~own abundant supplies and little pmnping, 

scientific information is needed as a basis for 

decisions. 

And the needs are here and now. The persistent increase in water demands ar<l the 

desirability of using ground-water reservoirs 

to supplement variable and increasingly expensive surface-water supplies and for storage of 

wastes mean that the detailed questions are 

being asked now, before we are ready to f.nswer 

them. Thus, many rule-of-thumb decisions are 

having to be made and will have to be made in 

the future, though there will be situations 

when any decision about ground water ,.vill be 

considered too risky until additional information becomes available. To avert the tragedy 

of misuse or nonuse of the ground-water reservoirs for lack of enough information to make 

rational decisions, every effort must be made 

to close the information gap as rapidly as 

possible. 

A two-pronged attack is called for. Or~ is to 

accelerate as rapidly as possible the coverage of 

areas of interest by reconnaissance or slightly 

more detailed ground-water studies. Such 

studies provide enough information for planning and undertaking many uses of ground 

water, and they are relatively straightfo""ward. 

About half the area of the conterminous United 

States plus all of Hawaii (but only a minor 

fraction of Alaska) has been covered by such 

studies. 
On the other hand, there are not enough 

trained hydrogeologists to complete the studies 

in time to avoid the risk of many incorrect 

decisions and choices. This means that increasing emphasis must be put also on the second 

prong-research. Research is the means by 

which techniques are developed for (1) improving, accelerating, and reducing the cost of the 

areal studies; and (2) learning how to extrapolate information from one area or situation 

to another on the basis of fundamental principles, instead of on the basis of empirical measurements that may prove to have little transfer 

value. 

Financing and staffing a ground-water research effort is no less a problem now than it 

was in 0. E. Meinzer's day, when he complained 

(Meinzer, 1934, p. 30-31) that the urgent demands for information on ground water in 

specific areas made it difficult to devote even 

a small part of the total effort to the research 

that might, in the end, prove to be worth ten 

or a hundred times as much as an equivalent 

expenditure on routine areal studies. Then, 

there were only a few dozen hydrogeologists in 

the United States. Now, there are more than a 

thousand, but the information needs have 

grown even faster than the number of groundwater specialists. 

Hence, it becomes necessary to pick and 

choose carefully among the many topics on 

which research might be done, in order to 

concentrate on those aspects that would :nneet 

the most urgent needs and promise the greatest payoff. A recent paper points out some of 

these (McGuinness, 1967; see also McGuinness, 1964; Hackett, 1966). 

ARTIFICIAL RECHARGE 

Natural replenishment of ground water in 

the conterminous United States amounts to an 

average of a few inches per year over the area 

of 3 million square miles-say, 3 inches, or 75 

millimeters, over 8 million square kilometers. 

This is enough that ground-water discharge 

accounts for perhaps one-third to two-fifths 

(W. B. Langbein, oral commun., 1950) of the 

total streamflow, which averages 81f2 inches 

(215 mm) per year (Langbein and others, 1949, 

p. 5; see also Busby, 1963). 

This natural ground-water recharge represents an enormous amount of water, on the 

6 

order of 400 billion gallons per day (11f2 billion 

m~/day). But in many areas it w~uld be 

desirable to increase greatly the natural rate, 

either to prevent the depletion of a valuable 

aquifer or to store surplus flood">vater for later 

use. 

Artificial recharge is practiced on a rather 

large scale in California and to a much smaller 

extent elsewhere in the United States. The total 

rate is not known, but at present it is on the 

order of several hundred millio:rt cubic meters 

per year, or perhaps even a th'.1usand million 

cubic meters per year or more-a cubic kilometer! This is a respectable an1ount of water, 

but if ground-water reservoirs are to do what 

is expected of them, the total y•ill have to be 

increased to several tens of cubic kilometers 

per year. 

It will not be easy to reach ther~ totals. Under 

favorable conditions, recharge by spreading 

surplus water over permeable alluvial fans 

can be done for a dollar or a few dollars per 

thousand cubic meters. (See paners annotated 

by Todd, 1959, such as that by Clyde, 1951.) 

But such favorable conditions a~e absent in 

many places where artificial recl'arge would be 

desirable. In such places, artificial recharge at 

a substantial rate will be either impossible, if 

suitable aquifers are missing, o1· at least relatively expensive, if the water must be injected 

through basins occupying valuable land, or 

through wellR. Recharge through wells could 

easily cost a cent to a few ce:nts per cubic 

meter, which might make the practice unfeasible for storing large quantities of floodwater 

during the brief time that it is available. 

The need for, and the promise of, artificial 

recharge are so great as to justjfy substantial 

expe:;.ditures for both research on techniques 

to reduce the cost and on arf~al studies to 

locate and evaluate aquifers suitable for recharging. This is one of the highest priority 

needs in the field of ground-water hydrology. 

PERMEABILITY DISTRIBUTfON 

Prediction and, then, manag~ment of the 

operation of hydrologic system~· require that 

suitable models be devised. That such models 

can be devised is due to the fact. that the two 

hydraulic parameters controlling ground-water 

flow-permeability and storage: coefficient--

can be measured quantitatively by means of 
pumping tests and other techniques. However, 

we can never afford as many test holes and 

pumping tests as would be desirable to determine permeability distribution with the desired accuracy. Research is critically needed, 

therefore, on the principles of sedimentation, 

and of other determinants of permeability, that 

will enable extrapolation and interpolation of 

scattered point data in such a way as to form 

a model more realistic than one based on simple 

proportioning of values between points. Some 

research is underway, including that by R. R. 

Bennett, P. M. Brown, and their colleagues in 

the Geological Survey, but much more is needed. 

VERTICAL PERMEABILITY 

Cross bed permeability ("vertical" permeability because most aquifers are sedimentary 

and most are nearly horizontal) is important 

to long-term predictions of aquifer response 

because of leakage of water to, from, or between aquifers through beds of low permeability. Yet, no economical technique of measuring vertical permeability has been devised. R. 

W. Stallman and his colleagues in the Geological Survey have been trying a number of 

new techniques to get around the time, expense, 

and uncertainties involved in measurements of 

actual vertical flow under field conditions. Some 

of these techniques involve measurements of 

pressure and temperature with sensitive instruments. The pressure measurements reflect 

the vertical permeability of fine-grained materials to air, which can be interpreted in terms 

of water permeability. The temperature measurements show the movement of water at rates 

so low that direct measurement would be 

difficult. 

STORAGE COEFFICIENT 

Besides permeability (or, rather, transmissibility, or transmissivity-the overall permeability of a whole aquifer), the parameter essential to flow prediction is the storage coefficient 

-the quantity of water that comes out of, or 

goes into, storage when the ground-water head 

is lowered or raised. The Theis equation for 

nonsteady-state flow (Theis, 1935) is based on 

heat-flow theory, in which it is assumed that 

heat comes out of, or goes into, storage instantaneously with a change in temperature. 

For simplicity the Theis equation assumes that 

7 

water behaves in the same way, but in reality 

it does not; a perceptible time is required for 

water to flow out of, or into, pores, sc"'lle of 

which are very small. So far, there is no 

practical way of predicting and allowing for: 

this delay, which is important when the equation must be solved for very short periods of 

withdrawal or recharge to avoid interference 

from boundary effects-for example, the presence of a recharging stream or imperneable 

bedrock wall not far from the test well. At 

present, the principal research underv,.ay to 

try to "get a handle" on this problem is that 

by J. F. Poland and others, in a project entitled 

"Mechanics of Aquifers," which is ained at 

the causes of land subsidence due to withdrawal 

and application of irrigation water in the San 

Joaquin Valley, Calif. Work being done by 

Rubin (1968) also is of interest. 

UNSATURATED ZONE 

The unsaturated zone, or zone of aeration, 

has a host of unsolved problems. Water movement in it is more complex than in the saturated · 

zone, principally because the flux occurs in 

both liquid and vapor phases and because the 

permeability changes with moisture c'1ntent. 

There is not space here to do more than point 

out that there is need for a theory of flovr based 

on fundamental characteristics of eartr materials that can be measured at reasonatle cost 

in time and money. Flow measurements at 

present are mainly empirical and of limited 

transfer value because it is not even known 

which soil characteristics are the important 

controls of flow. An unusually impressive justification for research on the unsaturaocrl zone 

is that it is largely in that zone that possibilities 

exist for reduction of the huge loss of water 

by evapotranspiration. A small percentage reduction in that loss would add many millions of 

cubic meters of water per day to the world's 

available supply. Also, this zone is of crucial 

importance in both natural and artificial recharge. 

GEOCHEMICAL PROBLEMS 

There are many important research targets 

which basically are chemical--effects of injection of water (or wastes) into an aquifer 

whose water chemistry is substantially different 

from that of the injected fluid, acid mine drain-
age, and evaluation of saline water as both a 

potential resource for desalting and a threat to 

fresh-water supplies in coastal areas. Desalination of salty water is already practical in some 

areas of high water cost, and it is bound to 

become more important. Studies of saline 

ground waters are even more expensive than 

those of fresh water, but they must be made 

nevertheless. 

GEOPHYSICS 

Both surface and borehole geophysical techniques include important research targets. Both 

serve to extend at relatively low cost the results of test drilling, a critical need in view of 

the low unit value of water and the desirability 

of obtaining hydrologic data at minimum cost. 

The borehole work is especially promising in 

this regard. Among recent developments are 

some in which hydrologic knowledge contributes significantly to the understanding of 

seismic phenomena (Cooper and others, 1965) 

-an interesting departure from the common 

situation where geophysical knowledge developed in another field, such as petroleum 

geology, is adapted for hydrologic use. 

CONCLUSION 

The list of research needs in ground-water 

hydrology and subsurface hydrology in general 

could be extended and detailed indefinitely. 

The field by no means is being neglected. In 

the United States some 600 research projects 

are underway in "subsurface hydrology~'­

ground water, water in soils, water and plants, 

and ground-water management-and several 

hundred more are closely related to the subsurface in some way. But the needs for both 

research and areal hydrologic descriptions are 

still great. It has yet to be confirmed that groundwater reservoirs are worth all this effort, but 

at least the hundreds of scientists engaged in 

these projects in the United States, and the 

hundreds more in other countries, are convinced that they are. By the time of the next 

Geological Congress the outcome should be 

less in doubt. I, for one, am among those who 

think that the outcome will be favorable. I 

believe that, little l>y little, the balance will. be 

swung in favor of t;cientific rather than ruleof-thumb techniques of answering questions 

8 

about ground water-questions that are among 

the most important now being put to man in his 

struggle to control his environment. 

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Inv. Atlas HA-212. 

Clyde, G. E., 1951, Utilization of natural underground 

water storage reservoirs: Soil and Water Conserv. 

Jour., v. 6, no. 1, p. 15-19. 

Cooper, H. H., Jr., Bredehoeft, J. D., Papadopulos, 

I. S., and Bennett, R. R., 1965, The response of 

well-aquifer systems to seismic waves: Jour. 

Geophys. Research, v. 70, no. 16, p. 3915-3926. 

Ferris, J. G., Knowles, D. B., Brown, R. H., and Stallman, R. W., 1962, Theory of aqnifer tests: U.S. 

Geol. Survey Water-Supply Paner 1536-E, p. 

69-174. 

Hackett, 0. M., 1966, Ground-water research in the 

United States: U.S. Geol. Survey Circ. 527, 8 p. 

Jacob, C. E., 1945, Correlation of gro•,nd-water levels 

and precipitation on Long Island, N.Y., Pt. 2, 

Correlation of data: Am. Geophy~- Union Trans., 

1944, pt. 6, 928-939. 

Langbein, W. B., and others, 1949, Annual runoff in the 

United States: U.S. Geol. Survey Circ. 52, 14 p. 

McGuinness, C. L., 1963, The role of ground water in 

the national water situation, with State summaries based on reports by district offices of the 

Ground Water Branch: U.S. Geol. Survey WaterSupply Paper 1800, 1121 p. 

---1964, Ground-water research of the U.S. Geological Survey: U.S. Geol. Survey Circ. 492, 7 p. 

---1967, Ground-water research in the U.S.A.: 

Earth Sci. Rev., Amsterdam, T'-e Netherlands, 

Elsevier Pub. Co., no. 3, p. 181-202. 

Meinzer, 0. E., 1934, The history and development of 

ground-water hydrology: Washington [D.C.] Acad. 

Sci. Jour., v. 24, no. 1, p. 6-32. 

Rubin, Jacob, 1968, Theoretical analys~s of two-dimensional flow of water in unsaturr-.ted and partly 

unsaturated soils: Soil Sci. Soc. America Proc. 

(in press). 

Skibitzke, H. E., 1967, New horizon~ in hydrologic 

studies: Indiana Univ. Water Res')urces Research 

Center, Conf. on Water Geology and the Future, 

Bloomington 1964, p. 109-113. 

Theis, C. V., 1935, The relation between the lowering of 

the piezometric surface and the rate and duration 

of discharge of a well using ground-water storage: 

·Am. Geophys. Union Trans., 1935, pt. 2, p. 519-524. 

---1941, The effect of a well on the flow of a 

nearby stream: Am. Geophys. Union Trans., 1941, 

pt. 3, p. 734-737. 

Todd, D. K., 1959, Annotated bibliogra:ohy on artificial 

recharge of ground water through 1954: U.S. Geol. 

Survey Water-Supply Paper 1477, 115 p. 

-tr U.S. GOVERNMENT PRINTING OFFICE: 1378 0- 261-221!35