The Effect of 

Artesian-Pressure Decline 

on Confined Aquifer 

Systems and its Relation 

to Land Subsidence

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1779-T
The Effect of 

Artesian-Pressure Decline 

on Confined Aquifer 

Systems and its Relation 

to Land Subsidence

By J. H. GREEN

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1779-T

An appraisal of the relation of 

compaction of confined aquifer 

systems to porosity and the 

coefficient of storage

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402
CONTENTS

Page

Abstract_______._______________________________ Tl

Introduction ______________________________________________________ 1

Aquifer compaction ________________________________________________ 2

Effects of compaction on artesian-aquifer systems._____________________ 5

Porosity________________________________________________________ 5

Ground-water storage.___________________________________________ 10

References________________________________________________________ 11

ILLUSTRATIONS

Page 

FIGURE 1. Areas of land subsidence in California caused by decreased

artesian pressure._____________________________________ T2

2. Computed compaction and actual subsidence at core hole

7S/1E-16C6 and hydrograph of nearby water well __________ 3

3. Computed compaction and actual subsidence at core hole

6S/2W-24C7 and hydrograph of nearby water well. _______ 4r

4. One-dimensional consolidation and rebound curves and method

of computing An from effective load and void ratio________ 6

TABLES

Page 

TABLE 1. Porosity decrease in core hole 7S/1E-16C6, computed from

typical consolidation curves___________________-__-----_ T7

2-4. Porosity decrease in core holes, computed from typical consolidation curves:

2. Core hole 6S/2W-24C7___-______-_-__-__---------- 7

3. Core hole 16/15-34N1______________________ 8

4. Core hole 23/25-16Nl___--___-___-_--_____-__------- 8

5. Porosities of samples from core holes in the Santa Clara Valley. 9

m
CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

THE EFFECT OF ARTESIAN-PRESSURE DECLINE ON

CONFINED AQUIFER SYSTEMS AND ITS RELATION TO

LAND SUBSIDENCE

By J. H. GKEEN

ABSTRACT

Ground water in the Southwestern United States is derived chiefly from 

unconsolidated to semiconsolidated alluvial deposits. Where these deposits 

contain confined water, they may be susceptible to compaction and related landsurface subsidence, if artesian pressures are reduced.

Compaction of artesian-aquifer systems can be estimated from core tests if 

the artesian-pressure decline is known. Compaction occurs chiefly in the finer 

grained deposits; porosity decrease is greater near the top of the confined aquifer 

than near the bottom.

Because most of the compaction of these aquifer systems is permanent, the 

storage coefficient during the initial decline of artesian pressure greatly exceeds 

the storage coefficient during a subsequent pressure decline through the same 

depth range, after an intervening period of pressure recovery.

INTRODUCTION

Throughout the Southwestern United States, ground-water resources are derived chiefly from unconsolidated and semiconsolidated 

alluvial sediments. Owing to the nature of alluvial deposition, the 

ground-water reservoir is likely to contain one or more extensive and 

thick fine-grained beds that confine or partially confine underlying 

aquifers and form a confined aquifer system.

If artesian pressure is reduced, the resultant increase of grain-tograin load in the aquifer system may cause compaction of deposits 

and an equal subsidence of the overlying land surface. The subsidence at any given location is related to the subsurface lithology and 

to the magnitude and duration of pressure decline, and it appears to be 

mostly inelastic and permanent. Careful releveling of surface bench 

marks is adequate to detect subsidence of tenths of a foot.

Figure 1 shows the areas of California where land subsidence is 

known to have resulted from the lowering of artesian pressure.

Tl
T2 CONTRIBUTIONS TO THE HYDEOLOGY OF THE UNITED STATES

FIGURE 1. Areas of land subsidence (solid black) in California caused by decreased 

artesian pressure. Dotted line outlines Central Valley.

Eight core holes have been drilled in California to study the causes 

of aquifer compaction and land-surface subsidence: four in the Los 

Banos-Kettleman City area, two in the Tulare-Wasco area, and two 

in the Santa Clara Valley (fig. 1). The core holes are from 760 to 

2,200 feet deep, and most pentrate the full thickness of sediments 

tapped by wells in the artesian-aquifer systems.

Core samples for laboratory testing were taken at predetermined 

intervals. Tests included consolidation characteristics, physical and 

hydrologic properties, and petrographic analysis. The consolidation 

testing, from which data were gathered for this report, was done by 

the Earth Laboratory of the U.S. Bureau of Reclamation, Denver, 

Colo.

AQUIFER COMPACTION

Analysis of consolidation-test data and the known artesian-pressure 

decline in the same locality provide a means for computing aquifer 

compaction and land-surface subsidence. The method of computing 

aquifer compaction from consolidation tests and artesian-pressure 

decline has been described by Miller (1961). The following two examples illustrate the correspondence between actual land-surface subsidence and the computed aquifer-system compaction in the Santa 

Clara Valley, described in more detail elsewhere (Green, 1962).
ARTESIAN-PRESSURE DECLINE AND LAND SUBSIDENCE T3

Figure 2 shows the computed compaction and actual subsidence 

at core hole 7S/1E-16C6 and the hydrograph of a nearby water well. 

A comparison of actual subsidence to the water-level change shows 

that aquifer compaction was most pronounced at times when artesian 

pressure was declining and was lower than in any prior period. Compaction progressed rapidly between 1915 and 1934 and between 1948 

and 1960.

From 1948 to 1951, rapid artesian-pressure decline at core hole 

7S/1E-16C6 was accompanied by a rapid rate of subsidence. After 

1951, the artesian pressure fluctuated widely but continued a slow 

decline. The lack of bench-mark leveling in 1951 prevents a shortterm analysis of pressure change and subsidence. However, compaction computations indicate a quick response of subsidence to pressure decline, and the sinking of bench mark A149 from 1948 to 1960 

probably is more irregular than is shown in figure 2.

Subsidence stopped or slowed considerably when artesian pressure 

(either rising or falling) was above the prior low (fig. 2). Between 

the middle 1930's and 1943, when artesian pressure was increasing, 

subsidence was extremely slow compared to the period before 1934, and 

may have ceased entirely by 1940.. The subsidence net was not releveled between 1940 and 1948, and the change in elevation of bench 

mark A149 in that critical period is not known. However, the cessation of subsidence by 1940 and the subsidence of 0.16 foot between 

1940 and 1948, interpreted in relation to artesian-pressure change, 

suggest stability from 1940 to 1947 and the renewal of subsidence in 

1947. This interpretation would imply that compaction of the aquifer 

system was complete for the artesian pressure and grain-to-grain load 

of 1940, and did not resume until the piezometric surface was drawn 

down below the 1940 level (in 1947).

AND COMPUTED DENCE 

DMPACTION, IN FEET

£* jcrtui r\> wc>-*

CO O

on

= 8

(ft

9 

191

^V^

^

^

^V]«// ^v \
y VNS^

\

^ >

NC/ \

,x\

 ^

Comput

 -

A

/

i

ed com

 . 

\

\

\

~Tx

V

>3ction -1

- 

VVX

V *

A A

~~"-:-

b

>

25

45 

65 

85 

105 

125 

145 

165

2 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965

FIGURH 2. Computed compaction and actual subsidence at core hole 7S/1B-16C6 

hydrograph of nearby water well.

as

^S

2t

and
T4 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Computed compaction for the 45-year period between 1915 and 

1960 was 7.56 feet and the actual subsidence was 7.42 feet; computed 

compaction agrees closely with actual subsidence.

Figure 3 shows the computed compaction and actual subsidence 

at core hole 6S/2W-24C7 and a hydrograph of a nearby water well.

A definite period of time is required for fine-grained sediments 

to compact and adjust completely to increased loading conditions. 

According to Terzaghi (1943), the time required for complete adjustment is proportional to the square of the thickness of the compacting layer. In beds of silty clay and clay, the rate of compaction 

depends upon the rate at which water can be squeezed from the pore 

spaces. Thick clay beds may require hundreds of years to adjust, but 

thinner beds may need only a few tens of years or less. Because 

consolidation tests for core hole 24C7 indicated a large amount of 

lagging long-term compaction, figure 3 also shows a plot of computed 

compaction less lag. Residual compaction (and the resultant subsidence lag) is the potential compaction remaining in the consolidating 

sediments at any given time after the effective stress (grain-to-grain 

load) has been increased. In figure 3, compaction lag is represented 

by the vertical distance between the lines showing "computed compaction, less lag" and "computed compaction, total."

Computed compaction, less lag, for the 1915-60 period was 10.95 

feet and the actual subsidence was 10.98 feet. (This degree of agreement should not be expected for all subsidence computations, but a reasonable approximation should be within 30 percent of the actual 

amount.) In both core holes, compaction as computed over long 

periods is more reliable than compaction computed for short periods. 

The longer periods allow more time for the accumulated residual 

compaction to occur. The relation of actual subsidence to the hydrograph shown in figure 3 is about the same as that shown in figure 2 for 

core hole 7S/1E-16C6.

1920 1925 1930 1935 1945 1950 1955 1960 1965

FIGURE 3. Computed compaction and actual subsidence at core hole 6S/2W-24C7 and 

hydrograph of nearby water well.
ARTESIAN-PRESSURE DECLINE AND LAND SUBSIDENCE T5

It is concluded that laboratory consolidation tests and known 

artesian-pressure declines can be used to estimate aquifer compaction 

and the resultant land-surface subsidence.

EFFECTS OF COMPACTION ON ARTESIAN-AQUIFER

SYSTEMS

As aquifer systems are reduced in volume by compaction due to 

artesian-pressure decline they undergo changes that influence their 

ability to store water: Porosity is reduced, and, if the aquifer system 

experiences more than a single stage of artesian-pressure decline 

through the same range, the storage coefficient during the first decline 

greatly exceeds that during subsequent declines.

POROSITY

When unconsolidated granular deposits are subjected to increased 

effective stress, rearrangement and deformation of mineral grains 

reduces the volume of the voids. The result is a porosity decrease 

in each compacting zone in the aquifer system.

The consolidation-test data and known artesian-pressure declines 

permit estimation of porosities at selected magnitudes of pressure 

decline. For compaction and porosity computations, the aquifer system is divided into relatively uniform lithologic zones by use of an 

electric log. Each zone is represented by the consolidation-test sample 

considered to be most typical of that zone. Consolidation-test results 

are plotted by the testing laboratory as graphs of void ratio versus load 

(load plotted on logarithmic scale). From these graphs, void ratio 

may be read directly for any value of applied load, or for any effective load on any part of the aquifer system. The approximate porosity 

decrease of any zone is computed from the relationship:

where An=porosity change,

e:=void ratio prior to load change, and 

e2 =void ratio after load change.

The average porosity decrease of the aquifer system undergoing compaction is the weighted average of the porosity decreases for all compacting zones.

Figure 4 shows a typical plot of void ratio versus log load and 

illustrates the method of computing An. Void ratio for a particular 

effective stress ("load" in fig. 4) can be determined more accurately 

by replotting the straight line segment of the curve on arithmetic
T6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES 

0.95

0.85

0.75

0.65

0.55

0.45

0.35

0.25

0.464 0.442

1.464 1.442 

A»= 0.317-0.307 = 0.010=1.

1 10 100 

LOAD, IN POUNDS PER SQUARE INCH

1000

FIGURE 4. One-dimensional consolidation and rebound curves and method of computing 

AM from effective load and void ratio.

coordinates to a larger scale. However, when the void ratio (or porosity) of a compacting zone several tens of feet thick is estimated 

from a laboratory consolidation test on one sample about 1 inch thick, 

the accuracy of the estimate is dependent on how representative the 

sample is of the entire zone. Obviously, in alluvial deposits, the 

estimate for the whole zone is only a rough approximation, and the 

reader should keep this in mind.

Table 1 shows the porosity decrease from 1915 to 1960 in compacting zones in core hole 7S/1E-16C6 in the Santa Clara Valley (fig. 

1), as computed from the laboratory consolidation tests and the 

change in artesian pressure. Because compaction occurs chiefly in the 

fine-grained sediments and is small to negligible in the sand and gravel 

zones, porosity decrease is shown only for the silt and clay zones. 

Porosity decrease ranges almost uniformly downward from 1.5 percent in the 245 to 335-foot zone to 0.2 to 0.3 percent near the bottom of
ARTESIAN-PRESSURE DECLINE AND LAND SUBSIDENCE T7

the core hole. Average porosity decrease for all compacting zones is 

0.9 percent. The analysis shows that decrease is greater in the upper 

zones than in the lower.

TABLE 1. Porosity decrease, in percent, in core hole 7S/1E-16C6, computed 

from typical consolidation curves

Zone depth (feet)

245-335 -----

335-390.... 

390-420 . ____ _________ -- 

420-435

435-475_ .- 

475-580 

580-670 

670-750. - 

750-785 

785-815 _____ . _ 

815-865... ____ -- - 

865-895.... __ -- 

895-930.. __ _ _____ -.- _ - _ _ _ . . -_-

930-1,000 _ .. 

1915

35.1

30.4

34.4

31.8

31.7

32.6

25.9

31.3

34.4

30.9

34.3

32.5

Porosity

1960

33.6

29.1

33.2

30.7

30.7

31.9

25.4

30.7

34.2

30.4

34.0

31.6

Decrease

1.5

1.3

1.2

1.1

1.6

.7

.5

.6

.2

.5

.3

.9

Computations based on soil mechanics theory indicated that most 

compaction at core hole 7S/1E-16C6 would occur within a few years 

after the reduction of artesian pressures and that residual compaction 

would be minor. Therefore, porosity decreases in table 1 are considered as approximate actual values for the period from 1915 to 1960.

Table 2 shows porosity decrease from 1915 to 1960 in compacting 

zones in core hole 6S/2W-2-1C7, also in the Santa Clara Valley. 

Again, porosity decrease is greatest at shallow depth and least near 

the bottom. At this core hole, computations based on data from the 

time-consolidation curves indicated an appreciable amount of residual 

compaction. The average porosity decrease of 1.2 percent for the 

compacting section represents the porosity decrease at a future time, 

if and when all residual compaction, due to the 1915-60 pressure 

decline, is accomplished.

TABLE 2. Porosity decrease, in percent, in core hole 68/2W-24C7, computed 

from typical consolidation curves

Zone depth (feet)

185-215

215-415 

415-550... ___ ------ .

550-680--. 

680-800- __ --. .-- .---

800-905--.- 

905-1,000 _______ ...

1915

35.9

37.8

32.7

37.0

37.9

37.2

ofi 7

1960

34.1

35.9

31.5

36.9

35.5

Decrease

1.8

1.9

1.2

.9

.9

.8

.6

1 O

Porosity
T8 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Tables 3 and 4 show computed porosity decrease in core holes 

16/15-34N1, in the Los Banos-Kettleman City area, and 23/25-16N1, 

in the Tulare-Wasco area. These areas are in the San Joaquin Valley, 

Calif, (fig. 1). Here also, compaction of the artesian-aquifer system 

reduces porosities more at shallow depth than near the bottom. At 

these locations, compaction computations (R. E. Miller and B. E. 

Lofgren, written communication, 1961) indicate that residual compaction is minor. Porosity decreases shown in tables 3 and 4 are 

considered actual values for the periods shown.

TABLE 3. Porosity decrease, in percent, in core hole 16/15-34N1, computed from 

typical consolidation ciirves

Zone depth (feet)

637-700.... ... ___ .... _ _. 

700-750....... ___ . ___ ....... __ ..........................

750-772........................................................

772-795 

795-842_____ __ _________________ ________________________

842-895-... _-__._- _.____-___ _.________________._.____

895-907- _ ___ _ ___ _ _--- ------ _ -.- _____ - _ __..

907-940 ___ __ -

940-980. _ - _ - ___ -._- ___ ...-_--.__..-.. ___ _____ _ __

980-1,130 _ 

1,130-1,200- _ ___ - __ ------- _ . .-- ....__. .-- ___ ....._

1,200-1,375--.. 

l,375-l,465_-_______ - ___ __

1,465-1,549. __. _._- 

1,549-1,720 

1905

48.4

33.7

40.6

40.8

33.0

40.4

32.8

42.0

30.5

40.6

31.9

35.0

39.2

37.5

Porosity

1955

47.2

32.4

39.4

39.6

3L9

39.3

31.7

40.7

29.6

40.3

31.0

34.4

38.7

36.7

Decrease

1.2

1.3

1.2

1.2

1.1

1.1

1.1

1.3

.9

.3

.9

.6

.5

.8

TABLE 4.- -Porosity decrease, in percent, at core hole 23/25-16N1, computed from 

typical consolidation curves

Zone depth (feet)

296-330-.... _ -. __ -.-- _____ - ________ ... .. _ ._

330-363._.___ _ _ 

363-420.. _ ........ __ 

420-615...... .- ____ ---. ________ ._ ___ .. ___ .._.

615-660-...- _ . .- __ ........ 

660-673... _-_ _. . __ . _ .. _ _.____.

673-707---- 

707-760 _ -_ _ . . __ ____ _ ...

1905

43.2

43.5

41.9

40.8

39.4

40.1

39.0

41.5

1961

40.5

40.8

39.8

38.7

37.5

38.3

37.1

39.4

Decrease

2.7

2.7

2.1

2.1

1.9

1.8

1.9

2.1

Consolidation tests (fig. 4) show that the relation of void ratio to 

load, on a semilogarithmic plot, is approximately linear after the 

preconsolidation load is exceeded. As loading progresses, more load 

is required to produce an equal decrease in void ratio and in porosity. 

Sediments near the bottom of the section, which have been under 

greater natural overburden load than those near the top, would require a greater change in effective load to produce uniform porosity 

decrease throughout the compacting aquifer system. Where residual 

compaction is not a factor, porosity reduction is greatest near the top
ARTESIAN-PRESSURE DECLINE AND LAND SUBSIDENCE T9

of the compacting aquifer system, because decreased artesian pressure 

causes a uniform increase of effective grain-to-grain load on the entire 

column of compacting sediments.

The rebound curve in figure 4 indicates an increase of void ratio, 

and consequent increase of porosity, when the sample is unloaded. 

When samples are taken from loaded conditions at depth, field effective stress is removed and they expand.

Laboratory porosity determinations made at, or near, atmospheric 

pressure reflect the unloading effect and indicate higher porosities 

than exist in the sediments at depth. Table 5 shows a comparison 

of porosities for individual depth zones of relatively uniform lithology 

determined by two methods: (1) from the void ratio of a representative sample loaded in a consolidometer, and (2) from averages of 

porosity values for samples at one or more depths in the zone, tested 

at atmospheric pressure. In every case the average porosity for the 

samples tested at atmospheric pressure is higher than that for the 

sample tested under load. The average difference for the two methods 

is 5.1 percent for the 17 zones listed in table 5.

The above comparison shows a need for all porosity determinations 

to be adjusted to pressures equal tojhose of the natural field conditions 

by means of consolidation-test data.

TABLE 5. Porosities, in percent, of samples from core holes in the Santa Clara

Valley

Zone depth (feet)

Porosity

From

consolidation tests J

From samples 

tested at 

atmospheric 

pressure 2

Difference 

<percent)

Core hole 6S/2W-24C7

185-215. ___ ... ____ . _ . _ . _ ..... ___ ..... .... .....

215-415.- _ - ___ -_ _ _

415-560... __ ........ ___ ._ _ ... . 

550-680. ___ . _______ .... _ - _ - _ . ___ ..

680-800.... ........ ____ _ ..... ...

800-905... _ _

905-1,000 

34.1

35.9

31.5

36.1

37.0

36.4

36.9

38.8

38.1

38.8

36.6

40.3

38.1

42.5

4.7

2.2

7.3

.5

3.3

1.7

5.6

Core hole 7S/1B-16C6

245-335.. __ _ ..... _ ........

335-390.. ___ 

390-420....- ........... ..... ... ...............

420-435. ___ _ 

475-580 __ 

670-750....- 

785-815. __ 

815-865. _____ -.--. _ . _ --------------- __ ..... ........

895-930 _ - - - 

930-1,000 _ . ____ ............

33.6

29.1

33.2

30.7

30.7

31.9

25.4

30.7

30.4

34.0

39.4

36.2

43.0

35.7

33.1

33.2

39.0

36.0

34.8

40.8

5.8

7.1

9.8

5.0

2.4

1.3

13.6

5.3

4.4

6.0

1 Porosity as of 1960 from field effective stress and consolidation curves, Earth Laboratory, U.S. Bur. 

Reclamation.

2 Average porosity of zone from samples tested at Denver Hydrologic Laboratory, U.S. Qeol. Survey.
T10 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

GROUND-WATER STORAGE

The amount of compaction of the aquifer system represents an 

equivalent reduction in pore space and in stored water. The volume 

of land subsidence caused by compaction, both inelastic and elastic, 

is equal to the volume of pore-space reduction. Therefore, the magnitude of land subsidence at any locality can be used to determine the 

approximate coefficient of storage. Although the gross storage coefficient, $, equals SW +SC , the volume of water produced by water expansion, /Sw, can be ignored in highly compressible aquifer systems because 

this volume is negligible compared to that produced by porosity reduction. At any locality, the component of storage derived from compaction of the artesian aquifer, JSC is equal to the amount of land 

subsidence (feet of water produced) divided by the reduction of artesian head, in feet (Poland, 1961, p. 53).

In the Santa Clara Valley, two periods of large-scale reduction of 

artesian pressure have occurred since 1915. The magnitude of subsidence near the two core holes is known for both these periods. 

Analysis of subsidence and artesian-pressure reduction indicates that 

the storage coefficient of the aquifer system was much greater during 

the first drawdown period (1915-34) than during the second drawdown period.

Figure 2 shows that about 100 feet of artesian-head decline at core 

hole TS/1E-16C6 produced about 3.7 feet of subsidence between 1912 

and 1934. The indicated coefficient of storage, $c , for the first drawdown phase is about 0.04. Between 1934 and 1943 the artesian head 

nearly recovered to its original level. The second drawdown phase, 

through the same range of pressure, began in 1943, and the artesian 

head declined about 85 feet, to the 1934 level, by 1948. During this 

period, land-surface subsidence was only about 0.09 foot. The indicated coefficient of storage, $c , for the second drawdown phase is about 

0.001. The coefficient of storage during the first drawdown phase 

exceeded that of the second by a ratio of about 40 to 1. Furthermore^ 

the coefficient of storage, /Sc , during the second drawdown phase has 

decreased to the same order of magnitude as the coefficient of storage 

obtained from a short-term pumping test.

Figure 3 shows that about 100 feet of artesian-head decline at 

core hole 6S/2W-24C7 produced about 5.3 feet of subsidence between 

1912 and 1934. The indicated coefficient of storage for the first drawdown phase is about 0.05. Between 1934 and 1944, the artesian head 

recovered about 65 feet above the 1934 level. The second drawdown 

phase, through the same range of pressure, began in 1944, and the 

artesian head declined about 65 feet, to the 1934 level, by 1948. During this period, land-surface subsidence was only about 0.29 foot. The
ARTESIAN-PRESSURE DECLINE AND LAND SUBSIDENCE Til

indicated coefficient of storage for the second drawdown phase is 

about 0.004. The coefficient of storage during the first drawdown 

phase exceeded that of the second by a ratio of 13 to 1.

Residual compaction (that due to the first drawdown phase, but 

not yet accomplished by 1944) at core hole 6S/2W-24C7 was part 

of the total compaction between 1944 and 1948 and caused the indicated 

storage coefficient for that period to be much larger than it would 

have been if the second drawdown phase had occurred after residual 

compaction had been completed.

If all compaction (and subsidence)' were elastic, the land surface 

would subside during periods of artesian-head decline and rebound 

an equal amount if head recovered to the initial level. The coefficient 

of storage would be the same for each repetition of drawdown through 

the same range of head decline. However, because the compaction is 

almost wholly inelastic and rebound is negligible, the coefficient of 

storage for the first drawdown phase greatly exceeds that of the second.

REFERENCES

Green, J. H., 1962, Compaction of the aquifer system and land subsidence in the

Santa Clara Valley, Calif.: U.S. Geol. Survey Prof. Paper 450-D, art. 172, p.

D175-D178. 

Miller, R. E., 1961, Compaction of an artesian aquifer system computed from

consolidation tests and decline in artesian head: U.S. Geol. Survey Prof. Paper

424-B, art. 26, p. B54-B58. 

Poland, J. F., 1961, The coefficient of storage in a region of major subsidence

caused by compaction of an aquifer system: U.S. Geol. Survey Prof. Paper

424-B, art. 25, p. B52-B54. 

Terzaghi, Karl, 1943, Theoretical soil mechanics: New York, John Wiley & Sons,

510 p.

U.S. GOVERNMENT PRINTING OFFICEM964