Land Subsidence 

in the United States 

Circular 1182 

U.S. Department of the Interior 

U.S. Geological Survey ________________ _ 
COVER 

Images are interferograms of 

Las Vegas Valley obtained 

from satellite radar signals. 

They show land movement 

over time. (See chapter on 

"The Role of Science" for 

more information.) 
LAND SUBSIDENCE IN THE UNITED STATES 

EDITED BY 

Devin Galloway 

David R. Jones 

S.E. Ingebritsen 

U.S. Geological Survey 

Circular 1182 

1999 
U.S. DEPARTMENT OF THE INTERIOR 

Bruce Babbitt 

Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat 

Director 

Reston, Virginia 1999 

The use of firm, trade, and brand names in this report is for 

identification purposes only and does not constitute endorsement by the U.S. Government. 

Free on application to the 

U.S. Geological Survey 

Information Services 

Box25286 

Denver, CO 80225-0286 

Library of Congress Cataloging-in-Publication Data 

Land subsidence in the United States I edited by Devin Galloway, David R. Jones, 

S.E. Ingebritsen. 

p. em.- (U.S. Geological Survey Circular; 1182) 

Includes bibliographical references. 

ISBN 0-607-92696-1 

1. Subsidence (Earth movements)-United States-Case studies. 

2. Groundwater-United States-Case studies I. Galloway, Devin L. 

II. Jones, David R. (David Richard) III. Ingebritsen, S.E. IV. Series. 

QE600.3.U6.L36 1999 

551.3 '07'0973-dc21 99-040089 
Foreword 

From the San Francisco Bay/Delta to the Florida Everglades and 

from upstate New York to Houston, people are dealing with a common problem in these diverse locations- land subsidence due to 

the withdrawal of ground water or the application of water at the 

land surface. These locations illustrate that subsidence is not an isolated problem: an area of more than 15,000 square miles in 45 States 

experience land subsidence. Using these locations and others as case 

studies, this report focuses on three principal processes causing land 

subsidence: the compaction of aquifer systems, the oxidation of organic soils, and the collapse of cavities in carbonate and evaporite 

rocks. The impacts of land subsidence, past and present, are illustrated, and most importantly, so is the value of science in effectively 

limiting damages from land subsidence. 

An important aspect of the USGS mission is to provide information 

that describes the Earth, its resources, and the processes that govern 

the availability and quality of those resources. With reports such as 

this Circular, the USGS seeks to broaden public understanding of 

land subsidence as an Earth process, and the serious impacts that 

subsidence can cause if those impacts are not understood, anticipated, 

and properly managed. By applying scientific understanding and 

engineering approaches to problems of land subsidence, our society 

will have solutions that can mitigate or eliminate the negative impacts of subsidence while allowing continued beneficial uses of water. It is our hope that this information will be helpful for concerned 

citizens, landowners, water users, water managers, and officials responsible for public investments and regulation of land and water use. 

For some readers, this report will be an end in itself in providing an 

understanding of the phenomena of land subsidence that satisfies 

their need to act as informed citizens or decision makers, or simply 

to satisfy their curiosity about an important Earth process. For 

other readers, we hope this report will be a gateway to the rich scientific literature on the subject of subsidence and strategies for the 

control of subsidence, through the references provided. 

Scientific understanding is critical to the formulation of balanced 

decisions about the management of land and water resources. This 

Circular coupled with ongoing data collection, basic research, and 

applications of that research to specific subsidence problems, constitute the USGS contribution toward wise management ofland 

subsidence as a part of effective and publicly beneficial land- and 

water-management strategies. 

~/A.~ Robert M. Hirsch 

Associate Director for Water Resources 

Acknowledgments 

We are particularly indebted to four U.S. Geological Survey colleagues who assisted in the planning and research that led to this report. Keith Prince, Stan Leake and Tom Holzer, long-time 

proponents of the scientific understanding of subsidence problems 

related to ground-water use, launched this effort. Francis Riley, a pioneer in field studies of land subsidence related to ground-water extraction, counselled us extensively on aquifer-system compaction. 

We are also especially grateful to the many key colleagues and cooperators who generously lent their expertise to review technical and 

nontechnical aspects of each of the case studies: Behzad Ahmadi, 

Tom Iwamura, and Cheryl Wessling (Santa Clara Valley Water District), and Eric Reichard (U.S. Geological Survey) for Santa Clara 

Valley, California; Gil Bertoldi (U.S. Geological Survey, retired), 

George Davis (U.S. Geological Survey, retired) and Harvey Swanson 

(California Division of Water Resources, retired) for San Joaquin 

Valley, California; Robert Gabrysch (U.S. Geological Survey, retired), 

and Ron Neighbors (Harris-Galveston Coastal Subsidence District) 

for Houston-Galveston, Texas; John Bell (Nevada Bureau of Mines 

and Geology), Gary Dixon (U.S. Geological Survey), and Michael 

Johnson (Las Vegas Valley Water District) for Las Vegas Valley, Nevada; Stan Leake (U.S. Geological Survey), and Herb Schumann 

(U.S. Geological Survey, retired) for Southern Arizona; Margit 

Aramburu (Delta Protection Commission), Lauren Hastings (U.S. 

Geological Survey), and M. Mirmazaheri (California Department of 

Water Resources) for the Sacramento-San Joaquin River Delta, California; Jud Harvey (U.S. Geological Survey), Carol Kendall (U.S. 

Geological Survey), and Jayantha Obeysekera (South Florida Water 

Management District) for the Florida Everglades; Jim Borchers (U.S. 

Geological Survey), Kathy Sanford (New York State Department of 

Environmental Conservation) and Richard Young (State University 

New York-Geneseo) for the Retsof Salt Mine Collapse, New York; 

and Mark Barcelo (Southwest Florida Water Management District), 

Craig Hutchinson (U.S. Geological Survey), William L. Wilson (Subsurface Evaluations Inc.), and Dan Yobbi (U.S. Geological Survey) 

for West-Central Florida. We are also grateful to our U.S. Geological 

Survey colleagues, Charles Heywood and Steven Phillips, for thorough and thoughtful reviews of the final chapter, Role of Science. Finally we thank Michelle Sneed (U.S. Geological Survey) for her final 

read-through of the Circular and for her constructive comments. 
\. 

Cover-ccillap; e sinkhole, West-Central Florida 
VII 

Contents 

1 Introduction 

7 PART I Mining Ground Water-Introduction 

15 Santa Clara Valley, California 

23 San Joaquin Valley, California 

35 Houston-Galveston, Texas 

49 Las Vegas, Nevada 

65 South-Central Arizona 

79 PART II Drainage of Organic Soils-Introduction 

83 Sacramento-San Joaquin Delta, California 

95 Florida Everglades 

107 PART III Collapsing Cavities-Introduction 

111 The Retsof Salt Mine Collapse 

121 Sinkholes, West-Central Florida 

141 The Role of Science 

159 Glossary 

165 References 
VIII 

Conversions 

This Circular uses English units. To determine metric values use 

the conversion factors listed below. 

MEASUREMENT MULTIPLY BY TOOBTAHT 

Length inch 25.4 millimeter 

foot 0.3048 meter 

mile 1.609 kilometer 

Area square foot 0.09290 square mete-r 

square mile 2.590 square kilometer 

acre 0.4047 hectare 

Volume acre foot 1233 cubic meter 

cubic foot 0.02832 cubic meter 

gallon 3.785 liter 

Mass ounce 28.35 gram 

pound 0.4536 kilogram 

ton (short) 0.9072 mega gram 

Temperature degree Fahrenheit °F-32 degree Cels :us 

1.8 

Vertical Datum 

In this report, "sea level" refers to the National Geodetic Vertical 

Datum of 1929 (NGVD of 1929)-a geodetic datum derivei from 

a general adjustment of the first-order level nets of both the United 

States and Canada, formerly called "Sea Level Datum of 192? :' 

"Mean sea level" is not used with reference to any particular vertical 

datum; where used, the phrase means the average surface of the 

ocean as determined by calibration of measurements at tidal stations. 
INTRODUCTION 

Land subsidence in the United States 

This earth fissure formed as a 

result of differential compaction of the aquifer system near 

Mesa, Arizona. 

L and subsidence is a gradual settling or sudden sinking of the 

Earth's surface owing to subsurface movement of earth 

materials. Subsidence is a global problem and, in the United 

States, more than 17,000 square miles in 45 States, an area roughly 

the size of New Hampshire and Vermont combined, have been direedy affected by subsidence. The prinicipal causes are aquifer-system compaction, drainage of organic soils, underground mining, 

hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991). More than 80 percent 

of the identified subsidence in the Nation is a consequence of our 

exploitation of underground water, and the increasing development 

of land and water resources threatens to exacerbate existing landsubsidence problems and initiate new ones. In many areas of the 

arid Southwest, and in more humid areas underlain by soluble 

rocks such as limestone, gypsum, or salt, land subsidence is an often-overlooked environmental consequence of our land- and water-use practices. 

In 1991, the National Research Council estimated that annual costs 

in the United States from flooding and structural damage caused by 

land subsidence exceeded $125 million. The assessment of other 

costs related to land subsidence, especially those due to groundwater withdrawal, is complicated by difficulties in identifying and 

mapping the affected areas, establishing cause-and-effect relations, 

assigning economic value to environmental resources, and by inherent conflicts in the legal system regarding the recovery of damages caused by resource removal under established land and water 

rights. Due to these "hidden'' costs, the total cost of subsidence is 

probably significantly larger than our current best estimate. 

We explore the role of underground water in human-induced land 

subsidence through illustrative case histories. Extraction and drainage of ground water play direct roles in land subsidence by causing 

the compaction of susceptible aquifer systems and the dewatering 

of organic soils. The catastrophic formation of sinkholes in susceptible earth materials, although fundamentally a natural process, can 
2 Subsidence in the United States 

Subsidence occurs worldwide 

Three famous examples of subsidence 

AQ UIFER-SYSTEM COMPACTION IN MEXICO CITY 

In Mexico City, rapid land subsidence caused by groundwater withdrawal and associated aquifer-system compaction has damaged colonial-era buildings, buckled highways, and disrupted water supply and waste-water 

drainage. Maximum rates of subsidence approach 2 feet per 

year and total subsidence during the 20th century is as great 

as 30 feet (New York Times International, January 29, 

1998). In the downtown area, the steel casings of wells drilled deep enough to penetrate beneath the subsiding aquifer 

system now protrude 20 feet or more above ground. The 

progressive sinking of the urban area has rendered the original waste-water drainage system ineffective, and forced 

construction of a new, deep, 124-mile-long sewer network. 

IC-SO IL SUBSIDENCE AND THE DUTCH LANDSCAPE 

It is said that "God created the world, but 

the Dutch created Holland." Near-sealevel marshlands in the western Netherlands began to be drained for agriculture 

between the 9th and 14th centuries, and 

by the 16th century the land had subsided to the extent that windmills were 

needed to artificially discharge water to 

the sea. The classic Dutch landscape of 

dikes, canals, and windmills reflects centuries of reclamation and consequent 

subsidence. Average subsidence rates 

have increased during the 20th century 

because of greatly improved drainage. 

DISSOLUTION-COLLAPSE FEATURES ON THE YUCATAN PENINSULA 

The low-lying Yucatan Peninsula of eastern 

Mexico is covered by a blanket oflimestone, 

and dissolution of the limestone by infiltrating 

rainwater has created a highly permeable aquifer, comparable to the Floridan aquifer of the 

Florida peninsula. Infiltration of rainwater is so 

rapid that there are no surface streams. For millennia, human civilizations relied on sinkholes 

formed by collapse of rock above subsurface 

cavities-locally known as cenotes-for water 

supply. Great troves of Mayan relics have been 

found in some cenotes. (Clive Ruggles, Leicester University. UK. 1986) 

Cenote at 

Chichen ltza, 

Mexico 
Introduction 

During the construction of a 

railroad northeast of Valdez, 

Alaska, the permafrost's thermal equilibrium was disrupted, 

causing differential thawing 

that warped the roadbed. The 

railroad was abandoned in 

1938, but subsidence has continued. 

Oil and gas removal in Long 

Beach, California caused subsidence. Levees were built to 

prevent flooding of the oil 

fields and port facilities . 

3 

also be triggered by ground-water-level declines caused by pumping, or by infiltration from reservoir impoundments, surface-water 

diversions, or storm runoff channels. The case histories illustrate 

the three basic mechanisms by which human influence on ground 

water causes land subsidence-compaction of aquifer systems, dewatering of organic soils, and mass wasting through dissolution 

and collapse of susceptible earth materials. We also examine the 

role that science and water-management groups play in mitigating 

subsidence damages. 

Several other types of subsidence involve processes more or less 

similar to the three mechanisms just cited, but are not covered in 

detail in this Circular. These include the consolidation of sedimentary deposits on geologic time scales; subsidence associated with 

tectonism; the compaction of sediments due to the removal of oil 

and gas reserves; subsidence of thawing permafrost; and the collapse of underground mines_ Underground mining for coal accounts for most of the mining-related subsidence in the United 

States and has been thoroughly addressed through Federal and 

State programs prompted by the 1977 Surface Mining Control and 

Reclamation Act. No such nationally integrated approach has been 

implemented to deal with the remaining 80 percent ofland subsidence associated with ground-water processes. 

Subsidence pits and troughs 

formed above the Dietz coal 

mines near Sheridan, Wyoming. The coal mines were 

in operation from the 1890s 

to the 1920s. 
4 

The organic soils of the 

Florida Everglades are 

quickly disappearing. 

An undeveloped aquifer system is in balance between recharge and discharge. Pumping 

.for urban or agricultural uses 

disrupts this balance and may 

cause subsidence to occur. 

Subsidence in the United States 

Sand and gravel 

(permeable aq uifer) 

~-----+--------------~ 

(!,_,'~ ~ Recharge Discharge 

Clay and si lt Ground water 

(aquitards) 

Mining ground water We begin with five case histories in which 

overdraft of susceptible aquifer systems has resulted in regional, 

permanent subsidence and related ground failures. In alluvial aquifer systems, especially those that include semiconsolidated silt and 

clay layers (aquitards) of sufficient aggregate thickness, long-term 

ground-water-level declines can result in a vast one-time release of 

"water of compaction" from compacting aquitards, which manifests 

itself as land subsidence. Accompanying this release of water is a 

largely nonrecoverable reduction in the pore volume of the compacted aquitards, and thus an overall reduction in the total storage 

capacity of the aquifer system. This "water of compaction" cannot 

be reinstated by allowing water levels to recover to their predevelopment status. The extraction of this resource for economic gain constitutes ground-water mining in the truest sense of the term. 

The five case studies demonstrate how agricultural and municipalindustrial ground-water use have combined to deplete critical 

ground-water resources and create costly regional-scale subsidence. 

We begin in the "Silicon Valley" in northern California, where early 

agricultural ground-water use contributed to subsidence that has 

increased flood risks in the greater San Jose area. Silicon Valley 

(properly the Santa Clara Valley) was the first place in the United 

States where subsidence due to ground-water pumpage was recognized; since the late 1960s, the ground-water resource there has 

been successfully managed to halt subsidence. In nearby San 

Joaquin Valley, the single largest human alteration of the Earth's surface topography resulted from excessive ground-water pumpage to 

sustain an exceptionally productive agriculture. In the HoustonGalveston area in Texas, early production of oil and gas, and a long 

history of ground-water pumpage, have created severe and costly 

coastal-flooding hazards and affected a critical environmental resource-the Galveston Bay estuary. In Las Vegas Valley ground-water depletion and subsidence have accompanied the conversion of a 

desert oasis into a thirsty and fast-growing metropolis. Finally, in 

south-central Arizona, importation of Colorado River water and 

conversion of water-intensive agriculture to lower-water-demand 

urban land uses has helped to partly arrest subsidence and forestall 

further fissuring of the Earth's surface. 
Introduction 

Cover collapse sinkhole in 

Winter Park, Florida, 1981 

5 

Drainage of organic soils Land subsidence invariably occurs when 

organic soils-soils rich in organic carbon-are drained for agriculture or other purposes. The most important cause of this subsidence 

is microbial decomposition which, under drained conditions, readily 

converts organic carbon to carbon-dioxide gas and water. Compaction, desiccation, erosion by wind and water, and prescribed or accidental burning can also be significant factors. 

The total area of organic soils in the United States is roughly equivalent to the size of Minnesota, about 80,000 square miles, nearly half 

of which is "moss peat" located in Alaska (Lucas, 1982). About 70 

percent of the organic-soil area in the contiguous 48 states occurs in 

northerly, formerly glaciated areas, where moss peats are also common (Stephens and others, 1984). Moss peat is composed mainly of 

sphagnum moss and associated species. It is generally very acidic 

(pH 3.5 to 4) and, therefore, not readily decomposed, even when 

drained. However, where moss peat is amended for agricultural cultivation, for example through fertilization and heavy application of 

lime to raise the pH, it can decompose nearly as rapidly as other 

types of organic soils. 

Our two case studies of organic-soil subsidence focus on examples 

of rapid subsidence ( 1 to 3 inches/year) caused by decomposition of 

the remains of shallow-water sedges and reeds. In the SacramentoSan Joaquin Delta of California and the Florida Everglades, continuing organic-soil subsidence threatens agricultural production, 

affects engineering infrastructure that transfers water supplies to 

large urban populations, and complicates ongoing ecosystem-restoration efforts sponsored by the Federal and State governments. 

Collapsing cavities The final two case studies deal with the sudden 

and sometimes catastrophic land subsidence associated with localized collapse of subsurface cavities-sinkholes. This type of subsidence is commonly triggered by ground-water-level declines caused 

by pumping and by enhanced percolation of water through susceptible rocks. Collapse features tend to be associated with specific rock 

types having hydrogeologic properties that render them susceptible 

to dissolution in water and the formation of cavities. Evaporite minerals (salt, gypsum and anhydrite) and carbonate minerals (limestone and dolomite) are susceptible to extensive dissolution by 

water. Salt and gypsum are, respectively, almost 7,500 and 150 times 

more soluble than limestone, the rock type often associated with 

catastrophic sinkhole formation. 

Evaporite rocks underlie about 35 to 40 percent of the United States, 

although in many areas at depths so great as to have no discernible 

effect at land surface. Natural solution-related subsidence has occurred in each of the major salt basins (Ege, 1984) throughout the 

United States. The high solubilities of salt and gypsum permit cavities to form in days to years, whereas cavity formation in carbonate 

bedrock is a very slow process that generally occurs over centuries to 

millennia. The slow dissolution of carbonate rocks favors the stability and persistence of the distinctively weathered landforms known 

as karst. Carbonate karst landscapes comprise more than 40 percent 
6 Subsidence in the United States 

of the humid United States east of the longitude of Tulsa, Oklahoma 

(White and others, 1995). Human activities can facilitate the formation of subsurface cavities in these susceptible materials and trigger 

their collapse, as well as the collapse of pre-existing subsurface cavities. Though the collapse features tend to be highly localized, their 

impacts can extend beyond the collapse zone via the potential introduction of contaminants to the ground-water system. Our two cavity-collapse case studies-Retsof, New York and west -central 

Florida-focus on human-induced cavity collapses in salt and limestone, respectively. 

The role of science In a final section we discuss the role of science 

in defining subsidence problems and understanding subsidence 

processes. A combination of scientific understanding and careful 

management can minimize the subsidence that results from devdoping our land and water resources. 
PART I 

Mining Ground Water 

Areas where subsidence has 

been attributed to groundwater pumpage 

California 

Antelope Valley 

Coachella Valley 

Elsinore Valley 

La Verne area 

Lucerne Valley 

M ojave River Basin 

Oxnard Plain 

Pomona Basin 

Sacramento Valley 

Salinas Valley 

San Benito Valley 

San Bernardino area 

San Gabriel Valley 

San Jacinto Basin 

San Joaquin Valley 

San Luis Obispo area 

Santa Clara Valley 

Temecula Valley 

Santa Clara Valley, California 

San Joaquin Valley, California 

Houston-Galveston, Texas 

Las Vegas, Nevada 

South-Central Arizona 

P ermanent subsidence can occur when water stored beneath 

the Earth's surface is removed by pumpage or drainage. The 

reduction of fluid pressure in the pores and cracks of aquifer 

systems, especially in unconsolidated rocks, is inevitably accompanied by some deformation of the aquifer system. Because the granular structure-the so-called "skeleton" -of the aquifer system is not 

rigid, but more or less compliant, a shift in the balance of support for 

the overlying material causes the skeleton to deform slightly. Both 

the aquifers and aquitards that constitute the aquifer system undergo 

deformation, but to different degrees. Almost all the permanent subsidence occurs due to the irreversible compression or consolidation 

of aquitards during the typically slow process of aquitard drainage 

(Tolman and Poland, 1940). This concept, known as the aquitarddrainage model, has formed the theoretical basis of many successful 

subsidence investigations.* 

Idaho 

Raft River area Colorad o 

Denver area 

New Je r sey 

Atlantic City-Oceanside area 

Barnegat Bay-New York Bay 

coastal area 

WolfValley _______ -l 

South C en t rai ____ ----'""-

Arizona ....__-LJ--.., 

AvraValley 

East Sa~ River Valley 

Eloy Basin 

D Major alluvial aquifer systems in 

the conterminous United States 

Gila Bend area 

H arquahala Plain 

San Simon Valley 

St anfield Basin 

Tucson Basin 

(Modif1ed from Clawges and Price, 1999) 

West Salt River Valley 

Willcox Basin 

Texas 

H ouston-Galveston 

G e o r gia 

Savannah area 

H ueco Bolson-EI Paso, Juarez 

• Studies of subsidence in the Santa Clara Valley (Tolman and Poland, 1940; Poland and Green, 1962; Green, 1964; Poland and Ireland, 1988) and San Joaquin Valley (Poland, 1960; Miller, 1961 ; 

Ri ley, 1969; Helm, 1975; Poland and others, 1975; Ireland and others, 1984) in California established the theoretical and field application of the laboratory derived principle of effective stress and 

theory of hydrodynamic consolidation to the drainage and compaction of aquitard s. For reviews of the history and application of the aquitard drainage model see Holzer (1998) and Ri ley (1998). 
8 Mining Ground Water 

When water levels drop, due mainly 

to seasonal increases in ground-water 

pumping, some support for the overlying material shifts from the pressurized fluid filling the pores to the granular skeleton of t he aquifer system. 

When ground water is recharged 

and water levels rise, some support for the overlying material 

shifts from the granular skeleton 

to the pressurized pore fluid. 

... ------------.---- 1--,._.~-------.!--1 Land surface 

,.-.---~--r-:--~-...., Land surface 

Unconfined [ • s~~ ~~d ~r:~~~f · · · aquifer • · ·· · 

compresses the skele1--...,.--:--· ton by contracting the 

pone spaces, causing 

some lowering of the 

land surface. Confined 

aquifer 

Depth 

to water 

Mostly recoverable (elastic) deformation was observed during 

and following a pumping test 

near Albuquerque, New Mexico. 

Changes in the water level due 

to cyclic pumping were accompanied by alternating cycles of 

compression and expansion of 

the aquifer system. 

A measure of the change in applied stress is the change in water level. 

Time 

Chaf'g " 

dept.!- •o wat~r 

('eet b£IOW 

land surf =c) 

Applied stress 

measured as 

change in 

depth to water 

(feet below 

land surface) 

. • . ·-: ~ -."-"" ._ -- ~ ... -- -- .. • 

. . .. ·-. 

.· · ... 

_ ........ 

Under the decreased 

load the pone spaces 

and the skeleton exContracting aquiferPone system skeleton 

space 

Clay 

particle 

Decreased fluid pressure 

causes the skeleton to 

contract, creating some 

small subsidence of land 

surface. 

Expanding aquifersystem skeleton 

Increased fluid pressure 

expands the skeleton, 

creating some small uplift 

of land surface. 

REVERSIBLE DEFORMATION OCCURS IN ALL AQUIFER SYSTEMS 

The relation between changes in ground-water levels and compression of the aquifer system is based on the principle of effective stress 

first proposed by Karl Terzaghi (Terzaghi, 1925). By this principle, 

when the support provided by fluid pressure is redueed, such as when 

ground-water levels are lowered, support previously provided by the 

pore-fluid pressure is transferred to the skeleton of the aquifer system, which compresses to a degree. Conversely, when the pore-fluid 

pressure is increased, such as when ground water recharges the aqui-

•OO -

0 

60 -

0 

Routine operational pumping 

Compacoor 

- 0.0008 (fcoet) 

50 100 ISO 

Days since pumping test began 

During pumping the aquifer Day 54 

system compresses. ~ 

Slope represents aquifer-"v ". 

system compressibility > During recovery 

the aquifer system 

expands. -

0.004 0.008 

During routine operational 

pumping the aquifer system 

expands and contracts 

through cycles of water-level 

recovery and drawdown. 

0.012 0.016 

Compaction (feet) (Heywood, 1997) 
Introduction 

When long-term pumping 

lowers ground-water levels 

and raises stresses on the 

aquitards beyond the preconsolidation-stress thresholds, 

the aquitards compact and 

the land surface subsides permanently. 

Land surface 

:·: .... • •. 0 .• •• 

Granular aquitard 

skeleton defining fiuidfilled pore spaces 

storing ground water 

9 

fer system, support previously provided by the skeleton is transferred to the fluid and the skeleton expands. In this way, the skeleton 

alternately undergoes compression and expansion as the pore-fluid 

pressure fluctuates with aquifer-system discharge and recharge. 

When the load on the skeleton remains less than any previous maximum load, the fluctuations create only a small elastic deformation of 

the aquifer system and small displacement ofland surface. This fully 

recoverable deformation occurs in all aquifer systems, commonly 

resulting in seasonal, reversible displacements in land surface of up 

to 1 inch or more in response to the seasonal changes in groundwater pumpage. 

INELASTIC COMPACTION IRREVERSIBLY ALTERS 

THE AQUIFER SYSTEM 

The maximum level of past stressing of a skeletal element is termed 

the preconsolidation stress. When the load on the aquitard skeleton 

exceeds the preconsolidation stress, the aquitard skeleton may undergo significant, permanent rearrangement, resulting in irreversible 

compaction. Because the skeleton defines the pore structure of the 

aquitard, this results in a permanent reduction of pore volume as the 

pore fluid is "squeezed" out of the aquitards into the aquifers. In 

confined aquifer systems subject to large-scale overdraft, the volume 

of water derived from irreversible aquitard compaction is essentially 

equal to the volume of subsidence and can typically range from 10 

to 30 percent of the total volume of water pumped. This represents a 

one-time mining of stored ground water and a small permanent 

reduction in the storage capacity of the aquifer system. 

-_ ·_ .. · __ -_ ·_ ·. · __ --__ · -_ -_ -_ ·_- -_ 1

_ ·_ t-__ ... J Recoverable land subsidence caused by 

1' reversible elastic deformation I J Permanent land subsidence caused by 

1--L-an_d_s_uri~a~c~e_.L____,.., irreversible inelastic deformation 

Rearranged, compacted granular aquitard 

skeleton with reduced 

porosity and groundwater storage capacity 

Compaction of the aquifer system 

is concentrated in the aquitards. 

Time 

Long-term decline in water level 

modulated by the seasonal cycles 

of ground-water pumpage 
10 

Aquitard Drainage and Aquifer-System Compaction 

The Principle of Effective Stress 

Mining Ground Water 

This principle describes the relation between changes in water levels and deformation of the aquifer system. 

Land surface 

Foran y arbitrary plane below the 

able, the total stress repreby the weight of the overock and water is balanced 

pore-fluid pressure and the 

anular or effective stress. 

water t Aquifer system sen ted 

TOTAL STRESS lying r 

(weight or load of the by the 

overlying rocks intergr 

and water) 

crT I crT= 

, 

cret t p 

EFFECTIVE PORE-FLUID 

STRESS STRESS 

(portion of the total (portion of the total stress that is borne stress that is borne 

by the solid grains by the water filling 

in the skeleton of the pores of the 

the rock matrix) rock matrix ) 

' ' 

Bedrock 

Confined 

aquifer 

system 

Land surface 

Thick aquitard 

(lateral ly 

extensive) 

Thick aquitard 

(laterally 

discontinuous) 

PROLONGED CHANGES IN GROUND-WATER LEVELS INDUCE SUBSIDENCE 

Prior to the extensive development of 

ground-water resources, water levels are 

relatively stable-though subject to seasonal and longer-term climatic variability. 

Confined 

aquifer 

system 

Land surface 

Bedrock 

The weight of the overlying 

rock and water is balanced by 

the pore-fiuid pressure and the 

intergranular or effective stress. 

water 

During development of groundwater resources, water levels decline 

and land subsidence begins. 

confined aquifers reduces fiuid 

pressures (p ). As the total stress 

(crT) remains nearly constant, a 

portion of the load is shifted from 

the confined fiuid to the skeleton 

of the aquifer system, increasng 

the effective stress (cre ) and 

causing some compression. 

After ground-water pumping slows 

or decreases, water levels stabilize but 

land subsidence may continue. 

Under the principle of effective 

stress, the compaction of a thick 

sequence of interbedded aquifers 

and aquitancs can proceed only 

as rapidly as pore pressures 

throughout the sequence can decay towanc equilibrium wrth reduced pressures in the pumped 

aquifers. Most of the land subsidence occurs as a result of the 

permanent compaction of the 

aquitancs, which may be delayed 

due to their slow drainage. 
Introduction 

More than 2.5 feet of permanent 

(inelastic) compaction was observed near Pixley, San Joaquin 

Valley, California during a 10-year 

period. 

The high summer demand for irrigation water combined with the 

normally wetter winters causes 

ground-water levels to fluctuate 

in response to seasonal pumpage 

and recharge. The annual cycles 

of alternating stress increase and 

decrease are accompanied by 

cycles of compression and slight 

expansion of the aquifer system. 

Compression proceeds most rapidly when the stress is larger than 

the preconso/idation stress threshold. Beyond this threshold almost 

all of the compression is perm anent (inelastic) and attributed to 

the compaction of fine-grained 

aquitards. 

Applied stress 

measured as 

change in 

depth to water 

(feet below 

land surface) 

" ... the term aquitard has been coined 

to describe the less-permeable beds in a 

stratigraphic sequence. These beds may 

be permeable enough to transmit water 

in quantities that are significant in the 

study of regional ground-water flow, but 

their permeability is not sufficient to 

allow the completion of production wells 

within them." 

- Freeze and Cherry,l979 

11 

I... OP-.. >?C.tiO 

lfe ct) 

1960 1962 1964 1966 1968 

140 ----------------------------------- Inelastic 

M~xt,d"~l Elastic/ \ ~ -., 

limb 

--.----.--~.---,r~-,----,----,---

0.8 1.6 2.4 3.2 

Displacement measured as cumulative compaction (feet) 

----- Relative expansion Relative compression -

(Riley. 1969) 

Aquitards play an important role in compaction 

In recent decades increasing recognition has been given to the critical role of aquitards in the intermediate and long-term response of 

alluvial aquifer systems to ground-water pumpage. In many such 

systems interbedded layers of silts and days, once dismissed as nonwater yielding, comprise the bulk of the ground-water storage capacity of the confined aquifer system! This is by virtue of their 

substantially greater porosity and compressibility and, in many 

cases, their greater aggregate thickness compared to the more transmissive, coarser-grained sand and gravel layers. 

Because aquitards are by definition much less permeable than aquifers, the vertical drainage of aquitards into adjacent pumped aquifers may proceed very slowly, and thus lag far behind the changing 

water levels in adjacent aquifers. The duration of a typical irrigation 

season may allow only a modest fraction of the potential yield from 

aquitard storage to enter the aquifer system, before pumping ceases 

for the season and ground-water levels recover in the aquifers. Typically, for thick aquitards, the next cycle of pumping begins before 

the fluid pressures in the aquitards have equilibrated with the previous cycle. The lagged response within the inner portions of a thick 

aquitard may be largely isolated from the higher frequency seasonal 

fluctuations and more influenced by lower frequency, longer-term 

trends in ground-water levels. Because the migration of increased 

internal stress into the aquitard accompanies its drainage, as more 

fluid is squeezed from the interior of the aquitard, larger and larger 

internal stresses propagate farther into the aquitard. 

When the internal stresses exceed the preconsolidation stress, the 

compressibility increases dramatically, typically by a factor of 20 to 
12 Mining Ground Water 

Aquitard Drainage and Aquifer-System Compaction 

The Theory of Hydrodynamic Consolidation 

The theory describes the delay in draining aquitards when water lbvels are lowered in adjacent aquifers, as well as 

the residual compaction that may continue long after water levels are initially lowered. 

During a 90-year period (1908-1997) of ground-water development in the Antelope Valley, California, the response of 

water levels in two thick aquitards lags the declining water level in the aquifer. A laterally discontinuous aquitard draining 

0 

200 

Depth to water 

400 (feet below 

land surface) 

600 

Some downward 

All upward fiow fiow through 

through aquitard aquitard 

r----1 

0 40 80 

Flowing artesian 

conditions in the 

confined aquifer system 

Depth to water 

(feet below land surface) 

RESIDUAL COMPACTION 

Significant amounts of compaction began occurring in the late 

1950s after water lev els in the aquifers had fallen some 60 feet. 

Initially, most of the compaction occured in the faster-draining 

thin aquitards within the aquifers. Subsequently most of the 

compaction occured in the two thickest and most slowly draining aquitards. Despite stabilization of ground-water levels in 

the aquifers, more than 0.3 feet of compaction has occurred 

since 1990, due to residual compaction. 

Simulations predict that another 1.3 feet of compaction may ultimately occur even if ground-water levels remain at 1997levels. 

*These results from an aquifer system in Antelope Valley. Mojave Desert, Califomia are 

based on field measurements and computer simulations of aquitard drainage. They 

illustrate the history of ground-water-level changes and compaction in the aquifers and 

aquitards throughout the period of ground-water resource development 1908-97. 

(Michelle Sneed, USGS. written communication, 1998) 

from both upper and lower faces approaches fluid-pressure 

equilibrium with the adjacent aquifers more rapidly than an 

overlying laterally extensive aquitard that has a complex 

drainage history, including a gradient reversal.* 

120 

Compaction 

(feet) 

0 

2 

3 

Land surface 

--Thick aquitard (laterally 

extensive, confini ng unit 

transitions from singlydraining to doubly-drain-

~ ing around 1968) 

Thick aquitard 

(lateral ly di scontinuous. doubly-draining) 

J Unconfined 

aquifer 

Confined 

aquifer 

system 

Compaction 

(percentage of total) 

0 

SIMULATED 

25 

50 

75 

100 

1900 1920 1940 1960 1980 2000 
Introduction 

Hydrodynamic lag, which is a delay 

in the propagation of fluid-pressure 

changes between the aquifers and 

aquitards, can be seen at this site in 

the Antelope Valley, Mojave Desert, 

California. 

The responses to changing water 

levels following eight decades of 

ground-water development suggest 

that stresses directly driving much of 

the compaction are somewhat insulated from the changing stresses 

caused by short-term water-level 

variations in the aquifers. 

Depth to 

water 

(feet below 

land surface) 

13 

100 times, and the resulting compaction is largely nonrecoverable. 

At stresses greater than the preconsolidation stress, the lag in aquitard drainage increases by comparable factors, and concomitant 

compaction may require decades or centuries to approach completion. The theory of hydrodynamic consolidation (Terzaghi, 1925)-

an essential element of the "aquitard drainage model" -describes 

the delay involved in draining aquitards when heads are lowered in 

adjacent aquifers, as well as the residual compaction that may continue long after drawdowns in the aquifers have essentially stabilized. Numerical modeling based on Terzaghi's theory has 

successfully simulated complex histories of compaction observed in 

response to measured water-level fluctuations (Helm, 1978). 

Compaction progresses steadily 

despite water-level recoveries. 

Further water-level declines do not 

cause accelerated subsidence. 

Seasonal high water levels 

(Michelle Sneed, USGS, written communication, 1998) 

SANTA CLARA VALLEY, CALIFORNIA 

A case of arrested subsidence 

30Miles 

F======;---- 30 Ktlometers 

CALIFORNIA 

S.E. lngebritsen and David R. Jones 

U. S. Geological Survey, Menlo Park, California 

The Santa Clara Valley is part of a structural trough that extends about 90 miles southeast from San Francisco. The northern third of the trough is occupied by the San Francisco Bay, the central third by the Santa Clara Valley, and the southern third by the San Benito Valley. The northern Santa Clara Valley, 

roughly from Palo Alto to the Coyote Narrows ( 10 miles southeast 

of downtown San Jose), is now densely populated and known as 

"Silicon Valley;' the birthplace of the global electronics industry. 

In the first half of this century, the Santa Clara Valley was intensively 

cultivated, mainly for fruit and vegetables. The extensive orchards, 

dominated by apricots, plums, cherries, and pears, led local boosters 

to dub the area a Garden of Eden or "The Valley of Heart's Delight:' 

In the post-World War II era (circa 1945- 1970), rapid population 

growth was associated with the transition from an agriculturally 

based economy to an industrial and urban economy. The story of 

land subsidence in the Santa Clara Valley is closely related to the 

changing land and water use and the importation of surface water to 

support the growing urban population. 

(Air Flight Service) 
16 

(George E. Hyde & Co. 19 15- 192 1. 

Bancroft Library, UC Berkeley) 

This free-flowing artesian well was 

capped to prevent waste (191 0). 

Mining Ground Water 

The Santa Clara Valley was a 

premier fruit growing region 

in the early part of the 20th 

century. The landscape was 

dotted with family orchards, 

each with its own well (note 

well house far right). 

The Santa Clara Valley was the first area in the United States where 

land subsidence due to ground-water withdrawal was recognized 

(Tolman and Poland,1940). It was also the first area where organized remedial action was undertaken, and subsidence was effectively halted by about 1969. The ground-water resource is still 

heavily used, but importation of surface water has reduced groundwater pumping and allowed an effective program of ground-water 

recharge that prevents ground-water levels from approaching the 

historic lows of the 1960s. The unusually well-coordinated and effective conjunctive use of surface water and ground water in the 

Santa Clara Valley is facilitated by the fact that much of the Valley is 

served by a single water-management agency, the Santa Clara Valley Water District. 

GROUND-WATER PUMPING SUPPLIED ORCHARDS AND, 

EVENTUALLY, CITIES 

The moderate climate of the Santa Clara Valley has distinct wet and 

dry periods. During the wet season (November to April), average 

rainfall ranges from a high of about 40 inches in the low, steep 

mountain ranges to the southwest to a low of about 14 inches on 

the valley floor-rates that are generally insufficient to support 

specialty crops. Early irrigation efforts depended upon local diversions of surface water, but the acreage that could be irrigated in this 

manner was very limited. By the 1860s, wells were in common use. 

Recharge areas 

Confined 

aquifer 

Water 
Santa Clara Valley, California 17 

Maximum annual depth to water 

as in ll ne rd nt sa e ______ ~L~an=d=su~ri~a~ce~--

In the late 1800s construction of railroads, 

refrigerator cars, and improved canning 

techniques gave farmers access to the growing California and eastern markets for perishable crops. The planting of orchards and 

associated ground-water pumping increased 

rapidly into the 1900s. 

Depth to 

water 

(feet below 120 

land surface) 

~JrJ_ 

vv~r I 1 In the late 1880s most wells in the area between downtown San Jose and Alviso and 

along the Bay northwest and northeast of 

Alviso were artesian. That is, water flowed 

240 

1920 1940 

Acre-Feet 

Hydrologists frequently use the term acrefeet to describe a volume of water. One 

acre-foot is the volume of water that will 

cover an area of one acre to a depth of one 

foot. The term is especially useful where 

large volumes of water are being described. 

One acre-foot is equivalent to 43,560 cubic 

feet, or about 325,829 gallons! 

200 

1960 

;, 

1980 

freely without needing to be pumped. In fact, there was substantial 

waste of ground water from uncapped artesian wells. The widespread artesian conditions were due to the natural hydrogeology of 

the Santa Clara Valley. Water levels in the artesian wells rose above 

the land surface because they tapped confined aquifers that have 

permeable connections to higher-elevation recharge areas on the 

flanks of the Valley but are overlain by low-permeability clay layers. 

By 1920, two-thirds of the Santa Clara Valley was irrigated, including 

90 percent of the orchards, and new wells were being drilled at the 

rate of 1, 700 per year (California History Center, 1981). By the late 

1920s, about 130,000 acre-feet of ground water was pumped annually to irrigate crops and support a total population of about 100,000. 

Ground-water levels drop 

Ground water was being used faster than it could be replenished. As 

a result, water levels were dropping and artesian wells becoming increasingly rare. By 1930, the water level in a formerly artesian USGS 

monitoring well in downtown San Jose had fallen 80 feet below the 

land surface. 

Between 1920 and 1960 an average of about 100,000 acre-feet per 

year of ground water was used to irrigate crops. Nonagricultural use 

of ground water began to increase substantially during the 1940s, 

and by 1960 total ground-water withdrawals approached 200,000 

acre-feet per year. In 1964 the water level in the USGS monitoring 

well in downtown San Jose had fallen to a historic low of 235 feet 

below the land surface. 

2 

Ground-water pumpage in 

northern Santa Clara Valley 

Ground-water 

;t~'C"' pumpage 100 Population 

(thousands of (millions of 

acre feet) residents) 

o r ---r-- --r---,- 0 

1920 1940 1960 1980 2000 
18 

These photographs of the South Bay Yacht Club 

in Alviso show dramatic evidence of subsidence. 

During the 33-year period, 

subsidence ranged from 2 

feet under the Bay and its 

tideland to 8 feet in San Jose 

and Santa Clara. 

Total land subsidence, which 

probably began in the 1920s 

and continued to 1969 or 

later, is likely greater than 

shown on this map. 

San Francisco Bay 

Palo Alto. 

0 5 Miles 

~=::;----' 

5 Kilometers 

Land-surface attitude 

(feet above sea level) 

\ 

1914-The Yacht Club 

(building to the right) is 

practically at sea level. 

1978-The Yacht Club is 

now about 10 feet below 

sea level, and a high levee 

keeps bay water from inundating Alviso. 

\ 

Mining Ground Water 

(Santa Clara Valley Water District) 

MASSIVE GROUND-WATER WITHDRAWAL 

CAUSED THE GROUND TO SUBSIDE 

Substantial land subsidence occurred in the northern Santa Clara 

Valley as a result of the massive ground-water overdrafts. Detectable subsidence of the land surface (greater than 0.1 feet) took 

place over much of the area. The maximum subsidence occurred in 

downtown San Jose, where land-surface elevations decreased from 

about 98 feet above sea level in 1910 to about 84 feet above sea level 

in 1995. 

Lands adjacent to the southern end of San Francisco Bay sank from 

2 to 8 feet by 1969, putting 17 square miles of dry land below the 

high-tide level. The southern end of the Bay is now ringed with 

dikes to prevent landward movement of saltwater, and flood-controllevees have been built to control the hayward ends of stream 

Lines of equal land subsidence (feet), 1934 to 

196 7 in the northern 

Santa Clara Valley. 

I 

'Oo 

channels. The stream channels must now 

be maintained well above the surrounding 

land in order to provide a gradient for flow 

to the Bay. In the land that has sunk below 

the high-tide level, local storm discharge 

must be captured and pumped over levees 

in order to prevent widespread flooding. 

The fact that Santa Clara Valley was subsiding became generally known in 1933, 

when bench marks in San Jose that were 

established in 1912 were resurveyed and 

found to have subsided 4 feet. This finding 

motivated the U.S. Coast and Geodetic 

Survey to establish a network of bench 

marks tied to stable bedrock on the edges 

of the Valley. The bench-mark network 

was remeasured many times between 1934 

and 1967, and forms the basis for mapping 

subsidence. 

(Modified from Poland and Ireland, 1988) 
Santa Clara Valley, California 

Land subsidence was a result of 

intensive ground-water pumping 

and the subsequent drop in water levels. Once pumping was 

stabilized by the introduction of 

imported surface water, subsidence was arrested. 

19 

Subsidence had to be stopped 

In 1935 and 1936, the Santa Clara Valley Water District built five 

storage dams on local streams to capture storm flows. This permitted controlled releases to increase ground-water recharge through 

streambeds. Wet years in the early 1940s enhanced both natural and 

artificial recharge. Although subsidence was briefly arrested during 

World War II, these measures proved inadequate to halt water-level 

declines over the long term, and, between 1950 and 1965, subsidence 

resumed at an accelerated rate. In 1965, increased imports of surface 

water allowed the Santa Clara Valley Water District to greatly expand 

its program of ground-water recharge, leading to substantial recovery of ground-water levels, and there has been little additional subsidence since about 1969. 

In fact, as of 1995, water levels in the USGS monitoring well in 

downtown San Jose were only 35 feet below land surface, the highest 

levels observed since the early 1920s. A series of relatively wet years 

in the mid -1990s even caused a return to artesian conditions in 

some areas near San Francisco Bay. Some capped and long-forgotten 

wells near the Bay began to leak and were thereby rediscovered! 

Subsidence in the Santa Clara Valley was caused by the decline of artesian pressures and the resulting increase in the effective overburden load on the water-bearing sediments. The sediments compacted 

under the increasing stress and the land surface sank. Most of the 

compaction occurred in fine-grained clay deposits (aquitards), 

which are more compressible, though less permeable, than coarsergrained sediments. The low permeability of the clay layers retards 

and smooths the compaction of the aquifer system relative to the 

water-level variations in the permeable aquifers. Since 1969, despite 

water-level recoveries, a small amount of additional residual com-

-------------------- 100 - 200 

~c ~ Altitude of 

bench mark 

r dow1town 

Ground-water 

pumpage and 

- I 00 recharge 

) t 

W t r 

(f ) I W 

1920 

111 1n USGS monitoring 

well in downtown San jose 

1940 

Subsidence slowed briefiy 

due to a recharge program 

instituted by the Santa 

Clara Valley Water District. 

1960 1980 

Reduced ground-water pumping 

and recharge using imported surface 

water has raised ground-water levels 

and effectively arrested subsidence. 

From 1960 to 1963 subsidence 

attained its greatest average rate 

(about 8 inches per year). 

San Jos~ 

(feet above 

sea lev I) 

- o 

(thousands of 

acre-feet) 
20 

The South Bay aqueduct 

conveys water from the Sacramento-San Joaquin Delta 

to the Santa Clara Valley. 

(Santa Clara Valley Water D1stnct) 

Water imports allow water 

managers to raise groundwater levels by reducing net 

ground-water extraction. 

Depr 0 

wat ~ 

If t bel w 

!r sur c 

G 

lliJ -

240 

1920 1940 

Mining Ground Water 

paction and subsidence has accrued. The total subsidence has been 

large and chiefly permanent, but future subsidence can be controlled if ground-water levels are maintained safely above their subsidence thresholds. 

Surface water is delivered for use in the Valley 

To balance Santa Clara Valley's water-use deficit, surface water has 

been imported from northern and eastern California via aqueducts-Hetch Hetchy (San Francisco Water Department, 1951-), 

the California State Water Project (1965-), and the Federal San 

Felipe Water Project (1987-). Much of the imported water also 

feeds into various local distribution lines. But presently about onefourth of the water imported by the Santa Clara Valley Water District (about 40,000 of the 150,000 acre-feet total) is used for 

ground-water recharge. 

The aquifer systems are used for natural storage and conveyance, in 

preference to constructing expensive surface-storage and conveyance systems. In order to avoid recurrence of the land subsidence 

that plagued the Valley prior to 1969, ground-water levels are maintained well above their historic lows, even during drought periods. 

For example, ground-water levels beneath downtown San Jose 

were maintained even during the major California droughts of 

1976-77 and 1987-91. In order to avoid large ground-water overdrafts, the Water District aggressively encourages water conservation during drought periods. Per-capita water use under current 

conditions is much lower than in the agrarian past. Today, about 

350,000 acre-feet of surface and ground water meet the annual requirements of a countywide population of about 1,600,000, and 

per-capita water use is only about one-fifth of the 1920 level. 

The economic impact can only be approximated 

The direct costs of land subsidence in the Santa Clara Valley include the cost of constructing levees around the southern end of 

San Francisco Bay and the bayward ends of stream channels, mainTotal imported surface water 

t 1960 1980 

Water 

IOO imports 

0 

(thousands of 

acre-feet) 

Hetch Hetchy water State water Federal water 
Santa Clara Valley, California 

Santa Clara Valley Water District 

Ground-water recharge system 

South Bay Aqueduct 

Surface water that is used for recharge is 

brought in by the South Bay Aq ueduct 

and the Santa Clara Conduit (San Feli pe 

Water Project). Hetch Hetchy water is 

not used for recharge. 

San Francisco Bay 

Palo Alto 

• 

Stevens Cr. Res. 

Winter rain water is stored in the 

reservoirs and later released, so 

that it can seep down through the 

gravel and sands of the creek beds. 

In addition, water is diverted from 

the creeks to adjacent percolation 

ponds, which also have the sand 

and gravel bottoms necessary for 

effective percolation. 

Lexington 

Res. 

(Santa Clara Valley Water District) 

NATURAL CONDITIONS 

Conditions are favorable for recharge in the upper reaches 

of several streams because there is an abundance of coarse 

sand and gravel deposits and the aquifer system is generally 

unconfined; that is, fluid pressure in the aquifer is not confined by any overlying lenses oflow-permeability clay. 

Nearer to the Bay, sediments tend to be finer-grained, and the 

exploited ground-water system is generally confined by lowpermeability materials that impede recharge. 

RECHARGE FACILITIES 

The first percolation facilities in the Santa Clara Valley were 

built in the 1930s. They relied on capturing local surface 

runoff, and proved 'inadequate to keep pace with the rate 

of ground-water extraction. The volume of artificial recharge 

was increased significantly when additional imported surface water became available in 1965. Artificial recharge rates 

The current recharge program includes 

I 0 reservoirs, 393 acres of percolation 

ponds, and 159 miles of conduits 

and pipelines. 

Chesbro Res. 

UvasRes. J • 

Santa Clara Conduit 

(San Felipe Water 

• One or mo re 

recharge ponds 

-- Pipelines 

0 5 Miles 

r------1 

0 5 Kilometers 

in the 1970s were sufficient to reverse ground-water level 

declines and arrest subsidence. 

COST-BENEFIT 

In 1984, a cost-benefit approach was used to estimate the 

value of artificial ground-water recharge in the Santa Clara 

Valley (Reichard and Bredehoeft, 1984). The benefits of 

reduced ground-water pumping costs and reduced subsidence were found to be greater than the total costs of continuing the artificial recharge program. A second analysis 

compared the costs of artificial recharge with the cost of a 

surface system that would achieve the same storage and 

conveyance of water. The costs of artificial recharge proved 

to be much less than the costs of an equivalent surface 

system. 

21 
22 

TOTAL: $131 million 

Bay levees 

($1 03 million) Channel levees ($1 0 m) 

Direct costs of land subsidence in the Santa Clara Valley 

in 1979 dollars. 

This view looking into a typical 

collapsed well screen shows the 

damage caused by compaction. 

This photograph was made by 

lowering a light into the well, 

followed by a camera; the 

crumpled vertical ribbing of the 

steel well screen produced this 

radiating effect. 

Drainage pumps ($4 m) 

Water-well repair ($7.6 m) 

0 JO inches 

Mining Ground Water 

taining salt-pond levees, raising grades for railroads and roads, enlarging or replacing bridges, enlarging sewers and adding sewage 

pumping stations, and constructing and operating storm-drainage 

pumping stations in areas that have subsided below the high-tide 

level. Most of these direct costs were incurred during the era of active subsidence. In 1981 Lloyd C. Fowler, former Chief Engineer of 

the Santa Clara Valley Water District, estimated the direct costs of 

subsidence to be $131,100,000 in 1979 dollars, a figure that translates to about $300,000,000 in 1998 dollars. The ongoing cost of 

maintaining levees and pumping facilities can also be attributed 

mainly to subsidence. In fact, as of this writing, the U.S. Army Corps 

of Engineers is building a substantial system for flood control along 

the lower Guadalupe River channel, with design requirements (and 

associated expense) influenced by past subsidence. 

Some of Fowler's estimates of direct costs deserve further explanation. Land subsidence was estimated to have damaged or destroyed 

about 1,000 wells in the 5-year period 1960 to 1965, and the cost estimate was based on the cost of repair. By the 1960s most large wells 

in the Santa Clara Valley extended to depths of 400 feet or more. 

Many well casings were buckled or collapsed by the compaction of 

clay lenses at depths more than 200 feet below the land surface. The 

compacting clay caused the casing to buckle and eventually collapse. 

The cost estimate cited for the Bay levees as of 1979 applies only to 

the publicly maintained flood-protection levees, and likely underestimates the total cost. An additional, unknown cost was incurred by 

a salt company that maintained levees on 30 square miles of salt 

ponds within the original bayland area. Land subsidence has permanently increased the risk of saltwater flooding in case of levee breaks 

and the potential for saltwater intrusion of shallow aquifers. 

Careful management will continue 

The Santa Clara Valley Water District is currently managing the 

ground-water basin in a conservative fashion in order to avoid further subsidence. Their management strategy depends on continued 

availability of high-quality surface water from State and Federal 

projects that import water from massive diversion facilities in the 

southern part of the Sacramento-San Joaquin Delta. As we describe 

in another case study, these diversion facilities themselves are threatened by land subsidence within the Delta. Thus the prognosis for 

land subsidence in the Santa Clara Valley depends in part on subsidence rates and patterns in the Delta. Because much of California relies on large-scale interbasin water transfers, subsidence and waterquality issues in many parts of the State are complexly interrelated. 
SAN JoAQUIN VALLEY, CALIFORNIA 

Largest human alteration of the Earth's surface 

CALIFORNIA 

0 80 Miles 

0 80 Kilometers 

Devin Galloway and Francis S. Riley 

U.S. Geological Survey, Menlo Park, California 

M ining ground water for agriculture has enabled the San 

Joaquin Valley of California to become one of the world's 

most productive agricultural regions, while simultaneously contributing to one of the single largest alterations of the 

land surface attributed to humankind. Today the San Joaquin Valley 

is the backbone of California's modern and highly technological 

agricultural industry. California ranks as the largest agricultural 

producing state in the nation, producing 11 

percent of the total U.S. agricultural value. 

The Central Valley of California, which 

includes the San Joaquin Valley, the Sacramento Valley, and the Sacramento-San 

Joaquin Delta, produces about 25 percent 

of the nation's table food on only 1 percent 

of the country's farmland (Cone, 1997). 

In 1970, when the last comprehensive surveys of land subsidence were made, subsidence in excess of 1 foot had affected 

more than 5,200 square miles of irrigable 

land-one-half the entire San Joaquin 

Valley (Poland and others, 1975). The 

maximum subsidence, near Mendota, was 

more than 28 feet. 

Approximate location of maximum subsidence in United 

States identified by research 

efforts of Joseph Poland (pictured). Signs on pole show approximate altitude of land 

surface in 1925, 1955, and 

1977. The pole is near benchmark S661 in the San Joaquin 

Valley southwest of Mendota, 

California. 
24 

1' 

s ~ / 

-y 1.-- Cantua C . • 

0 

Pacific 

Ocean 

C' 

0' 

0 40 Miles 

!:==:::'::::-____J 

0 40 Kilometers 

'<-" 

• Bakersfield 

Mining Ground Water 

Since the early 1970s land subsidence has continued in some locations, but has generally slowed due to reductions in ground-water 

pumpage and the accompanying recovery of ground-water levels 

made possible by supplemental use of surface water for irrigation. 

The surface water is diverted principally from the Sacramento-San 

Joaquin Delta and the San Joaquin, Kings, Kern and Feather Rivers. 

Two droughts since 1975 have caused surface-water deliveries in 

the valley to be sharply curtailed, and demonstrated the valley's 

vulnerability to continued land subsidence when ground-water 

pumpage is increased. 

The history of land subsidence in the San Joaquin Valley is integrally linked to the development of agriculture and the availability 

of water for irrigation. Further agricultural development without 

accompanying subsidence is dependent on the continued availability of surface water, which is subject to uncertainties due to climatic 

variability and pending regulatory decisions. 

Land subsidence in the San Joaquin Valley was first noted in 1935 

when I. H. Althouse, a consulting engineer, called attention to the 

possibility of land subsidence near the Delano (Tulare-Wasco) 

area. The process was first described in print by Ingerson (1941, p. 

40-42), who presented a map and profiles ofland subsidence based 

on comparison ofleveling of 1902,1930, and 1940. Four types of 

subsidence are known to occur in the San Joaquin Valley. In order 

of decreasing magnitude they are (1) subsidence capsed by aquifersystem compaction due to the lowering of ground-water levels by 

sustained ground-water overdraft; (2) subsidence caused by the 

hydrocompaction of moisture-deficient deposits above the watertable; (3) subsidence related to fluid withdrawal from oil and gas 

fields; and (4) subsidence related to crustal neotectonic movements. Aquifer-system compaction and hydrocompaction have 

significantly lowered the land surface in the valley since about the 

1920s, and our review of the subsidence problems there is limited 

to these two primary causes. 

THE SAN JOAQUIN VALLEY IS PART 

OF A GREAT SEDIMENT-FILLED TROUGH 

The San Joaquin Valley comprises the southern two-thirds of the 

Central Valley of California. Situated between the towering Sierra 

Nevada on the east, the Diablo and Temblor Ranges to the west, 

and the Tehachapi Mountains to the south, the valley occupies a 

trough created by tectonic forces related to the collision of the Pacific and North American Plates. The trough is filled with marine 

sediments overlain by continental sediments, in some places thousands of feet deep, deposited largely by streams draining the 

mountains, and partially in lakes that inundated portions of the 

valley floor from time to time. More than half the thickness of the 

continental sediments is composed of fine-grained (clay, sandy clay, 

sandy silt, and silt) stream (fluvial) and lake (lacustrine) deposits 

susceptible to compaction. 
San Joaquin Valley, California 

Meltwater from the Sierra 

snowpack recharges ground 

water in the San Joaquin Valley and supplies surface water during the dry summer 

months. 

25 

The valley floor, comprising about 10,000 square miles, is arid to 

semiarid, receiving an average of 5 to 16 inches of rainfall annually. 

Most of the streamflow in the valley enters from the east side in 

streams draining the western Sierra Nevada, where much of the 

(California Department ofWater Resources) precipitation occurs as snow. The San Joaquin River begins high in 

the Sierra Nevada and descends onto the valley floor, where it takes 

a northerly flow path toward the Sacramento-San Joaquin Delta. 

On its course northward to the Delta it collects streamflow from 

the central and northern portions of the valley. The southern valley 

receives streamflow from the Kings, Kaweah, and Kern Rivers, 

which issue from steeply plunging canyons onto broad, extensive 

alluvial fans. Over many thousands of years, the natural flow of 

these rivers distributed networks of streams and washes on the 

slopes of the alluvial fans and terminated in topographically closed 

sinks, such as Tulare Lake, Kern Lake, and Buena Vista Lake. 

Streams draining the drier western slopes and Coast Ranges adjacent to the valley are intermittent or ephemeral, flowing only episodically. Precipitation and streamflow in the valley vary greatly 

from year to year. 

Pumping for irrigation altered the ground-water budget 

Ground water occurs in shallow, unconfined (water table) or partially-confined aquifers throughout the valley. Such aquifers are 

particularly important near the margins of the valley and near the 

toes of younger alluvial fans. A laterally extensive lacustrine clay 

known as the Corcoran Clay is distributed throughout the central 

and western valley. The Corcoran Clay, which varies in thickness 

from a feather edge to about 160 feet beneath the present bed of 

Tulare Lake, confines a deeper aquifer system that comprises finegrained aquitards interbedded with coarser aquifers. Most of the 

subsidence measured in the valley has been correlated with the 

distribution of ground-water pumpage and the reduction of water 

levels in the deep confined aquifer system. 
26 

PREDEVELOPMENT 

Ground water flowed from 

the mountains toward the 

center of the valley where it 

discharged into streams or 

through evapotranspiration. 

POSTDEVELOPMENT 

Ground water flows generally 

downward and toward 

pumping centers. 

Cor<:ora{l clay 

(~ooflning la:zer) 

Mining Ground Water 

Under natural conditions before development, ground water in the 

alluvial sediments was replenished primarily by infiltration through 

stream channels near the valley margins. The eastern-valley streams 

carrying runoff from the Sierra Nevada provided most of the recharge for valley aquifers. Some recharge also occurred from precipitation falling directly on the valley floor and from stream and 

lake seepage occurring there. Over the long term, natural replenishment was dynamically balanced by natural depletion through 

ground-water discharge, which occurred primarily through evapotranspiration and contributions to streams flowing into the Delta. 

The areas of natural discharge in the valley generally corresponded 

with the areas of flowing, artesian wells mapped in an early USGS 

investigation (Mendenhall and others, 1916). Direct ground-water 

outflow to the Delta is thought to have been negligible. 

Today, nearly 150 years since water was first diverted at Peoples Weir 

on the Kings River and more than 120 years after the first irrigation 

colonies were established in the valley, intensive development of 

ground-water resources for agricultural uses has drastically altered 

the valley's water budget. The natural replenishment of the aquifer 

systems has remained about the same, but more water has discharged than recharged the aquifer system; the deficit may have 

amounted to as much as 800,000 acre-feet per year during the late 

1960s (Williamson et al. , 1989). Most of the surface water now being 

imported is transpired by crops or evaporated from the soil. The 

amount of surface-water outflow from the valley has actually been 

River 
San Joaquin Valley, California 

By pumping the vast reserves 

of ground water, farmers have 

developed the San Joaquin 

Valley into a major agricultural 

region. 

27 

reduced compared to predevelopment conditions. Ground water in 

the San Joaquin Valley has generally been depleted and redistributed 

from the deeper aquifer system to the shallow aquifer system. This 

has created problems of ground-water quality and drainage in the 

shallow aquifer system, which is infiltrated by excess irrigation water 

that has been exposed to agricultural chemicals and natural salts 

concentrated by evapotranspiration. 

A STABLE WATER SUPPLY IS DEVELOPED FOR IRRIGATION 

In the San Joaquin Valley, irrigated agriculture surged after the 1849 

Gold Rush and again in 1857, when the California Legislature 

passed an act that promoted the drainage and reclamation of riverbottom lands (Manning, 1967). By 1900, much of the flow of the 

Kern River and the entire flow of the Kings River had been diverted 

through canals and ditches to irrigate lands throughout the southern part of the valley (Nady and Laragueta, 1983). Because no significant storage facilities accompanied these earliest diversions, the 

agricultural water supply, and hence crop demand, was largely limited by the summer low-flows. The restrictions imposed by the need 

for constant surface-water flows, coupled with a drought occurring 

around 1880 and the fact that, by 1910, nearly all the available surface-water supply in the San Joaquin Valley had been diverted, 

prompted the development of ground-water resources. 

The first development of the ground-water resource occurred in 

regions where shallow ground water was plentiful, and particularly 

where flowing wells were commonplace, near the central part ofthe 

valley around the old lake basins. Eventually, the yields of flowing 

(Californta Department ofWater Resources) WellS diminished aS Water levels Were reduced, and it became neCeSSary to install pumps in wells to sustain flow rates. Around 1930, the 

development of an improved deep-well turbine pump and rural 

electrification enabled additional ground-water development for 

irrigation. The ground-water resource had been established as a 

reliable, stable water-supply for irrigation. Similar histories were 

repeated in many other basins in California and throughout the 

Southwest, where surface water was limited and ground water was 

readily available. 

Overhead and flood irrigation supply water to a wide 

range of crops. 

(California Department of 

Water Resources) 
28 

Extraction of ground water for 

irrigation in the San joaquin Valley 

8----------------------

Pumpage L - ---------------- --------- (millions of ) 

acre-feet) • ~ 

Pumpage 

(millions of 

acre-feet) 

0 ~-· I 

1900 1920 

I 

1940 

Total ground-water extractions 

for irrigation, 1955 

I 

1960 

40 ----------------------

0------ United States California San Joaquin --•-

Valley 

Qoseph F. Poland, U.S. Geological Survey. 

written communication, ca 1957) 

Mining Ground Water 

WATER WITHDRAWAL CAUSED LAND SUBSIDENCE 

Shortly after the completion of the Delta-Mendota Canal by the 

U.S. Bureau of Reclamation in 1951, subsidence caused by withdrawal of ground water in the northern San Joaquin Valley had 

begun to raise concerns, largely because of the impending threat to 

the canal and the specter of remedial repairs. Because of this threat 

to the canal, and in order to help plan other major canals and engineering proposed for construction in the subsiding areas, the 

USGS, in cooperation with the California Department of Water 

Resources, began an intensive investigation into land subsidence in 

the San Joaquin Valley. The objectives were to determine the causes, 

rates, and extent ofland subsidence and to develop scientific criteria for the estimation and control of subsidence. The USGS concurrently began a federally funded research project to determine 

the physical principles and mechanisms governing the expansion 

and compaction of aquifer systems resulting from changes in aquifer hydraulic heads. Much of the material presented here is drawn 

from these studies. 

In 1955, about one-fourth (almost 8 million acre-feet) of the total 

ground water extracted for irrigation in the United States was 

pumped in the San Joaquin Valley. The maximum changes in water 

levels occurred in the western and southern portions of the valley, 

in the deep confined aquifer system. More than 400 feet of waterChange in water-table altitude 

from 1860 to spring 1961 

Change in water level in the deep confined 

aquifer system from 1860 to spring 1961 

0 

0 

Ground-water level change (feet) 

Ooto40 ]Rise 

Ooto40 J D 40 to I 00 Decline 

D Greater than I 00 

!' 

• Bakersfield 

0 

0 

'\ -y 

J' 

--' 

L .. 

\ \ 

/ " ·, "'/- -L. 

' 0~ -,. ~~ 

o> ,· 

,_~---------------. ~ Ground-water level change (feet) ~~ 

D 0 to 40 J Rise • ·'!.>l., 

• Bakersfield 

, ..... 

D B~;:o I 20 to200 

~20 l 

Decline 

0200to 300 

D Greater than 300 

(Williamson and others. 1989) 
San Joaquin Valley, California 

By 1971 the growing use 

of imported surface-water 

supplies surpasses the use 

of local ground-water supplies, but the effects of 

drought reverse this trend 

in 1977. 1200 

-

29 

Surface water 

Ground water 

-

Annual total 

water use 600 

(millions of - -

acre-feet) 

Land subsidence from 1926 to 1970 

0 40Miles 

~===;-___j 

40 Kilometers 

0 

·. 

·. 

' 

G' (1l 

r-'-----""-0' ___ ·_. -:-- ,\\ --<. ••• ) ( 

Subsidence (feet) "J> •. 

~ D Less than I ~<:/"c \ • Bakersfield D I to4 ···!.. 

04to8 

O sto 12 

D 12to 16 

D 16to 24 

D Greater than 24 

Decline 

(Modified from Poland 

and others, 1975) 

- -

[ i· :J :J 

m I l ~ 0 0 

a. a. 

~ ~ I I I I I I 0 

1968 1970 1972 1974 1976 

level decline occurred in some west -side areas in the deep aquifer 

system. Until1968, irrigation water in these areas was supplied 

almost entirely by ground water. As of 1960, water levels in the deep 

aquifer system were declining at a rate of about 10 feet per year. 

Western and southern portions of the valley generally experienced 

more than 100 feet of water-level decline in the deep aquifer system. Water levels in the southeastern and eastern portions of the 

valley were generally less affected because some surface water was 

also available for irrigation. In the water-table aquifer, few areas 

exceeded 100 feet of water-level decline, but a large portion of the 

southern valley did experience declines of more than 40 feet. In 

some areas on the northwest side, the water-table aquifer rose up to 

40 feet due to infiltration of excess irrigation water. 

Accelerated ground-water pumpage and water-level declines, principally in the deep aquifer system during the 1950s and 1960s, 

caused about 75 percent of the total volume of land subsidence in 

the San Joaquin Valley. By the late 1960s, surface water was being 

diverted to agricultural interests from the Sacramento-San Joaquin 

Delta and the San Joaquin River through federal reclamation 

projects and from the Delta through the newly completed, massive 

State (California) Water Project. Less-expensive water from the 

Delta-Mendota Canal, the Friant-Kern Canal, and the California 

Aqueduct largely supplanted ground water for crop irrigation. 

Ground-water levels began a dramatic period of recovery, and subsidence slowed or was arrested over a large part of the affected area. 

Water levels in the deep aquifer system recovered as much as 200 

feet in the 6 years from 1967 to 1974 (Ireland and others, 1984). 

When water levels began to recover in the deep aquifer system, 

aquifer-system compaction and land subsidence began to abate, 

although many areas continued to subside, albeit at a lesser rate. 

During the period from 1968 to 197 4, water levels measured in an 

observation well near Cantua Creek recovered more than 200 feet 

while another 2 feet of subsidence continued to accrue. This apparent contradiction is the result of the time delay in the compaction 
30 

To supplement local groundwater supplies, the California 

Aqueduct (left) conveys water 

from the Delta to the dry southern valleys. 

When water levels recover, 

compaction and land subsidence can abate. 

Mining Ground Water 

(California Department ofWater Resources) 

of the aquitards in the aquifer system. The delay is caused by the 

time that it takes for pore-fluid pressures in the aquitards to equilibrate with the pressure changes occurring in the aquifers, which 

are much more responsive to the current volume of ground-water 

being pumped (or not pumped) from the aquifer system. The time 

needed for pressure equilibration depends largely on the thickness 

and permeability of the aquitards. Typically, as in the San Joaquin 

Valley, centuries will be required for most of the pressure equilibration to occur, and therefore for the ultimate compaction to be realized. Swanson (1998) states that "Subsidence is continuing in all 

historical subsidence areas . . . , but at lower rates than before .. . :' 

Since 197 4, land subsidence has been greatly slowed or largely arrested but remains poised to resume. In fact, during the severe 

lOO --------------------~ ro gh ~-------- ro ht~-----

Compaction 

(i nches) 

1960 1970 1980 1990 

(Modified from Swanson, 1998) 

Some elastic expansion of the aquifer 

system has occured, but the compacted materials can never return to 

their pre-compacted thickness. 
San Joaquin Valley, California 

In the major subsiding areas, 

subsidence has continued except for a slight leveling off in 

the mid 1970s. 

Subsidence 

(volume measured, 

millions of acre-feet) 

12 

1930 1940 

1.- • Fresno «'L 

Bakersfield. 

1950 

7 

() 

7 

Major subsiding areas 

in the San joaquin Valley 

A 1-to-3 ratio between subsidence and pumpage in area A 

refiects the portion of total 

_.;: pumpage that was initially derived from water of compaction. 

Lt PJt ::1 c JrrL at ve 

purlj.l""e 1A 

I /it 0 f t 

1960 1970 

(Modified from Poland and others, 1975) 

31 

droughts in California in 1976-77 and 1987-91, diminished deliveries of imported water prompted some water agencies and farmers, especially in the western valley, to refurbish old pumping 

plants, drill new wells, and begin pumping ground water to make 

up for cutbacks in the imported water supply. The decisions to renew ground-water pumpage were encouraged by the fact that 

ground-water levels had recovered nearly to predevelopment levels. 

During the 197 6-77 drought, after only one-third of the peak annual pumpage volumes of the 1960s had been produced, groundwater levels rapidly declined more than 150 feet over a large area 

and subsidence resumed. Nearly 0.5 feet of subsidence was measured in 1977 near Cantua Creek. This scenario was repeated during the more recent 1987-91 drought. It underscores the sensitive 

dependence between subsidence and the dynamic state of imported-water availability and use. 

That a relatively small amount of renewed pump age caused such a 

rapid decline in water levels reflects the reduced ground-water storage capacity- lost pore space-caused by aquifer-system compaction. It demonstrates the nonrenewable nature of the resource 

embodied in the "water of compaction:' It emphasizes the fact that 

extraction of this resource, available only on the first cycle of largescale drawdown, must be viewed, like more traditional forms of 

mining, in terms not only of its obvious economic return but also 

its less readily identifiable costs. 
32 

Hydrocompaction 

Compaction near the surface 

Hydrocompaction-compaction due to wetting- is a 

near-surface phenomenon that produces land-surface 

subsidence through a mechanism entirely different from 

the compaction of deep, overpumped aquifer systems. 

Both of these processes accompanied the expansion of irrigated agriculture onto the arid, gentle slopes of the alluvial 

fans along the west side and south end of the San Joaquin 

Valley. Initially, the distinction between them, and their 

relative contributions to the overall subsidence problem, 

were not fully recognized. 

In the 1940s and 50s farmers bringing virgin valley soils 

under cultivation found that standard techniques of flood 

irrigation caused an irregular settling of their carefully 

graded fields, producing an undulating surface of hollows 

and hummocks with local relief, typically of3 to 5 feet. 

Where water flowed or ponded continuously for months, 

very localized settlements of 10 feet or more might occur 

on susceptible soils. These consequences of artificial wetting seriously disrupted the distribution of irrigation water 

and damaged pipelines, power lines, roadways, airfields, 

and buildings. In contrast to the broad, slowly progressive 

and generally smooth subsidence due to deep-seated aquifer-system compaction, the irregular, localized, and often 

rapid differential subsidence due to hydrocompaction was 

readily discernible without instrumental surveys. Recognition of its obvious impact on the design and construction 

of the proposed California Aqueduct played a major role 

in the initiation in 1956 of intensive studies to identify, 

characterize, and quantify the subsidence processes at 

work beneath the surface of the San Joaquin Valley. 

Mining Ground Water 

Hydrocompaction produces an 

undulating surface of hollows and 

hummocks with local relief, typically 

of 3 to 5 feet. In this view of a 

furrowed field, the hollows are filled 

with irrigation water. 

MECHANISMS OF COMPACTION 

WERE ANALYZED 

The mechanisms and requisite conditions for hydrocompaction, initially known as "near-surface subsidence," were 

investigated by means oflaboratory tests on soil cores from 

depths to 100 or more feet, and by continuously flooded 

test plots equipped with subsurface benchmarks at various 

depths and, in some cases, with soil-moisture probes. 

The combined field and laboratory studies demonstrated 

that hydrocompaction occur~ed only in alluvial-fan sediments above the highest prehistoric water table and in 

areas where sparse rainfall and ephemeral runoff had never 

Hydrocompaction caused surface 

cracks and land subsidence at experimental Test Plot B, Fresno County. 
San Joaquin Valley, California 

Mudflow containing 

hydrocompactible sediments, western Fresno 

County (1961) 

penetrated below the zone subject to summer desiccation 

by evaporation and transpiration. Under these circumstances the initial high porosity of the sediments (often 

enhanced by numerous bubble cavities and desiccation 

cracks) is sun-baked into the deposits and preserved by 

their high dry strength, even as they are subjected to the 

increasing load of 100 or more feet of accumulating overburden. In the San Joaquin Valley, such conditions are 

associated with areas of very low average rainfall and infrequent, flashy, sediment -laden runoff from small, relatively 

steep upland watersheds that are underlain by easily erodable shales and mudstones. The resulting muddy debris 

flows and poorly sorted stream sediments typically contain 

montmorillonite clay in proportions that cause it to act, 

when dry, as a strong interparticulate bonding agent. 

When water is first applied in quantities sufficient to penetrate below the root zone the clay bonds are drastically 

weakened by wetting, and the weight of the overburden 

crushes out the excess porosity. The process of densifying 

to achieve the strength required to support the existing 

overburden may reduce the bulk volume by as much as 10 

percent, the amounts increasing with increasing depth and 

overburden load. 

Most of the potential hydrocompaction latent in anomalously dry, low-density sediments is realized as rapidly as 

the sediments are thoroughly wetted. Thus the progression 

of a hydrocompaction event is controlled largely by the 

rate at which the wetting front of percolating water can 

move downward through the sediments. A site underlain 

by a thick sequence of poorly permeable sediments may 

continue to subside for months or years as the slowly 

descending wetting front weakens progressively deeper 

deposits. If the surface water source is seasonal or intermittent, the progression is further delayed. 

Localized compaction beneath a water-filled pond or ditch 

often leads to vertical shear failure at depth between the 

water-weakened sediments and the surrounding dry material. At the surface this process surrounds the subsiding 

flooded area with an expanding series of concentric tensional fissures having considerable vertical offset-a 

severely destructive event when it occurs beneath an engineered structure. 

The hazards presented by hydrocompaction are somewhat 

mitigated by the fact that the process goes rapidly to completion with the initial thorough wetting, and is not subject 

to reactivation through subsequent cycles of decreasing 

and increasing moisture content. However, if the volume 

of water that infiltrates the surface on the first wetting cycle 

is insufficient to wet the full thickness of susceptible deposits, then the process will propagate to greater depths on 

subsequent applications, resulting in renewed subsidence. 

Also, an increase in the surface load such as a bridge footing or a canal full of water can cause additional compaction 

in prewetted sediments. 

Studies undertaken in the mid- 1950s 

led to a better understanding of hydrocompaction and to the identifi cation of long reaches of the Californ ia 

Aqueduct route that were underlain 

by deposits susceptible to hydrocompaction. Construction of the 

aqueduct through these reaches was 

preceded by prewetting, and thus 

compacting to a nearly stable state, 

the full thickness of susceptible deposits beneath the aqueduct alignment. These measures added more 

than two years and tens of millions 

of dollars to the cost of the project. 

Prewetting a new section of 

the California Aqueduct to 

precompact shallow deposits 

susceptible to hydrocompaction 

(near toe of Moreno Gulch, 1963) 

33 
34 Mining Ground Water 

MANY COSTS OF LAND SUBSIDENCE ARE HIDDEN 

The economic impacts of land subsidence in the San Joaquin Valley 

are not well known. Damages directly related to subsidence have 

been identified, and some have been quantified. Other damages 

indirectly related to subsidence, such as flooding and long-term 

environmental effects, merit additional assessment. Some of the 

direct damages have included decreased storage in aquifers, partial 

or complete submergence of canals and associated bridges and pipe 

crossings, collapse of well casings, and disruption of collector drains 

and irrigation ditches. Costs associated with these damages have 

been conservatively estimated at $25,000,000 (EDAW-ESA, 1978). 

These estimates are not adjusted for changing valuation of the dollar, and do not fully account for the underreported costs associated 

with well rehabilitation and replacement. When the costs of lost 

property value due to condemnation, regrading irrigated land, and 

replacement of irrigation pipelines and wells in subsiding areas are 

included, the annual costs of subsidence in the San Joaquin Valley 

soar to $180 million per year in 1993 dollars (G. Bertoldi and S. 

Leake, USGS, written communication, March 30, 1993). 
HousToN-GALVESTON, TExAs 

Managing coastal subsidence 

TEXAS 

0 20 Miles 

,------, 

0 20 Kilometers 

Laura S. Coplin 

U.S. Geological Survey, Houston, Texas 

Devin Galloway 

U.S. Geological Survey, Menlo Park, California 

T he greater Houston area, possibly more than any other 

metropolitan area in the United States, has been adversely 

affected by land subsidence. Extensive subsidence, caused 

mainly by ground-water pumping but also by oil and gas extraction, 

has increased the frequency of flooding, caused extensive damage to 

industrial and transportation infrastructure, motivated major investments in levees, reservoirs, and surface-water distribution facilities, and caused substantial loss of wetland habitat. 

Although regional land subsidence is often subtle and difficult to 

detect, there are localities in and near Houston where the effects are 

quite evident. In this low-lying coastal environment, as much as 10 

feet of subsidence has shifted the position of the coastline and 

changed the distribution of wetlands and aquatic vegetation. In fact, 

the San Jacinto Battleground State Historical Park, site of the battle 

that won Texas independence, is now partly submerged. This park, 

about 20 miles east of downtown Houston on the shores of 

Galveston Bay, commemorates the April21, 1836, victory of Texans 

led by Sam Houston over Mexican forces led by Santa Ana. About 

100 acres of the park are now under water due to subsidence, and 

A road (below right) that provided access to the 

San Jacinto Monument was closed dt.Je to flooding caused by subsidence. 
36 

_, 

'"' \ 

I 

r"' 

,, 

\ " ..... , \ ' ...... .,/___....;..,. 

I 

I 

\ HARRIS 

I 

I 

I 

\ Coasta I 

~ Cll'fA ,. n 

' / 

1

, FORT BEND 

\ 

I COUNTY ) 

'~ \ / '-./ 

0 20 Miles 

0 20 Kilometers 

Mining Ground Water 

part of the remaining area must now be protected from the Bay by 

dikes that trap local rain water, which must then be removed by 

pumps. At many localities in the Houston area, ground-water 

pumpage and subsidence have also induced fault movement, leading to visible fracturing, surface offsets, and associated property 

damage. 

Growing awareness of subsidence-related problems on the part of 

community and business leaders prompted the 1975 Texas legislature to create the Harris-Galveston Coastal Subsidence District," ... 

for the purpose of ending subsidence which contributes to, or precipitates, flooding, inundation, and overflow of any area within the 

District ... :'This unique District was authorized to issue (or refuse) 

well permits, promote water conservation and education, and promote conversion from ground-water to surface-water supplies. It 

has largely succeeded in its primary objective of arresting subsidence in the coastal plain east of Houston. However, subsidence has 

accelerated in fast -growing inland areas north and west of Houston, 

which still rely on ground water and, partly as a result, the Fort 

Bend Subsidence District was created by the legislature in 1989. 

THE FLAT, HUMID GULF COAST IS PRONE TO FLOODING 

The Houston-Galveston Bay area includes a large bay-estuary-lagoon system consisting of the Trinity, Galveston, East, and West 

Bays, which are separated from the Gulf of Mexico by Pelican Island, Galveston Island, and the Bolivar Peninsula. Tidal exchange 

occurs between the Gulf and bay system through the barrier-island 

and peninsula complex. 

The Houston climate is subtropical; temperatures range from 45° to 

93° Fahrenheit and on average about 47 inches of rain falls each 

year. The humid coastal plain slopes gently towards the Gulf at a 

Galveston \ c, o 

""~.,. 

' o\ ~\ (j 

rate of about 1 foot per mile. Two major 

rivers, the Trinity and San Jacinto, and 

many smaller ones traverse the plain before 

discharging into estuarine areas of the bay 

system. Another large river, the Brazos, 

crosses the Fort Bend Subsidence District 

and discharges directly into Galveston Bay. 

The same warm waters of the Gulf of 

Mexico that attract recreational and commercial fishermen, and other aquatic enthusiasts, are conducive to hurricanes and 

tropical storms. The Texas coast is subject to 

a hurricane or tropical storm about once 

every 2 years (McGowen and others, 1977). 

Storm tides associated with hurricanes have 

reached nearly 15 feet in Galveston . The 

flat-lying region is particularly prone to 

flooding from both riverine and coastal 

sources, and the rivers, their reservoirs, and 
Houston-Galveston, Texas 

Galveston Bay near 

Goose Creek 

Homes at Greens Bayou were 

flooded during a storm in 

June 1989. 

37 

an extensive system of bayous and manmade canals are managed as 

part of an extensive flood-control system. 

Land subsidence contributes to flooding 

Land subsidence in the Houston-Galveston area has increased the 

frequency and severity of flooding. Near the coast, the net result of 

land subsidence is an apparent increase in sea level, or a relative sealevel rise: the net effect of global sea-level rise and regional land subsidence in the coastal zone. The sea level is in fact rising due to 

regional and global processes, both natural and human-induced. 

The combined effects of the actual sea-level rise and natural consolidation of the sediments along the Texas Gulf coast yield a relative 

sea-level rise from natural causes that locally may exceed 0.08 inches 

per year (Paine, 1993). Global warming is contributing to the 

present-day sea level rise and is expected to result in a sea-level increase of nearly 4 inches by the year 2050 (Titus and Narayanan, 

1995). However, during the 20th century human-induced subsidence has been by far the dominant cause of relative sea-level rise 

along the Texas Gulf Coast, exceeding 1 inch per year throughout 

much of the affected area. This subsidence has resulted principally 

from extraction of ground water, and to a lesser extent oil and gas, 

from subsurface reservoirs. Subsidence caused by oil and gas production is largely restricted to the field of production, as contrasted to the 

regional-scale subsidence typically caused by ground-water pumpage. 

HOUSTON'S GROWTH WAS BASED ON OIL 

AND GAS INDUSTRIES 

Since 1897, when the population was about 25,000, the Houston area 

has experienced rapid growth, spurred on by the discovery of oil 

and establishment of the Port of Houston. In 1907 the first successful oil well was drilled, marking the beginning of the petrochemical 

industry that provided the economic base on which the Houston 

area was built and still stands. In 1925 Houston became a deep-water port when the U.S. Army Corps of Engineers completed dredging 

the Houston Ship Channel across Galveston Bay, up the lower 

reaches of the San Jacinto River, and along Buffalo Bayou to Hous-
38 

Houston (downtown can be 

seen top center) owes much of 

its development to the Houston 

ship channel, which is flanked 

by petrochemical industries and 

shipping facilities . 

Annu 

grvLr d-w 

pun >af\ 

fT'illicns vf 

~ re ~~ ) 

06 

04 

02 

Following the opening of the 

Houston Ship Channel in 1915, 

large water-consuming oil refineries were constructed. 

Mining Ground Water 

ton. Easy access to the Gulf via the ship channel, and the discovery 

of additional oilfields, triggered major industrial development 

along the ship channel in Baytown-La Porte, Pasadena, Texas City, 

and Houston. The region and industry have continued to grow, and 

the Houston-Galveston area currently has a population of about 3 

million people that is projected to grow to 4.5 million by the year 

2010. Nearly half of all U.S. petrochemical production occurs in the 

greater Houston area. The Port of Houston is the second largest port 

(by tonnage shipment) in the nation, eighth largest in the world, 

and handles more commodities for Mexico than all Mexican ports 

combined. Subsidence to the east of Houston has recently been 

arrested by substituting imported surface water supplies for much 

of the ground-water pumpage, but fast growing areas to the west 

and north, which still depend largely on ground water, are actively 

subsiding. 

"'"' 0.4 

Surface-water use (approximate) Population 

(millions of 

residents) 

Industries were established 

in the early 1940s to support the war effort, and 

after World War II industry 

and population continued 

to grow. 

- 0.2 

Surface water from Lake Water from Lake Livingston was first deHouston became available livered to the Ethyl Corporation in late 

in 1954, and ground-water 1976 and most of the industrial converpumpage was temporarily sion to surface water occurred in 1977. 

reduced. 

(Compiled from Jorgenson, 1961: Gabrysch. 1987: and 

Houston-Galveston Coastal Subsidence District, 1996) 
Houston-Galveston, Texas 

Goose Creek oil field 

Prolific oil production produced the region 's first major subsidence 

Most subsidence in the Houston area has been caused by 

ground-water withdrawal, but the earliest subsidence was 

caused by oil production. In fact, the subsidence of the Goose 

Creek oil field on Galveston (San Jacinto) Bay was the first 

"In 1917 a prolific oil field was developed near the mouth 

of Goose Creek, and during 1918 and subsequent years, 

millions of barrels of oil were removed from beneath its 

surface. Beginning in 1918 it became apparent that the 

Gaillard Peninsula, near the center of the field, and other 

nearby low land was becoming submerged. Elevated plank 

roadways or walks were built from the mainland to the 

derricks. Derrick floors had to be raised. Vegetation was 

flooded and killed, and finally all of the peninsula disappeared beneath the water ... The maximum measured 

subsidence is now more than 3 feet and the area affected is 

2'12 miles long by 1112 

miles wide ... Outside 

this area no change in 

elevation can be 

detected ... . " 

- Pratt and johnson, 1926 

Between 1918 and 1926 

subsidence was measured 

around Goose Creek oilfield. Lines of equal subsidence (feet) for an 8-year 

period are shown in grey 

lines-for a 1-year period, 

in black lines. 

"Submerged land in Texas belongs to the state and only the 

state can grant oil and gas leases on submerged lands. 

Consequently, when Gaillard Peninsula became submerged, 

the state claimed title to it and sought not only to dispossess 

the fee owner and the oil and gas lessee, but also to recover 

from them the value of the oil and gas removed from the 

premises subsequent to the time when the land became 

submerged. The question was taken into court and finally 

Pratt and Johnson (1926) also noted that the subsided volume, 

calculated based on the difference between current and initial 

topography, amounted to about 20 per cent of the produced 

volume of oil, gas, water, and sand. 

FAULTING FOLLOWED SUBSIDENCE 

" ... cracks appeared in the ground running beneath houses, 

across streets, and through lawns and gardens .... recurrent 

movement along them resulted in dropping the surface of 

the ground on the side toward the oil field .. . The movements were accompanied by slight earthquakes which 

shook the houses, displaced dishes, spilled water, and disturbed the inhabitants generally." 

-Pratt and johnson, 1926 

subsidence attributed to subsurface-fluid withdrawal to be 

described in the scientific literature. A dispute over the legal 

status of the land submerged by subsidence caused Texas 

courts to formally recognize the process. 

0 20 Miles i===:::;--__J 0 20 Kilometers 

"There can be no doubt, .. . that the contours 

show correctly the essential fact that a local 

'dishing' of the earth's surface has occurred in 

the Goose Creek region, the central area of 

greatest subsidence corresponding approximately with the center of the oil field." 

-Pratt and johnson, 1926 

a decision was rendered in favor of the defendants, that is, the 

claim of the state of Texas was denied, and the present owners 

continue in possession. The basis for the decision was the 

court's acceptance that the subsidence at Goose Creek (which 

the defendants admitted) was caused by an act of man, 

namely, the removal of large volumes of oil, gas, water, and 

sand from beneath the surface." 

This photograph taken 

about 1926 shows a 

'fault fissure' in Pelley, 

one-half mile north of 

the oil fields. To the 

left of the fault, the 

ground had dropped 

about 16 inches. 

-Pratt and johnson, 1926 

39 
40 

Subsidence trends reflect 

patterns of resource development that shifted inland 

from coastal oil and gas extraction to ground-water 

extraction for municipal and 

industrial supplies. 

0 

Line of equal land 

subsidence 

30 Miles 

30 Ki lometers 

1906-1943 

1973-1995 

(Harris-Galveston Coastal Subsidence District) 

Mining Ground Water 

Subsidence trends are related to patterns 

of ground-water and oil-and-gas extraction 

Land subsidence first occurred in the early 1900s in areas where 

ground water, oil, and gas were extracted and has continued 

throughout the 20th century due primarily to ground-water pumpage. The patterns of subsidence in the Houston area closely follow 

the temporal and spatial patterns of subsurface fluid extraction. 

Prior to the early 1940s there was localized subsidence caused 

chiefly by the removal of oil and gas along with the attendant brine, 

ground water and sand in oilfields such as Goose Creek. Near Texas 

City the withdrawal of ground water for public supply and industry 

caused more than 1.6 feet of subsidence between 1906 and 1943. 

This period also marked the beginning of a slow but steady development of ground-water resources that constituted the sole water supply for industries and communities around the Ship Channel, 

including Houston. By 1937 ground-water levels were falling in a 

growing set of gradually coalescing cones of depression centered on 

the areas of heavy use. Until1942, essentially all water demand in 

Houston was supplied by local ground water. By 1943 subsidence 

had begun to affect a large part of the Houston area although the 

amounts were generally less than 1 foot. 

A period of rapid growth in the development of ground-water resources was driven by the expansion of the petrochemical industry 

and other allied industries in the early 1940s through the late 1970s. 

By the mid-1970s, 6 or more feet of subsidence had occurred 

throughout an area along the Ship Channel between Bayport and 

Houston, as a result of declining ground-water levels associated with 

the rapid industrial expansion. During this time, subsidence problems took on crisis proportions, prompting the creation of the Harris-Galveston Coastal Subsidence District. By 1979 up to 10 feet of 

subsidence had occurred, and almost 3,200 square miles had subsided more than 1 foot. 

In the 1940s upstream reservoirs and canals allowed the first deliveries of surface water to Galveston, Pasadena, and Texas City, but 

ground water remained the primary source until the 1970s. The city 

of Galveston began converting to surface water supplied from Lake 

Houston in 1973, and in the late 1970s the cities of Pasadena and 

Texas City converted to surface water from Lake Livingston, a reservoir on the Trinity River. 

Since the late 1970s subsidence has largely been arrested along the 

Ship Channel and in the Baytown-LaPorte and Pasadena areas due 

to a reduction in ground-water pumpage made possible by the conversion from ground-water to surface-water supplies. By 1995, total 

annual ground-water pumpage in the Houston area had declined to 

only 60 percent of peak amounts pumped during the late 1960s; 

within the jurisdiction of the Harris-Galveston Coastal Subsidence 

District, ground-water pumpage constituted only 25 percent of peak 

amounts. However, as subsidence in the coastal area was stabilizing, 
Houston-Galveston, Texas 41 

The Harris-Galveston Coastal 

Subsidence District has arrested subsidence along the 

western margins of Galveston 

Bay by substituting imported 

water for ground water. A 

new challenge is to manage 

ground-water use north and 

west of Houston where water 

levels are declining and subsidence is increasing. 

Change in ground-water levels in wells in the 

Evangeline aquifer, 1977-1997 

Continuing subsidence 

1974-1997 

Subsidence 

Subsidence 

(feet) 

Some arrested subsidence 

1974-1997 

0.8 -,--,.---,--r=---,--

1975 1985 1995 

(feet) 1.2 ---_,_ ___ _ 

Line of equal 

ground-water 

level change 

(interval 40 fe 

1975 1985 1995 

Extensometer sites 

for measuring subsidence 

@ Addicks 

@ Lake Houston 

©Baytown 

@ Clear Lake 

@ Texas City 

subsidence inland-north and west of Houston-was accelerating. 

In this region ground-water levels have declined more than 100 

feet in the Evangeline aquifer between 1977 and 1997, and more 

than 2.5 feet of subsidence was measured near Addicks between 

1973 and 1996. 

Texas Gulf Coast Aquifer System 

The Evangeline aquifer is the principal source of freshwater 

Most of the ground water pumped in the Houston-Galveston area comes from the Chicot and Evangeline aquiferspart of a vast coastal aquifer system that extends throughout 

the margin of the coastal plain of Texas and Louisiana into 

Florida. Most of the supply wells are completed in the upper 

1,000 to 2,000 feet of the aquifers, where freshwater is available. Saltwater, originally in the aquifers and subsequently 

flushed by freshwater following sea-level recession, now 

The Evangeline is recharged directly by 

precipitation and surface runoff where it 

crops out north of Houston. 

North I 

Recharge 

encroaches on deeper portions of the aquifers. An interface 

between the saltwater and the overlying freshwater slopes 

landward from the Galveston coast. Historically, saltwater 

encroachment in both aquifers has been exacerbated by 

lowered ground-water levels, especially near the coast. 

Ground-water quality, levels, and aquifer-system compaction are being closely monitored to minimize any detrimental effects related to overdrafting the ground-water supply. 

A weak hydraulic connection between shallow 

ground water, the Chi cot aquifer, and t he Evangeline 

aquifer allows the vertical movement of water into 

and between the aquifers. 

Gulf of Mexico 

Sea 

level ~ Chicot aquifer 

I South 

----- Interface between salt 

and freshwater 

Land surface _

1 600 altitude 

(feet above 

sea level) -3200 

Saltwater Burkeville confining layer 

jasper aquifer 

-4800-'---------------------- - --'--

0 20Mi 

,-----, 

0 20 Km 
42 

In 1983 Brownwood was 

flooded after hurricane Alicia 

produced a storm surge up to 

11 feet. 

(Harris·Galveston Coastal Subsidence District) 

Water from Galveston Bay inundated subsiding land and flooded 

homes in Baytown (1960). 

' I 

Mining Ground Water 

Subsidence increases the frequency and intensity of flooding 

Located along a low-lying coast that is subject to tropical storms, 

the Houston area is naturally vulnerable to flooding. In coastal 

areas, subsidence has increased the amount of land subject to the 

threat of tidal inundation. Flooding by tidal surges and heavy rains 

accompanying hurricanes may block evacuation routes many 

hours before the storms move inland, endangering inhabitants of 

islands and other coastal communities. The increased incidence of 

flooding in coastal areas eventually led to the growing public 

awareness of subsidence and its costs. 

The fate of the Brownwood subdivision of Baytown affords a particularly dramatic example of the dangers of coastal subsidence. 

Brownwood was constructed, beginning in 1938, as an upper-income subdivision on wooded lots along Galveston Bay (Holzschuh, 

1991). At that time the area was generally 10 feet or less above sea 

level. By 1978 more than 8 feet of subsidence had occurred. 

"The subdivision is on a small peninsula bordered by three bays. {It} is 

a community of about 500 single-unit family houses. Because of 

subsidence, a perimeter road was elevated in 1974 to allow ingress and 

egress during periods of normal high tide {about 16 inches}, and to 

provide some protection during unusual high tide. Pumps were installed 

to remove excess rainfall from inside the leveed area. Because of 

subsidence after the roadway was elevated, tides of about { 4 feet} will 

cause flow over the road. The United States Army Corps of Engineers 

studied methods to protect the subdivision from flooding. The cost of a 

levee system was estimated to be about $70 million. In 1974, the Army 

Corps estimated that it would cost about $16 million to purchase 442 

homes, relocate 1,550 people, and convert {750 acres} of the peninsula 

into a park. This proposed solution was approved by the Congress of 

the United States and provided necessmy funding. However, the project 

required that a local sponsor (the City ofBaytown) should approve the 

project, provide 20 per cent of the funds ($3 million) and agree to 

maintain the park. By the time the first election to fund the project was 

held on 23] uly 1979, the cost estimate had increased to $3 7. 6 million, 

of which the local share was $7.6 million. The proposal was defeated, 

and two days later 12 inches of rain fell on Brownwood causing the 

flooding of 187 homes. Another bond election was held on 9 January 

1980 and again the proposal was defeated. Accepting the residents' 

decision, Baytown officials began planning the sale of $3.5 million 

worth of bonds to finance the first stage of a fifteen-year, $6.5-million 

programme to upgrade utilities in the subdivision. Meanwhile, those 

who own the houses generally also owe mortgages and cannot afford to 

purchase other homes. Although they continue to live in the subdivision 

many have to evacuate their homes about three times each year." 

- Gabrysch, 1983 

The year that article was published, Hurricane Alicia struck a final 

blow to Brownwood. All homes in the subdivision were abandoned. Today, most of the subdivision is a swampy area well-suited 

for waterfowl; egrets and scarlet ibis are often seen. 
Houston-Galveston, Texas 

An abandoned house in the 

Brownwood subdivision 

43 

Subsidence also exposes inland areas to 

increased risks of flooding and erosion by 

altering natural and engineered drainageways (open channels and pipelines) that 

depend on gravity-driven flow of stormrunoff and sewerage. Differential subsidence, depending on where it occurs with 

respect to the location of drainageways, may 

either reduce or enhance preexisting gradients. Gradient reductions decrease the rate 

of drainage and thereby increase the chance 

of flooding by storm-water runoff. Gradient reversals may result in 

ponding or backflow of sewage and stormwater runoff. In some 

areas, the drainage gradients may be enhanced and the rate of 

drainage may be increased. In terms of flooding risk, this may have 

a beneficial effect locally but an adverse effect downstream. For 

open channels, the changing gradients alter streamflow characteristics leading to potentially damaging consequences of channel erosion and sediment deposition. 

Wetlands are being lost to subsidence 

Galveston Bay is one of the most significant bay ecosystems in the 

Nation. The estuary is Texas' leading bay fishery and supports vibrant recreation and tourism industries. Sixty-one percent of the 

Bay's 232 miles of shoreline is composed of highly productive fringing wetlands but, mainly because of subsidence, more than 26,000 

acres of emergent wetlands have been converted to open water and 

barren flats (White and others, 1993). Subsidence has also contributed to a significant loss of submerged aquatic vegetation (mostly 

seagrass) since the 1950s. Some bay shorelines have become more 

susceptible to erosion by wave action due to loss of fringing wetlands. At the same time, the reduction in sediment inflows to the 

bay system resulting from construction of reservoirs along tributary rivers slows the natural rebuilding of shorelines. Because of the 

combined and interrelated effects of relative sea-level rise, loss of 

wetlands, and reduced sediment supply, the shoreline is eroding at 

an average rate of 2.4 feet per year (Paine and Morton, 1986). As the 

water level rises, marsh along the shoreline is drowned. When residential, commercial, or industrial development is located near the 

shoreline, the potential for the landward migration of marshes is 

eliminated. The result is a reduction in wetland habitats, which provide the foundation for commercial and recreational fisheries. 

The most extensive changes in wetlands have occurred along the 

lower reaches of the San Jacinto River near its confluence with Buffalo Bayou. This area had subsided by 3 feet or more by 1978, resulting in submergence and changes in wetland environments that 

progressed inland along the axis of the stream valley. Open water 

displaced riverine woodlands and swamps. Trends along the lower 

reaches of other rivers, bayous, and creeks have been similar, resulting in an increase in the extent of open water, loss of inland marshes 
44 

Wetlands were lost to 

inundation resulting 

from subsidence in 

the lower reaches of 

the San Jacinto River. 

--San jacinto River 

\ 

Buffalo Bayou 

Coastal subsidence allows 

shorelines to move landward 

causing the demise of some 

coastal woodlands. 

Galveston 

Bay 

(Galveston Bay Information Center; TAMUG) 

1956 1979 ,----------------. 

Mining Ground Water 

(Whrte and others. 1993) 

Dwater 

D Fluvial woodlands 

and swamps 

D Fresh to brackish 

marshes 

0 5.000 feet 

r------1 

0 I ,500 meters 

and woodlands and, in some areas, the development of new 

marshes inland from the encroaching waters. 

The health and productivity of the bay ecosystem depends on the 

presence of key habitats like salt marshes, but also on the mix of 

river and bay water. Many species of fish, wildlife, aquatic plants, 

and shellfish in Galveston Bay depend on adequate freshwater inflows for survival. The estuary is adapted to highly variable inflows 

of freshwater. For instance, oysters prefer somewhat salty water, but 

need occasional surges of freshwater. The volume, timing, and 

quality of freshwater inflows to the estuary are key factors. 

The increasing demand for surface-water supplies, motivated in 

recent years by efforts to mitigate land subsidence, has led to construction of reservoirs and diversions that have reduced the sediments and nutrients transported to the bay system (Galveston Bay 

National Estuary Program, 1995). Controlled releases from surface 

impoundments such as Lake Livingston and Lake Houston have 

changed the natural freshwater inputs to the bay system; the high 

flows are lower, the low flows are higher, and peak flows are delayed 

by about 1 month. As a result, the amount of mineral sediment 

being delivered by streams to the wetlands has been reduced, limiting some of the natural accretion of wetlands. 

Normally, the process of wetland accretion is self-regulated 

through negative feedback between the elevation of the wetland 

and relative sea level. When wetland elevations are in balance relative to mean sea level, periodic and frequent tidal inundations mobilize sediment and nutrients in the wetland in a way that favors 

vegetative growth and a balance between sediment deposition and 

erosion. Subsidence may upset this balance by submerging the 

wetland. The drowned wetland cannot support the same floral 

community, loses its ability to trap sediment as before, and is virtually unregulated by relative sea-level changes. These changes impact the natural processes in the bay and related ecosystems, which 

evolved with the rhythm of the unregulated streams and rivers. 
Houston-Galveston, Texas 

Subsidence activates faults 

Fault creep related to water-level declines 

Many faults exist in the Houston-Galveston area, both 

regional-scale "down-to-the-coast" faults that represent slow 

sliding of the land mass towards the Gulf of Mexico and 

local structures associated with oil fields (see sidebar on the 

Goose Creek oil field) (Holzshuh, 1991). Since the late 

1930s, 86 active faults with an aggregate scarp length of 

about 150 miles have offset the land surface and damaged 

buildings and highways in the metropolitan area (Holzer 

and Gabrysch, 1987). The scarps typically grow by seismic 

creep at rates of up to 1 inch per year (Holzer, 1984). 

Monitoring of fault creep, water levels, and land subsidence 

has demonstrated a clear cause-and-effect relation. The 

fault movement is caused by water-level decline and associated subsidence. In the 1970s, a period of water -level 

recovery began in the eastern part of the Houston area, due 

A house in Baytown near 

Brownwood was damaged 

by fault creep. 

(Holzer and Gabrysch. circa 1987) 

to delivery of imported surface water and associated 

reduction of ground-water pump age. Fault creep stopped 

or slowed in the area of water-level recovery, but continued 

unabated in the area of ongoing water-level decline. 

Vertical displacements at eight selected fault-monitoring sites in the Houston 

area show a pattern related to water-level declines and land subsidence. 

Displacement on faults is greater 

in areas where water levels were 

declining and subsidence was active. 

scarp 

lncrei1ng he1ght ~~A 

D1splacement 

(1nches) ~ C 8 

~[ r-r-r""'D""" 1978 1980 1982 1984 

Land subsidence dashed 

where estimated 

Lines of equal land 

subsidence (feet), 

1978 to 1983, and 

(Holzer and Gabrysch, 1987) 

Displacement on faults is less in areas 

whe re water levels were recovering 

and subsidence was largely anrested. 

E 

F 

~ ----- ".,H 

- 1978 1980 1982 1984 

T 

Increasing scarp height 

Displacement (i nches) 

l 

SUBSIDENCE IS ACTIVELY MANAGED 

45 

Public awareness of subsidence and its causes increased along with 

the frequency of coastal flooding. In the late 1960s groups of citizens began to work for a reduction in ground-water use. State legislators became educated about the problem, and in May 1975 the 

Texas Legislature passed a law creating the Harris-Galveston 

Coastal Subsidence District, the first district of its kind in the 

United States. The unprecedented Subsidence District was authorized as a regulatory agency, with the power to restrict ground-water 

withdrawal by annually issuing or denying permits for large-diameter wells, but was forbidden to own property such as water-supply 

and conveyance facilities. 
46 

Increasing ground-water 

pumpage landward, west and 

north of Houston, has caused 

additional, ongoing subsidence. 

In areas to the east and south 

of Houston, regulatory action 

by the Harris-Galveston 

Coastal Subsidence District has 

reduced ground-water pumpage, thus dramatically slowing 

subsidence. 

Annual 0.2 

pumpage 

(mill ions of 

acre-feet) 

0 

1980 1985 1990 1995 

Pumpage in northwestern zones 

of the subsidence district 

- Harris-Galvestion 

Coastal Subsidence 

District 

Line of equal land 

subsidence, 1978- 1995 

(interval 0.5 feet) 

Annual 0.2 -

pumpage 

(mill ions of 

acre-feet) 

Pumpage in southeastern 

zones of the subsidence 

district 

1980 1985 1990 1995 

(Harris-Galveston Coastal Subsid ence District) 

Mining Ground Water 

The initial (1976) Subsidence District plan recognized the critical 

situation in the coastal areas and was designed to have an immediate impact there. Surface water from the recently completed Lake 

Livingston reservoir on the Trinity River was used to convert industry along the Houston Ship Channel from ground water to surface 

water. Subsidence in the Baytown-Pasadena area soon slowed dramatically. Earlier imports of surface water from Lake Houston on 

the San Jacinto River, to the east side of Houston, had locally and temporarily halted water-level declines, but were insufficient to keep pace 

with the growing demand. The additional water supplied from Lake 

Livingston was sufficient to significantly reduce ground-water use 

and ultimately did lead to a recovery of water levels over a large area. 

In the eastern part of the greater Houston region, near the bay system, subsidence has been controlled by conversion from groundwater to imported surface-water. However, subsidence is 

accelerating to the west, where ground-water use has increased. 

Thus, the area of active subsidence has shifted from the low-lying, 

tide-affected areas towards higher elevations inland. 

A devastating flood in 1984 on Brays Bayou, a major watershed in 

southwest Houston, renewed concern about the effects of subsidence in inland areas. It was recognized that flood control and subsidence control should be coordinated to minimize flood damages. 

During the 1989 legislative session, the Fort Bend Subsidence District was created to manage and control subsidence in Fort Bend 

County. 

In 1992, the Harris-Galveston Coastal Subsidence District adopted 

a regulatory action plan to reduce ground-water pumpage by 80 

percent no later than the year 2020. Due to the high cost of constructing distribution lines westward across the metropolitan area, 

the plan was to be implemented in phases, allowing time to design, 

finance, and construct surface-water importation facilities. The two 

subsidence districts will cooperate to ensure coordinated planning 

of the conversion from ground water to surface water. 

The direct and indirect costs of subsidence 

The low elevation, proximity to bays and the Gulf of Mexico, dense 

population, and large capital investment make it likely that the 

Houston-Galveston area has been more significantly impacted by 

subsidence than any other metropolitan area in the United States. 

The actual economic cost of subsidence is hard to quantify, and 

most published estimates are necessarily vague. For example, 

Gabrysch ( 1983) stated that "many millions of dollars" have been 

spent reclaiming land submerged by tidal water, elevating structures 

such as buildings, wharves and roadways, and constructing levees to 

protect against tidal inundation; further, "millions of dollars" are 

spent on repairing damage due to fault movement. One conservative estimate for the period 1969 to 1974 placed the average annual 

cost to property owners at more than $31,000,000 in 1975 dollars 

(Jones, 1976) or about $90,000,000 in 1998 dollars. 
Houston-Galveston, Texas 

After the completion of Lake 

Houston in 1954, water distribution lines were constructed 

to convey surface water from 

Lake Houston to the Pasadena industrial area in order 

to supplement local groundwater supplies. 

USGS hydrologist measures water levels at an extensometer 

site, which also serves as a Continuous Operating Reference 

Station equipped with a GPS antenna and receiver to continuously monitor land subsidence. 

47 

(Harris-Galveston Coastal Subsidence District) 

The costs of such subsidence-related phenomena as the loss of wetlands are even more difficult to assess than property losses. Although some estimates could be made based on the changing value 

of commercial and recreational fisheries, it would be difficult to 

distinguish the influence of subsidence from that of other factors. 

Similarly, some fraction of the ongoing cost of flood prevention and 

flood-damage repair could fairly be attributed to subsidence. 

The most definitive published subsidence-damage estimates have to 

do with the costs of relocating dock facilities, constructing hurricane levees, and rectifying drainage problems at refineries along the 

Houston Ship Channel. For two refineries alone, the estimated total 

cost was $120,000,000 in 1976 dollars (Holzschuh, 1991), or about 

$340,000,000 in 1998 dollars. If these estimates are correct, it seems 

reasonable to suggest that subsidence-related damage to industrial 

infrastructure alone may run into the billions of dollars. 

Ongoing monitoring will help managers plan for the future 

Ongoing patterns of subsidence in the Houston area are carefully 

monitored. Compaction of subsurface material is measured continuously using 13 borehole extensometers (wells equipped with 

compaction monitors) at 11 sites throughout the region. Piezometers completed to different depths are used to simultaneously 

monitor water levels. The decreasing subsidence rates observed at 

sites in the eastern part of the region are a direct result of reducing 

local ground-water withdrawals through conversion to imported 

surface-water supplies. In contrast, measurements from the western 

part of the region reveal continuing subsidence. 

A network of 82 bench marks distributed throughout the two subsidence districts was installed in 1987 for determination of elevation 

changes using the Global Positioning System (GPS). The bench 

marks were resurveyed using GPS in 1995. The results of the measurements are the basis for the subsidence measured during the 

1987 to 1995 period. Continuous Operating Reference Stations 

(CORS), used to continuously monitor the elevation of three extensometers with GPS, are being maintained by the Harris-Galveston 

Coastal Subsidence District under the direction of the National 

Geodetic Survey (NGS). One of the CORS sites is in the NGS Na-
48 

1976 

SURFACE WATER 

As a percentage of the total, 

ground-water use has dropped 

significantly, but total water 

use is rising. 

TOTAL 

WATER 

USE 

0.826 million 

acre-feet 

1.0 16 million 

acre-feet 

Galveston at sunset 

Mining Ground Water 

tional Network. In addition to the fixed locations, portable GPS 

receivers mounted in trailers are used wherever subsidence measurements are needed. Each portable receiver can operate at up to 

four different sites each month. GPS is expected to be more costeffective for monitoring subsidence in the Houston area than constructing additional extensometers or surveying benchmarks using 

more traditional leveling techniques. 

Some controversy attends efforts to gradually achieve conversion to 

surface water on the north and west sides of Houston, mainly because the imported surface water is expected to cost about twice as 

much as the ground water that is currently used. Various local municipalities are contesting the timing and apportioning of costs 

(Houston Chronicle, 27 August 1997, "That sinking feeling hits 

northwest Houston"). 

Given the continuing rapid growth of Houston, there is also some 

long-term concern about securing sufficient surface-water supplies. 

State and local governments are already at work seeking to ensure 

that there will be enough water for the expected future population. 

The primary strategies aim to promote water conservation and ac- . 

quire supplies from East Texas reservoirs. In addition to the concerns of East Texas communities about water being exported to 

Houston, such water transfers have ecological effects on the coast 

and on the waterways through which the water is moved. 

The price of water is expected to gradually increase as population 

and economic growth increase demand. Many farmers will fmd it 

difficult to pay higher prices. This may lead to land-use changes in 

rural communities as farmers find new crops, turn to ranching, or 

give way to suburban development. Small businesses that support 

farms will be particularly vulnerable to these changes. 

Houston's continuing rapid growth means that subsidence must 

continue to be vigilantly monitored and managed. However, the 

region is better-positioned to deal with future problems than many 

other subsidence-affected areas, for several reasons: a raised public 

consciousness, the existence of well-established subsidence districts 

with appropriate regulatory authority, and the knowledge base provided by abundant historical data and ongoing monitoring. 

(Harris·Galveston Coastal Subsidence District) 
LAS VEGAS, NEVADA 

Gambling with water in the desert 

NEVADA 

20 Miles 

~===;-----' 

20 Kilometers 

M ich ael T. Pavelko, David B. Wood , an d 

Randell J. Laczniak 

U.S. Geological Survey, Las Vegas, Nevada 

L as Vegas Valley is the fastest growing metropolitan area in 

the United States (U.S. Department of Commerce, accessed 

July 27, 1999). The accelerating demand for water to support 

the rapid growth of the municipal-industrial sector in this desert 

region is being met with imported Colorado River System supplies 

and local ground water. The depletion of once-plentiful groundwater supplies is contributing to land subsidence and ground failures. Since 1935, compaction of the aquifer system has caused 

nearly 6 feet of subsidence and led to the formation of numerous 

earth fissures and the reactivation of several surface faults, creating 

hazards and potentially harmful impacts to the environment. 

In the near future, the current water supplies are expected not to 

satisfy the anticipated water demand. The federally mandated limit 

placed on imported water supplied from nearby Lake Mead, a reservoir on the Colorado River, will likely force a continued reliance on 

ground water to supplement the limited imported-water supplies. 

Water supply-and-demand dynamics in this growing desert community will likely perpetuate problems of land subsidence and related ground failures in Las Vegas Valley, unless some 

balanced use of the ground-water resource can be 

achieved. 
so 

Wednesday Oct. 11th 1848 

[ ... ] Camped about midnight at a 

spring branch called Cayataus. Fair 

grass. Thisiswhatiscalledthe "Vegas". 

Thursday Oct. 12th 1848 

[. .. ]Staid {sic} in the camp we made 

last night all day to recruit the animals. 

They done finely. There is the finest 

stream of water here, for its size, I ever 

saw. The valley is extensive and I doubt 

not[,} would by the aid of irrigation be 

highly productive. There is water 

enough in this rapid little stream to 

propel a grist mill with a dragger run of 

stones! And oh! such water. It comes, 

too, like an oasis in the desert, just at 

the termination of a 50 m. {mile} stretch 

without a drop of water or a spear of 

grass. [. .. }" 

Orville C. Pratt (from The Journal of 

Orville C. Pratt, 1848 in Hafen and 

Hafen, 1954) 

Uunior League of Las Vegas Collection, University of Nevada. Las Vegas Library) 

Mining Ground Water 

"THE MEADOWS" WAS AN IMPORTANT DESERT OASIS 

Las Vegas Valley is located in southern Nevada and lies within both 

the Great Basin and Mojave Desert sections of the Basin and Range 

physiographic province. The arid, northwest-trending valley is 

bounded on the west by several mountain ranges and drains a 

1,564-square-mile watershed southeastward through Las Vegas 

Wash into Lake Mead. 

More than 24 inches of precipitation fall annually in the Spring 

Mountains bounding the valley to the west, but less than 4 inches 

of rain fall annually on the valley floor; measurable amounts 

(greater than 0.01 inch) seldom occur more than 30 days each year. 

Temperatures range from below freezing in the mountains to more 

than 120° F on the valley floor. There are typically more than 125 

days of 90° F or warmer temperatures each year in Las Vegas Valley 

(Houghton and others, 1975). 

The desert oasis of Las Vegas Valley has been a source of water for 

humans for more than 13,000 years. Native Americans of the 

Mojave and Paiute tribes were among the earliest known users. 

Named by an unknown trader for its grassy meadows, Las Vegas, 

Spanish for "the meadows;' was a watering stop along the Old 

Spanish Trail that connected the settlements in Los Angeles and 

Santa Fe. In 1844, the renowned explorer John C. Fremont stopped 

here and spoke of the waters as "two narrow streams of clear water, 

4 or 5 feet deep, with a quick current, from two singularly large 

springs" (Mendenhall, 1909). Others were similarly moved by the 

refreshing contrast of these welcome meadows in the otherwise 

barren landscape. 

The railroad initiates a period of rapid growth 

After failed attempts by Mormon settlers to mine lead from the 

nearby Spring Mountains and to establish farming in the valley, a 

flourishing ranch supported by springs and Las Vegas Creek was 

established in 1865 by Octavius Decatur Gass, a settler who had 

initially been attracted to the West by gold mining. In 1905, Montana Senator William Clark brought the San Pedro, Los Angeles 

and Salt Lake Railroad to the valley and established the small town 

of Las Vegas, a site chosen because of its central location between 

Los Angeles and Salt Lake City, and because of the water supply 

necessary to keep the steam locomotives running. 

Fremont Street, Las Vegas, looking west (ca. 1910) 
Las Vegas, Nevada 

The Las Vegas Land and Water 

Company, established in 1905, 

was the area's first water purveyor. 

Today Las Vegas sprawls across the valley. 

Urban growth in the Las Vegas 

Valley has soared in the last 

few decades. 

51 

As the railroad grew, so did Las Vegas and its thirst for water (Jones 

and Cahlan, 1975). To help meet the increasing demand, the Las 

Vegas Land and Water Company was formed ii11905. A new period 

of growth began in 1932 with the construction of Boulder Dam 

(later renamed Hoover Dam) and Lake Mead on the Colorado 

River, southeast of Las Vegas. Boulder Dam brought workers to Las 

Vegas from throughout America, and provided a seemingly unlimited supply of water and power in one of the most unlikely places. 

The wealth of land, water, and power resources attracted industry, 

the military, and gambling to the valley during the 1940s and 

19 50s. The population of Las Vegas was growing steadily, and by 

1971 the heightened water demand required importing additional 

water from Lake Mead through a newly constructed Southern Nevada Water Project pipeline. At present, Las Vegas Valley is home to 

1.2 million people, about two-thirds of Nevada's population, and 

hosts more than 30 million tourists each year. 

NEVADA 

Clark 

County 

\ 

Map area 

Population of Clark County 

(thousands of residents) 

1900 1920 1940 1960 1980 

~~ ~ M b, 

1907 1923 1952 1967 1972 1995 

1,000 

500 

0 IOMiles 

I 0 Kilometers 

(Data from Acevedo and others. accessed July 27. 1999) 
52 

In 1912, the Eglington well, 

one of several uncapped artesian wells, was allowed to 

flow freely. (It is shown here 

flowing at about 615 gallons 

per minute.) 

(Carpenter. 19 15) 

By 1938 the Eglington well had ceased 

flowing. The water 

level was then 3.3 

feet below land 

surface. 

(Livingston. 1941 ) 

Mining Ground Water 

BROWNING OF "THE MEADOWS" : 

DEMAND FOR WATER DEPLETES THE AQUIFER SYSTEM 

Prior to development in Las Vegas Valley, there was a natural, albeit 

dynamic, balance between aquifer-system recharge and discharge. 

Over the short term, yearly and decadal climatic variations (for example, drought and the effects of El Nino) caused large variations in 

the amount of water available to replenish the aquifer system. But 

over the long term, the average amount of water recharging the 

aquifer system was in balance with the amount discharging, chiefly 

from springs and by evapotranspiration. Estimates of the average, 

annual, natural recharge of the aquifer system range from 25,000 to 

35,000 acre-feet (Maxey and Jameson, 1948; Malmberg, 1965; 

Harrill, 1976; Dettinger, 1989). 

In 1907, the first flowing well was drilled by settlers to support the 

settlement of Las Vegas, and there began to be more ground-water 

discharge than recharge (Domenico and others, 1964). Uncapped 

artesian wells were at first permitted to flow freely onto the desert 

floor, wasting large quantities of water. This haphazard use of 

ground water prompted the State Engineer, W.M. Kearney, to warn 

in 1911 that water should be used " ... with economy instead of the 

lavish wasteful manner, which has prevailed in the past" (Maxey and 

Jameson, 1948). 

Intensive ground-water use led to steady declines in spring flows 

and ground-water levels throughout Las Vegas Valley. Spring flows 

began to wane as early as 1908 (Maxey and Jameson, 1948). By 1912 

nearly 125 wells in Las Vegas Valley ( 60 percent of which were flowing-artesian wells) were discharging nearly 15,000 acre-feet per year. 

Las Vegas' water supply has kept pace with the demand. 

360 ------------------------------1,200 

Annual ground-water 

discharge and 

imported water 

(thousands of 

acre-feet) 

240 ---------------------------+-1 

IMPORTED SURFACE WATER [Southern Nevada Water System 

l20 ___ Basic Magnesium, Inc. (BMI) 

GROUND-WATER DISCHARGE 

Range of natural recharge 

Las Vegas Valley Water Disctrict 

begins to utilize the BMI pipeline 

- 800 

- 400 

Population of 

Clark County 

(thousands of 

residents) 
Las Vegas, Nevada 

95 

Lines of equal water-level change, 

predevelopment to 1990 

(interval 50 feet) 

North I 

0 (Burbey.1 995) 

0 I 0 Kilometers 

By 1990 areas of the valley that 

had once supported flowing artesian wells experienced water level 

declines of more than 300 feet. 

Depth to 

water 

(feet below 

land surface) 

Anr>1..~ g. vL.rd· 

w,lter ~ · • 11p"~e 

'thous 1ds of 

(, .... rc-fcet~ 

-------- --------------- (, 

.:>0 

1950 1970 1990 

53 

Increasing pumpage 

through the 1960s caused 

water levels to drop 

throughout Las Vegas 

Valley. Presently, due to 

some stabilization in the 

pumpage amounts and 

artificial ground-water recharge programs, water 

levels are recovering in 

many areas of the valley. 

With the construction of Boulder Dam came development of the 

military and industrial sectors and a rapidly increasing demand for 

water. In 1942 a water pipeline was constructed to bring water from 

Lake Mead to the Basic Magnesium Project (now called Basic Management, Inc.) in the City of Henderson. This pipeline marked the 

first supplementation of Las Vegas Valley ground water and the beginning of surface-water imports to the valley. In 1955 the Las Vegas 

Valley Water District (LVVWD) began to use this pipeline to supplement the growing water demands. By this time, the amount of 

ground water pumped annually from wells had reached nearly 

40,000 acre-feet, surpassing the estimated natural recharge to the 

valley aquifer system (Mindling, 1971). By 1968 the annual groundwater pumpage in the valley reached nearly 88,000 acre-feet (Harrill, 

1976). 

In 1971, the capacity to import surface water into the valley was 

greatly expanded when a second, larger pipeline was constructed 

between Lake Mead and Las Vegas by the Southern Nevada Water 

Project (Harrill, 1976). However, despite the steady increases in imported surface-water deliveries, rising demand for water and federally stipulated limits on Lake Mead imports encouraged a continued 

dependence on the local ground-water resource. 

Ground-water levels decline as Las Vegas expands 

Between 1912 and 1944, ground-water levels declined at an average 

rate of about 1 foot per year (Domenico and others, 1964). Between 

1944 and 1963, some areas of the valley experienced water-level declines of more than 90 feet (Bell, 1981a). The City of North Las Vegas 

was the first area to experience large water-level declines but, as Las 

Vegas expanded, new wells were drilled, pumping patterns changed, 

and ground-water-level declines spread to areas south and west of 

the City of North Las Vegas. Between 1946 and 1960, the area of the 
54 

0 10 Miles 

Total pumpage by section, 

1968to 1995 

(thousands of acre-feet) 

D Less than I 0 

0 IOto 40 

D 40to 70 

D 70to 100 

D Greater than I 00 

i====;==:::'=;-----' 

0 

1964 

I 0 Kilometers 

(Data comp1!ed from unpublished Las Vegas Valley water 

usage reports, Nevada Department of Conservation 

and Natural Resources. Divison ofWater Resources) 

Mining Ground Water 

valley that could sustain flowing-artesian wells shrank from more 

than 80 square miles (Maxey and Jameson, 1948) to less than 25 

square miles (Domenico and others, 1964). By 1962, the springs 

that had supported the Native Americans, and those who followed, 

were completely dry (Bell, 1981a). 

Since the 1970s annual ground-water pumpage in the valley has 

remained between 60,000 and 90,000 acre-feet; most of that has 

been pumped from the northwestern part of the valley. By 1990 

areas in the northwest experienced more than 300 feet of decline, 

and areas in the central (including downtown and The Strip) and 

southeastern (Henderson) sections experienced declines between 

100 and 200 feet (Burbey, 1995). 

In 1996, imports from Lake Mead provided Las Vegas Valley with 

approximately 356,000 acre-feet of water (Co ache, 1996) and represented the valley's principal source of water. This amount included 56,000 acre-feet of return-flow credits for annual 

streamflow discharging into Lake Mead from Las Vegas Wash. 

DEPLETION OF THE AQUIFER SYSTEM CAUSES SUBSIDENCE 

Land subsidence and related ground failures in Las Vegas Valley 

were first recognized by Maxey and Jameson (1948) based on comparisons of repeat leveling surveys made by the USGS and the U.S. 

Coast and Geodetic Survey between 1915 and 1941. Since then, 

repeat surveys of various regional networks have shown continuous land subsidence throughout large regions within the valley. 

The surveys have revealed that subsidence continued at a steady 

rate into the mid -1960s, after which rates began increasing through 

1987 (Bell, 1981a; Bell and Price, 1991). Surveys made in the 1980s 

delineate three distinct, localized subsidence bowls, or zones, superimposed on a larger, valley-wide subsidence bowl. One of these 

smaller subsidence bowls, located in the northwestern part of the 

valley, subsided more than 5 feet between 1963 and 1987. Two 

1997 

These photographs of a 

protruding well just west of 

downtown Las Vegas show 

evidence of subsidence. The 

1964 photograph shows 

that the ground has subsided enough, relative to 

the well casing, to suspend 

the broken concrete foundation of the well head 

above land surface. Thirty 

three years later well head 

protrudes farther as the 

ground has continued to 

subside. 
Las Vegas, Nevada 

95 

........__ / Line of equal land subsidence, 

--......_ 1963- 1987 (interval I foot) 

--------

0 10 Moles 

0 I 0 Kilometers 

BenchMarks 

The determination of subsidence trends in 

time and in space is limited in part by the 

inherently sparse distribution of available 

bench marks from which comparisons can 

be made. Subsidence is determined by comparing two elevations made at a vertical 

reference point-a bench mark-at two 

different times. The destruction and loss of 

historical bench marks inevitably accompanies the march of time and cultural developments such as building and road construction. 

The loss of comparable reference points 

reduces the spatial detail of subsidence 

determinations and disrupts the continuity 

of subsidence monitoring unless care is 

taken to preserve bench marks. These factors have limited the spatial detail of subsidence maps in Las Vegas and will continue to 

pose serious challenges to subsidence monitoring in the years to come. In 1990 the 

Nevada Bureau of Mines and Geology established more than 100 new bench marks 

in Las Vegas Valley. 

Three subsidence bowls were 

identified between 1963 and 

1987. These bowls are caused by 

a combination of ground-water 

declines and the presence of 

compressible sediments in the 

aquifer system at these locations. 

">s1C'ence 

(teet) 

AnnuJI ground 

w1ter >unrpag.? 

( t>ou'anc- o' 

0 ·-===-=-::--Q)- --;"'I - .-----I 00 

2 

-50 

1930 1950 1970 1990 

(Subsodence data from Bell and Price, 1991) 

55 

Subsidence measured at 

two bench marks continued 

beyond 1970, although 

ground-water pumpage 

was slightly reduced. 

other localized subsidence bowls, in the central (downtown) and 

southern (Las Vegas Strip) parts of the valley, subsided more than 

2.5 feet between 1963 and 1987. The areas of maximum subsidence 

do not necessarily coincide with areas of maximum water-level declines. One likely explanation is that those areas with maximum 

subsidence are underlain by a larger aggregate thickness of finegrained, compressible sediments (Bell and Price, 1991). 

Aquifer-system compaction creates earth fissures and reduces 

storage 

All the impacts of subsidence in Las Vegas Valley have not yet been 

fully realized. Two important impacts that have been documented 

are (1) ground failures-localized ruptures of the land surface; and 

(2) the permanent reduction of the storage capacity of the aquifer 

system. Other potential impacts that have not been studied extensively are: 

• Creation of flood-prone areas by altering natural and engineered 

drainage ways; 

• Creation of earth fissures connecting nonpotable or contaminated 

surface and near-surface water to the principal aquifers; and 

• Replacement costs associated with protruding wells and collapsed 

well casings and well screens. 

All of these potential damages create legal issues related to mitigation, restoration, compensation, and accountability. 

Ground failures Earth fissures are the dominant and most spectacular type of ground failure associated with ground-water withdrawal 

in Las Vegas Valley. Earth fissures are tensile failures in subsurface 

materials that result when differential compaction of sediments 

pulls apart the earth materials. Buried, incipient earth fissures be-
56 

1 

Line of ~qual land 

subsiCience, 

1963- 1987 

(Interval I foot) 

0 10 Miles 

A 

0 I 0 IGiometers 

Earth fissures have occurred 

near areas of greater differential subsidence, and many fissures are associated with surface faults. 

(Data from Bell and Price. 1991) 

NW 

Mining Ground Water 

come obvious only when they breach the surface and begin to 

erode, often following extreme rains or surface flooding conditions. 

Earth fissures have been observed in Las Vegas Valley as early as 

1925 (Bell and Price, 1991), but were not linked directly to subsidence until the late 1950s (Bell, 1981a). Most of the earth fissures are 

areally and temporally correlated with ground-water level declines. 

Movement of preexisting surface faults has also been correlated to 

ground-water level changes and differential land subsidence in numerous alluvial basins (Holzer, 1979; Bell, 1981a; Holzer, 1984). In 

Las Vegas Valley, earth fissures often occur preferentially along preexisting surface faults in the unconsolidated alluvium. They tend to 

form as a result of the warping of the land surface that occurs when 

the land subsides more on one side of the surface fault than the 

other. This differential land subsidence creates tensional stresses 

that ultimately result in fissuring near zones of maximum warping. 

The association of most earth fissures with surface faults suggests a 

causal relationship. The surface faults may act as partial barriers to 

ground-water flow, creating a contrast in ground-water levels across 

the fault, or may offset sediments of differing compressibility. 

The associated land-surface displacements and tilts are often sufficient to damage rigid or precisely leveled structures. Damage to 

homes in a 241-home subdivision in the north -central part of the 

valley has already cost more than $6 million, and the total cost projections are in excess of $14 million (Marta G. Brovgn, City of North 

Las Vegas, written communication, 1997). Other damage related to 

fissuring includes cracking and displacement of roads, curbs, sidewalks, playgrounds, and swimming pools; warped sewage lines; 

ruptured water and gas lines; well failures resulting from shifted, 

sheared, and/ or protruded well casings; differential settlement of 

railroad tracks; and a buckled drainage canal (Bell, 1981b; Marta G. 

Brown, City of North Las Vegas, written communication, 1997). 

Earth fissures are also susceptible to erosion and can form wide, 

steep-walled gullies capable of redirecting surface drainage and 

creating floods and other hazards. Adverse impacts of ground failures may worsen as the valley continues to urbanize and more developed areas become affected. 

SE 

Earth-fissure zone 

0 ---:..: 19-=- 78::-----~ 1 

1979 

This cross section of the Eglington 

fault zone and accompanying fissure zone shows that /and-surface 

elevations on the upthrown side 

of the fault are decreasing due to 

subsidence. 

Change in land 

surface altitude 

(inches) 

1981 

I Kilometer 

f ----- Eglington fault 

I Mile (Modified from Bell, 1998) 
Las Vegas, Nevada 

A fissure displaces 

pavement (far right) 

and damages a 

building (near right) • 

on Harrison Street, 

Las Vegas. 

An estimated 187,000 acre-feet (61 billion 

gallons) of water (enough water to supply 

almost 10,000 households in Las Vegas for 

nearly 20 years) may have been derived 

from a permanent reduction in the storage 

capacity of the Las Vegas Valley aquifer 

system due to compaction of the aquifer 

system and land subsidence between 1907 

and 1996. 

c' 

...... . ~ 0 

I 

a:i 

....;.!,_ ___ _. l 

57 

Reduced storage capacity Reduction of storage capacity in the Las 

Vegas Valley aquifer system is another important consequence of 

aquifer-system compaction. The volume of ground water derived 

from the irreversible compaction of the aquifer system -"water of 

compaction" -is approximately equal to the reduced storage capacity of the aquifer system and represents a one-time quantity of water 

"mined" from the aquifer system. 

Loss of aquifer-system storage capacity is cause for concern, especially for a fast-growing desert metropolis that must rely in part on 

local ground-water resources. A study conducted by the Desert Research Institute (Mindling, 1971) estimated that, at times, up to 10 

percent of the ground water pumped from the Las Vegas Valley aquifer system has been derived from water of compaction. Assuming 

conservatively that only 5 percent of the total ground water pumped 

between 1907 and 1996 was derived from water of compaction, the 

storage capacity of the aquifer system has been reduced by about 

187,000 acre-feet. This may or may not be considered "lost" storage 

capacity: arguably, if this water is derived from an irreversible process, this storage capacity has been used in the only way that it could 

have been. In any case, producing water of compaction represents 

mining ground water from the aquifer system. Further, the reduced 

storage implies that, even if water levels recover completely, any future drawdowns will progress more rapidly. 

LAS VEGAS VALLEY IS UNDERLAIN BY A 

GROUND-WATER RESOURCE 

Las Vegas Valley is a sediment-filled structural trough that has 

formed over many millions of years through compression, extension, and faulting of the original flat-lying marine sediments that 

form the bedrock. Some bedrock blocks were down-dropped between the faults along the eastern and western margins of the presentday valley. 

Sediment eroded by wind and water from the surrounding bedrock 

highlands began filling the trough with gravel, sand, silt, and clay. 
58 

Most precipitation in the watershed falls in the mountains surrounding Las Vegas 

8 

0 20Miles 

0 20 Kilometers 

Boulder 

City 

Mining Ground Water 

During some of the wetter periods in the past 1 million years or so, 

extensive playa lakes and spring-fed marshes covered the lower 

parts of the valley floor, depositing variably thick sequences of 

fine-grained sediment (Mifflin and Wheat, 1979 and Quade et al., 

1995). Coarse-grained sand and gravel tend to rim the valley, forming alluvial fans and terraces, especially in the northern, western, 

and southern parts. The deposits generally thicken and become 

finer-textured toward the central and eastern part of the valley, 

where their total thickness exceeds 5,000 feet (Plume, 1989). 

Ground water flows through the aquifers 

Ground water is generally pumped from the upper 2,000 feet of 

unconsolidated sediments that constitute the aquifer system in the 

central part of the valley. The deeper aquifers, generally below 300 

feet, are capable of transmitting significant quantities of ground 

water, and have been referred to variously as the "principal;'"artesian;' or"developed-zone" aquifers (Maxey and Jameson, 1948; 

Malmberg, 1965; Harrill,1976; Morgan and Dettinger, 1996). In 

places, these principal aquifers are more than 1,000 feet thick and 

consist mainly of sands and gravels beneath the terraces along the 

margins of the valley. In the central and eastern parts, clays and 

silts predominate (Plume, 1989). Overlying the principal aquifers, 

in most places, is a 100-to-300 foot-thick section of extensive clay, 

sand, and gravel deposits known as the "near-surface reservoir:' The 

principal aquifers and the near-surface reservoir are separated by a 

variably-thick, laterally discontinuous aquitard, or confining unit. 

Much of the ground water found in the aquifer system originates as 

rain or snow falling on the Spring Mountains to the west or on the 

Sheep and Las Vegas Ranges to the northwest. Some of the precipitation infiltrates into the underlying bedrock through faults and 

fractures, eventually moving into the deposits comprising the principal aquifers. The remainder of the precipitation runs off onto the 

sloping alluvial terraces and rapidly enters the sand and gravel 

deposits, where it either recharges the underlying principal aquifers 

or is evaporated or transpired into the atmosphere. 

Near the margins of the valley, ground water moves freely through 

the coarse-grained sand and gravel deposits, but as it moves 

"The sJ tlement [subsidence] in Las 

Vegas Valley as a whole appears to be 

the result of compaction of the sediments 

of the valley fill, and the faults, ... are 

probably caused by the differential 

compaction of the fine-grained and 

coarse-grained sediments. " 

-1948, George B. Maxey and C. Harry jameson 
Las Vegas, Nevada 

Predevelopment 

Ground water was sustained 

by natural recharge, and excess ground water discharged 

through several springs and 

into the Las Vegas Wash. 

Postdevelopment 

Principal 

aquife rs 

Excessive pumping has caused 

the water table to drop and 

springs to dry up. Urban runoff has created a reservoir of 

poorer quality, potentially contaminated water just below 

the surface that now recharges 

the principal aquifers. 

59 

w 

Spring Mts. 

,....-:. Waterfiow 

Faults (arrow in dicates 

direction of relative movement) 

Near surface reservoir (from urban runoff) 

.I Some near-surface water 

E 

0 5 Miles (Generalized from Maxey and Jameson, 1948) 

0 5 Kilometers 

Vertical exaggeration ISx 

basinward it begins to encounter increasingly greater percentages 

of lower permeability, fine-grained clay and silt. The increasing 

proportion of fine-grained deposits retards lateral flow, and the 

low-permeability deposits effectively impede the vertical flow of 

ground water. As ground water recharges the aquifer system from 

the higher elevations, fluid pressures in the principal aquifers can 

build to create artesian conditions at lower elevations in the basin. 

Prior to development of the ground-water resource, artesian pressure in the aquifer system forced water slowly upward through 

confining zones and more rapidly along faults. Flow from these 

conduits formed the springs on the valley floor and supported 

thriving grassy meadows with an estimated annual flow of 7,500 

acre-feet (Malmberg, 1965). Most of the spring flow and precipitation falling on the valley floor was consumed by evapotranspiration, but some infiltrated downward into the surficial deposits. 

The changing balance between recharge and discharge 

Development of the ground-water resource to support the local 

population and its land uses drastically altered the way water cycles 

through the basin. The present water budget reveals that only a 

small fraction of the water used in Las Vegas Valley is actually consumed, and therefore removed from the water cycle, by domestic, 
60 Mining Ground Water 

WATER BUDGET 

Predevelopment 

Atmosphere 

T Natural i 50 

recharge Evaporation 100 

200 

Spnng 400 

Infiltration discharge 

i T Thousands of 

acre-feet 

Near-surface reservoir 

i Leakage Subsurface 

outflow 

Principal aquifers I ..,.._ 

"All data available from this and other 

studies strongly indicate that the 

quantities of water presently developed, 

if removed entirely from the groundwater reservoir on a permanent basis, 

would eventually result in critical 

depletion" 

-Domenico and others, 1964 

1000 

Present 

Atmosphere 

Evaporation Evaporation 

NaturaiT i 

Lake Mead 

recharge Municipal-industrial Wate~portsl sector ...-- '-------_j 

TUrban LasVegasWash Infiltration T runoff return flow l Surface water outflow 

Pumpage i I Near-surface reservoir I 

T Artificial recharge T Leakage Subsurface 

r'-------'--- ----'-------,

1 

o~ow 

Principal aquifers . ...--

agricultural and municipal/industrial uses. Most is either returned 

to the aquifer system, evaporated, or discharged into the Colorado 

River system. Large quantities of this generally poorer-quality water 

drain from overwatered lawns, public sewers, paved surfaces, and 

other drainage ways. Much of this urban runoff flows onto open 

ground where it evaporates, is transpired by plants, or recharges the 

near-surface reservoir. Large amounts of treated sewage water are 

discharged into the Colorado River system by way of the Las Vegas 

Wash. Ground water has been depleted in the principal aquifers and 

aquitards, causing land subsidence, while the shallow, near-surface 

reservoir has been recharged with poor-quality urban runoff. 

LAS VEGAS IS DEALING WITH A LIMITED WATER SUPPLY 

Managing land subsidence in Las Vegas Valley is linked directly to 

the effective use of ground-water resources. At present more ground 

water is appropriated by law and is being pumped in Las Vegas Valley than is available to be safely withdrawn from the ground-water 

basin (Nevada Department of Conservation and Natural Resources, 

1992; Coache, 1996). Historic and recent rates of aquifer-system 

depletion caused by overuse of the ground-water supply cannot be 

sustained without contributing further to land subsidence, earth 

fissures, and the reactivation of surface faults. 

In order to arrest subsidence in the valley, ground-water levels must 

be stabilized or maintained above historic low levels. Stabilization 

or recovery of ground-water levels throughout the valley will require that the amount of ground water pumped from the aquifers 

be less than or equal to the amount of water recharging the system. 

Eliminating any further decline will reduce the stresses contributing to the compaction of the aquifer system. Even so, a significant 

amount ofland subsidence (residual compaction) will continue to 

occur until the aquifer system equilibrates fully with the stresses 

imposed by lowered ground-water levels in the aquifers (Riley, 

1969). This equilibrium may require years, decades, or even centuries to be realized. 
Las Vegas, Nevada 

Replenishing the aquifer system artificially 

Las Vegas Valley Water District (L VVWD) and the City of North Las Vegas have 

developed artificial recharge programs 

The artificial recharge programs serve two primary purposes: 

• To store surplus imported surface water in the principal 

aquifers during winter months when demand is relatively 

low, so that it can later be pumped to supplement any shortfalls in the supply and delivery of imported water during 

the high-demand summer months 

• To replenish the principal aquifers, if only temporarily, thus 

raising ground-water levels and forestalling subsidence in 

the local area. 

80 

Recharging began in 1988 and by 

199 5 a total of nearly I I 5,000 

acre-feet of treated, imported 

Lake Mead water had been injected through more than 40 wells, at 

an annual rate of up to 25,000 

acre-feet. Additional recharge 

wells constructed since 199 5 have 

significantly enlarged the recharge 

area and increased the number of 

injection-well sites. 

Artificial recharge wells, 1995; 

/ 

City of North Las Vegas 

octc?~ o 

'{7 / las VegaS' 

Las Vegas Valley Water District ~ ~"' 

Volume of water recharged each year, 

replacing the equivalent pumped volume 

Volume of ground-water pumped each year 

Discharge ~ and recharge 

(thousands of 

acre-feet) 

40 

0 1 

1900 

If 

I I 

1920 

DISINFECTION BYPRODUCTS 

I 

1940 

The artificial recharge program poses a potential for contamination of the Las Vegas Valley aquifer system. The problem arises because it is necessary to disinfect the recharge 

water prior to injecting it through the wells into the aquifer 

system. Disinfection byproducts (DBPs), chiefly trihalomethanes (THMs), form when chlorine is introduced into the 

water-treatment process. The dissolved and particulate organic material in the water reacts with the chlorine and other halogens to form DBPs, of which THMs are specifically 

regulated by State and Federal standards. THMs have been 

shown to cause cancer in laboratory animals, and may pose 

other health risks to humans. Presently, the total THM maximum contaminant level allowed under the drinking-water 

standards is 100 ~-tg/1 (micrograms per liter), but the U.S. 

Environmental Protection Agency is strongly considering a 

lower limit. 

Native ground waters in arid alluvial basins are typically 

low in dissolved organics compared to surface waters, so 

that even if they are chlorinated prior to use, few if any 

THMs form. In contrast, the imported surface water is high 

in organics, and when it is disinfected before injection into 

the aquifer system, an average of 45 ~-tg/1 ofTHMs are produced. This concentration eventually becomes diluted within the aquifer. But when the mixture is pumped for use, 

disinfection is still needed, and the chlorine raises THM 

levels about 25 ~-tg/1, potentially near the drinking-water 

standard. To lower the THMs to acceptable levels, further 

treatment or blending (dilution) may be needed. 

1960 

Recharge of 

treated imported 

surface water 

Pumped 

ground 

water 

I 

2000 

Dual purpose slAt the end of the pumping/injec~ W 11 lA recharge seasonTHMs 

t1on wells 80 __ e ____ _,. continue to form in 

the ground water as 

------- - long as residual chlorine and organic 0 -------' material are available. 

Concentrations of 

trihalomethanes 

(THMs) 

(micrograms 

per liter) 

80 Well SA 

0 

80 We1134 

0 

80 Well38 

0----------------

Oct. Dec. Feb. Apr June Aug. Oct. 

1992 1993 

(Modified from Bemholtz 

and others. 1994) 

61 
62 

Average water 

level altitude 

(feet above 

sea level) 

Pumping with 

Pumping artificial recharge 2030 -------~---

1987 1989 1990 1992 

Water levels at the Las Vegas 

Valley Water District's main 

well field have increased with 

artificial recharge. 

Mining Ground Water 

The natural recharge is augmented "artificially" 

Since 1988, the LVVWD and the City of North Las Vegas have 

implemented artificial ground-water-recharge programs in an attempt to increase local water supplies during periods of high demand. These aquifer-recharge programs replenish the aquifers by 

injecting treated surface water imported from Lake Mead through 

dual-purpose wells. Water is recharged primarily during cooler 

months, when water demand is lowest, thereby raising groundwater levels above typical winter conditions. Recently, annual artificial recharge of nearly 20,000 acre-feet has succeeded in raising 

ground-water levels in some local areas to the extent that they are 

generally higher both at the beginning and end of the peak waterdemand (summer) season. 

Despite the ambitious efforts to artificially recharge the aquifer 

system, valleywide net ground-water pumpage still exceeds the 

estimated natural recharge. To minimize any future subsidence, 

some combination of increased recharge and reduced pumpage is 

needed, especially in areas prone to subsidence. These options depend largely on the seasonal availability of additional imported 

water, to compensate for any additional water recharged, and on 

the amount of reduced pumpage required to maintain groundwater levels above critical levels. 

Both the ground water and surface water of Nevada belong to the 

public and are managed on their behalf by the State of Nevada, the 

Colorado River Compact, and the Bureau of Reclamation. Nevada 

water law is founded on the doctrine of prior appropriation-"first 

in time, first in right" -which grants the first user of a water course 

a priority right to the water. All the surface- and ground-water resources in the valley are currently fully appropriated. The State 

Engineer has established a perennial yield of25,000 acre-feet for 

the Las Vegas Valley aquifer system (Malmberg, 1965; Nevada Dept. 

Of Conservation and Natural Resources, 1992), based on the minimum, average annual natural recharge to the aquifer system. Despite this legally established yield, more than 25,000 acre-feet have 

been pumped from the valley every year since 1945; a maximum 

yield of more than 86,000 acre-feet were pumped in 1968. As of 

This typical artificial recharge 

well has the dual function of 

pumping and injecting. (The tall 

object on the far right is the 

electric motor for the pump). 
Las Vegas, Nevada 

Water levels and compaction 

fluctuate seasonally in response to natural and artificial recharge and pumpage. 

During the winter and spring 

when demand for water is 

low, less ground water is pumped, and imported water is recharged into the aquifer system. 

During the summer when demand for water is high, more 

ground water is pumped, and 

less imported water is recharged into the aquifer system. 

To arrest subsidence, 

ground-water levels 

must recover or stabilize. 

Compaction, 

since Nov. 16, 1994 

63 

W1ter level 

(fc.c beow 

.met surfac ) 

lOO- - 0.4 (inches) 

1995 1996 1997 

(Data is from an extensometer 

site near central Las Vegas.) 

1996, State permits for an annual total of90,000 acre-feet had been 

issued (Coache, 1996), and in that year nearly 76,000 acre-feet, more 

than three times the perennial yield, were pumped. 

WATER MANAGERS ATTEMPT TO MEET 

GROWING WATER DEMAND 

A limit on the amount of water that can be imported from the Colorado River system, and a growing local water demand, make it difficult to reduce the present reliance on the local ground-water supply. 

At the current rate of ground-water extraction, there may be insufficient surplus of imported water to control land subsidence. Wateruse projections for southern Nevada have indicated that the region's 

available water supply likely will not meet projected demands beyond the year 2002, or 2006 provided responsible water-conservation programs are implemented (Water Resources Management 

Incorporated, 1991). After that time, the water supply will become 

extremely vulnerable to variability caused by droughts and potentially by contamination. 

It is uncertain whether Nevada will be able to acquire, on a permanent basis, any additional Colorado River system water beyond the 

current annual allocation of 300,000 acre-feet. To help prevent water 

shortages, and thereby reduce additional stress on the aquifer system, the Southern Nevada Water Authority (SNWA) is pursuing 

several avenues to increase the future supply of water to southern 

Nevada and Las Vegas Valley. Primary sources might include importation of both in-state and out-of-state water and ground-water 

banking. Water from the Virgin and Muddy Rivers and groundwater banking in southern Nevada and Arizona are leading options. 

Stormwater recovery and desalination are also being considered. 
64 Mining Ground Water 

Perhaps the most desirable option to the SNWA would be the 

"wheeling" of Virgin and Muddy River water. Under this scenario, 

river water that is legally available for use is allowed to continue to 

flow into Lake Mead, rather than being piped directly out of the 

rivers. This would allow the SNWA to obtain approximately an 

additionall20,000 acre-feet, without constructing a pipeline. 

"Wheeling" of this water, however, is technically not permitted, 

because any river water that reaches Lake Mead is legally considered to be part of Nevada's Colorado River system water apportionment of 300,000 acre-feet. If legal solutions cannot be achieved 

in favor of"wheeling" water, a legal, and costly, pipeline could divert this water before it reaches Lake Mead. 

Another important potential resource is ground-water banking, 

whereby aquifers could be artificially recharged with unused portions of Colorado River system water to be used during future 

high -demand periods. While this option is already being used in 

Las Vegas Valley, more water could be banked elsewhere in southern Nevada and, pending legal decisions, Nevada could buy water 

for banking from Arizona or other member states in the Colorado 

River Compact. 

Given these expanded options, the SNWA has projected that there 

will in fact be enough water to meet the demands of southern Nevada beyond the year 2025. 
SouTH -CENTRAL ARIZONA 

Earth fissures and subsidence complicate 

development of desert water resources 

I ' 

' c ' '!:> // 

~ / 

' ,c, ' c / (.) ~ ,, I 

I 00 Kilometers 

Michael C. Carpenter 

ARIZONA 

U.S. Geological Survey, Tucson, Arizona 

E arth fissures that rupture the Earth's surface and widespread 

land subsidence in deep alluvial basins of southern Arizona 

are related to ground-water overdrafts. Since 1900 ground 

water has been pumped for irrigation, mining, and municipal use, 

and in some areas more than 500 times the amount of water that 

naturally replenishes the aquifer systems has been withdrawn 

(Schumann and Cripe, 1986). The resulting ground-water-level 

declines-more than 600 feet in some places-have led to increased 

pumping costs, degraded the quality of ground water in many locations, and led to the extensive and uneven permanent compaction 

of compressible fine-grained silt- and day-rich aquitards. A total 

area of more than 3,000 square miles has been affected by subsidence, including the expanding metropolitan areas of Phoenix and 

Tucson and some important agricultural regions nearby. 

Earth fissures, a result of ground failure in areas of uneven or differential compaction, have damaged buildings, roads and highways, 

railroads, flood-control structures, and sewer lines. The presence 

and ongoing threat of subsidence and fissures 

forced a change in the planned route of the 

massive, federally-financed Central 

Arizona Project (CAP) aqueduct that 

has delivered imported surface water 

from the Colorado River to central 
66 

Annuat grour l-water 

pt..Mpage and 

•mported water 

d,'IVL te> 

(n·tllic.m of 

acre-feet) 

Arizona water use by sector, 1994 

TOTAL: 

6.8 million acre feet 

Agricultural 

~::===:=:::::1 Other 

Water sources, 1994 

rivers 

*In excess of 

natural recharge 

Power and mines 

N atural recharge 

Reclaimed water 

(Arizona Department ofWater Resources, 

accessed july 27. 1999) 

Ground-water 

pumpage 

After World War II the population 

of Arizona grew rapidly, especially 

in southern Arizona, where most 

ofthe population presently resides. 

Population 

growth--... 

Water deliveries begin from 

the Central Arizona Project 

Mining Ground Water 

- 4 

Population of Arizona - 2 (millions of residents) 

(Ground-water pumpage from Anning andTuet. 1994; imported water 

deliveries from Arizona Department of Water Resources; population 

data modified from U.S. Census Bureau) 

Arizona since 1985. In the CAP, Arizona now has a supplemental 

water supply that has lessened the demand and overdraft of 

ground-water supplies. Some CAP deliveries have been used in 

pilot projects to artificially recharge depleted aquifer systems. When 

fully implemented, recharge of this imported water will help to 

maintain water levels and forestall further subsidence and fissure 

hazards in some areas. 

GROUND WATER HAS SUSTAINED AGRICULTURE 

Irrigation is needed to grow crops in Arizona because of the low 

annual rainfall and the high rate of potential evapotranspirationmore than 60 inches per year. Precipitation in south-central Arizona 

ranges from as low as 3 inches per year over some of the broad flat 

alluvial basins to more than 20 inches per year in the rugged mountain ranges. Large volumes of water can be stored in the intermontane basins, which contain up to 12,000 feet or more of sediments 

eroded from the various metamorphic, plutonic, volcanic, and consolidated sedimentary rocks that form the adjacent mountains. 

Ground water is generally produced from the upper 1,000 to 2,000 

feet of the basin deposits, which constitute the aquifer systems. 

Ground water pumped from the aquifer systems became a reliable 

and heavily tapped source of irrigation water that fueled the development of agriculture during the early and mid-20th century. In 

many areas, the aquifer systems include a large fraction of finegrained deposits containing silt and clay that are susceptible to compaction when the supporting fluid pressures are reduced by pumping. 

CAP water sustains urban growth 

Pumping for irrigation began prior to 1900, and increased markedly 

in the late 1940s. By the mid -1960s the expected growth in the metropolitan Phoenix and Tucson areas, coupled with the already large 
South-Central Arizona 

Agriculture in Arizona requires 

intensive irrigation. 

(U.S. Bureau of ReclamatJon) 

LAND USE 

D Agricultural 

DUrban 

WATER-LEVEL DECLINE 

Predevelopment to 1980 

D Less than 50 feet 

Osoto 100 

D 100to200 

0 200to 300 

D 300to400 

D Greater than 400 

1(. AVI 

J 

VA!< 

67 

ground-water-level declines and worsening subsidence problems, 

prompted Arizona water officials to push for and receive congressional approval for the CAP. Since then, growth in the metropolitan 

areas has exceeded expectations, and municipal-industrial and domestic water use presently accounts for nearly 20 percent of 

Arizona's water demand. 

Subsidence fol lows water-level declines 

Subsidence first became apparent during the 1940s in several alluvial basins in southern Arizona where large quantities of ground 

water were being pumped to irrigate crops. By 1950, earth fissures 

began forming around the margins of some of the subsiding basins. 

The areas affected then and subsequently include metropolitan 

Phoenix in Maricopa County and Tucson in Pima County, as well as 

important agricultural regions in Pinal and Maricopa Counties near 

Apache Junction, Chandler Heights, Stanfield, and in the Picacho 

Basin; in Cochise County near Willcox and Bowie; and in La Paz 

County in the Harquahala Plain. By 1980 ground-water 

! 

levels had declined at least 100 feet in each of these areas 

and between 300 and 500 feet in most of the areas. 

Boundary of Basin and 

Range Physiographic 

Province in Arizona 

Alluvial deposits 

Consolidated rock 

Phoenix • Apache Junction 

• Ch.andler Heights fsa;,. .F <?,z_ v "'~ Picacho 

Basin Pli\FAL 

Tucson. 

0 100 Mi 

0 100 Km 

(Anderson and others. 1992) 
68 

Central Arizona Project (CAP) 

Delivering water to the interior basins 

The primary purpose of The Central Arizona Project (CAP) is 

to help conserve the ground-water resources of Arizona by 

extending the supply of Colorado River water to interior 

basins in Arizona that are heavily dependent on the already 

depleted ground-water supplies. A body oflegal doctrine 

collectively known as the "Law of the River" allots Arizona up 

to 2.85 million acre-feet of Colorado River water yearly, 

depending on availability. The Central Arizona Project was 

designed to deliver about 1.5 million acre-feet of Colorado 

River water per year to Maricopa, Pinal, and Pima Counties. 

Colorado River water fills the aqueduct at Lake Havasu near 

Parker and flows 336 miles to the San Xavier Indian Reservation southeast of Tucson, with the aid of pumping plants 

and pumping-stations with lifts that total about 3,000 feet. Of 

the more than 80 major customers, 75 percent are municipal 

or industrial, 13 percent are irrigation districts, and about 

, ) 

.

~· 2-\ , Lake 

\ Havasu 

Parker' 

Dam, 

Central Arizona 

Project (CAP) 

aqued uct 

o Pumping sites 

• Pilot recharge sites 

( ~ :--.1/?, · I MA PA~ so- "e_. 

I)~ :f~"' " .- ~~\1 

Co"ocy "~:, ~ :>J 

American Indian reservations ... ________ ~ 

served by CAP 

0 IOO Mi 

0 100 Km 

Mining Ground Water 

A segment of the CAP aqueduct snakes 

through the desert west of Phoenix. 

12 percent are Native American communities. CAP 

water was first delivered to Phoenix in 1986 and to 

Tucson in 1992. Having a higher salinity than the 

natural ground-water supplies it augments, CAP water 

is generally used in three ways- direct treatment and 

delivery; treatment, blending and delivery; and spread 

in percolation basins to artificially recharge the aquifer 

systems. Before it is distributed as drinking water, CAP 

water is disinfected and generally "softened." Of the 1.5 

million acre-feet annual capacity of the CAP, only about 

1 million acre-feet were being directly utilized as of 

1997. Much of the balance was used to augment natural 

aquifer-system recharge through artificial-recharge 

pilot projects, in order to store water for future use and 

mitigate water-level declines and limit subsidence. 

Land subsidence was first verified in south-central Arizona in 1948 

using repeat surveys of bench marks near Eloy (Robinson and 

Peterson, 1962). By the late 1960s, installation and monitoring of 

borehole extensometers at Eloy, Higley Road south of Mesa, and at 

Luke Air Force Base, as well as analysis of additional repeat surveys, 

indicated that land subsidence was occurring in several areas. The 

areas of greatest subsidence corresponded with the areas of greatest 

water-level decline (Schuman and Poland, 1970). 

By 1977, nearly 625 square miles had subsided around Eloy, where 

as much as 12.5 feet of subsidence was measured; another 425 

square miles had subsided around Stanfield, with a maximum sub-
South-Central Arizona 69 

Co np to 1 

(• f 

I -

1993 1994 1995 1996 1997 

sidence of 11.8 feet (Laney and others, 1978). Near Queen Creek, an 

area of almost 230 square miles had subsided more than 3 feet. In 

northeast Phoenix, as much as 5 feet of subsidence was measured 

between 1962 and 1982. By contrast, in the Harquahala Plain, only 

about 0.6 feet of subsidence occurred in response to about 300 feet 

of water-level decline, whereas near Willcox, more than 5 feet of 

subsidence occurred in response to 200 feet of water-level decline 

(Holzer, 1980; Strange, 1983; Schumann and Cripe, 1986). The relation between water-level decline and subsidence varies between and 

within basins because of differences in the aggregate thickness and 

compressibility of susceptible sediments. 

Data from a borehole extensometer site in the Tucson Basin shows how compaction can 

respond to water level changes. 

Seasonal fluctuations are related to patterns of groundwater pumping. 

Subsidence has occurred in basins with large water-level declines, but the relation between 

the magnitude of water-level 

decline and subsidence varies 

between and within basins. 

Representative profiles show 

that subsidence is greater near 

the center of basins, where the 

aggregate thickness of finegrained sediments is generally 

greater. 

0 

By 1992, ground-water level declines of more than 300 feet had 

caused aquifer-system compaction and land subsidence of as much 

as 18 feet on and near Luke Air Force Base, about 20 miles west of 

Phoenix. Associated earth fissures occur in three zones of differential subsidence on and near the base. Local flood hazards have 

greatly increased due to differential subsidence at Luke, which led to 

a flow reversal in a portion of the Dysart Drain, an engineered flood 

Luke AFB ,--, 

Land 

subsidence, 

1952- 1992 10 -

(feet) 

(Modified from Schumann. 1995) 

Land 

subsidence, 

1952- 1992 3 -

Land 

subsidence, 

- 3 1952- 1980 

(feet) (feet) 

12 ---------------------- ---------------------------- 12 

(Modified from Laney and others. 1978) (Modified from Anderson. 1988. 1989) 
70 

Fissures tend to develop 

near the margins of subsiding basins. 

Fissures have vertical sides, and 

typically first appear following severe rainstorms. Opening or 

movement is rarely more than 1 

inch in any particular episode, although erosion and collapse of 

the sides during the initial episode may leave a fissure gully 

more than 10 feet wide, 30 feet 

deep, and hundreds of feet long. 

The apparent 1-foot width of the 

fissure that opened on July 23, 

1976, near the Picacho Mountains, is due to erosion, collapse, 

and disintegration of downdropped blocks. Several blocks 

remain wedged about 1 foot below land surface. 

Mining Ground Water 

0 Water-level decline 

greater than I 00 feet 

0 Areas with fissures 

0 50 Mi 

50 Krn 

conveyance. On September 20, 1992, surface runoff from a rainstorm 

of 4 inches closed the base for 3 days. The sluggish Dysart Drain 

spilled over, flooding the base runways along with more than 100 

houses and resulted in about $3 million in damage (Schumann, 1995). 

EARTH FISSURES ARE COMMON IN MANY BASINS 

Some of the most spectacular examples of subsidence-related earth 

fissures occur in south-central Arizona. Earth fissures are the 

dominant mode of ground failure related to subsidence in alluvialvalley sediments in Arizona and are typically long linear cracks at 

the land surface with little or no vertical offset. The temporal and 

spatial correlation of earth fissures with ground-water-level deIn another fissure that opened July 

23, 1976, near the Picacho Mountains, an erosional gully 6 feet 

wide, 5 feet deep, and 20 feet 

long was cut in less than 16 hours. 

The head-cut gully developed perpendicular to the fissure in a wash 

on its upstream side. In subsequent storms, both the head-cut 

gully in the wash and the fissure 

were widened, deepened, and 

lengthened. It may take years or 

decades before a wash again carries water or sediment past a fissure that has cut across it. 
South-Central Arizona 71 

clines indicates that many of the earth fissures are induced, and are 

related to ground-water pumpage. More than 50 fissure areas had 

been mapped in Arizona prior to 1980 (Laney and others, 1978). 

Most fissures occur near the margins of alluvial basins or near exposed or shallow buried bedrock in regions where differential land 

subsidence has occurred. They tend to be concentrated where the 

thickness of the alluvium changes markedly. In a very early stage, 

fissures can appear as hairline cracks less than 0.02-inch wide interspersed with lines of sink-like depressions resembling rodent holes. 

When they first open, fissures are usually narrow vertical cracks less 

than about 1-inch wide and up to several hundred feet long. They 

Fissure formation 

Several theories explain the mechanism of fissure formation 

Several mechanisms have been proposed for earth fissures, 

the most widely accepted of which is differential compaction. 

As ground-water levels decline in unconsolidated alluvial 

basins, less compaction and subsidence occurs in the thinner 

alluvium near the margin of the basin than in the thicker 

alluvium near the deeper, central part of the basin. The 

tension that results from the differential compaction stretches 

the overlying sediment until it fails as a fissure. 

Differential compaction 

As the land surface subsides, 

all uvium stretches and eventually fails, generally in a 

region of abrupt change 

in alluvium thickness. 

Fissures are concentrated in 

areas where the thickness 

of the alluvium changes, such 

as near the margin of basins 

or where bedrock is near 

the surface. 

Bedrock 

OTHER POSSIBLE MECHANISMS 

Alluvium 

Not to scale 

Horizontal seepage stresses and rotation of a rigid slab over 

an incompressible edge are other mechanisms that have been 

suggested. The observation that new fissures have formed 

between existing fissures and the mountain front argues 

against these two hypotheses. Hydrocompaction, or collapse 

oflow-density soils upon complete wetting, and increased 

soil-moisture tension have also been suggested as possible 

mechanisms. Hydrocompaction in fact did occur during 

construction of sections of the CAP Aqueduct between the 

Picacho Mountains and Marana. 

Lateral stresses induce 

tension cracking. 

Surface water infi ltrates, dissolving 

the natural cement bonding t he 

soil, connecting hairline cracks, and 

further eroding and enlarging the 

fissure. 

Fissure progressively enlarges, capt uring surface runoff, sediment, 

and debris. Eventually vegetation 

establishes itself, creating a line of 

vegetation along the trace of t he 

fissure. 

Other proposed mechanisms include piping erosion, soil 

rupture during earthquakes, renewed faulting, collapse of 

caverns or mines, oxidation of organic soils, and diapirism. 

Piping (subsurface soil erosion) along the trace of a fissure 

certainly plays a part in the opening, progressive enlargement and subsequent development of fissure gullies. 

(Eaton and others, 1972; Carpenter, 1993) 
72 

Discovering Arizona's early fissures 

Two fissures, two scientists, and their one discovery 

On September 12, 1927, Professor R.J. Leonard from the 

University of Arizona visited and photographed an earth 

fissure south of the town of Picacho that was observed 

following a severe thunderstorm. After considering several 

possible causes for the fissure, Leonard tentatively concluded 

that an earthquake which had occurred on September 11, 

1927, 170 miles from Tucson, caused the fissure by triggering 

the release of preexisting, accumulated strain. Leonard, a 

mining engineer, was probably influenced by his knowledge 

of the occurrence of unusual cracks at the El Tiro Mine near 

Silver Bell, Arizona, about 20 miles to the south (Leonard, 

1929). 

Two months later on November 13, 1927, Professor A.E. 

Douglas, also from the University of Arizona, visited and 

photographed what he probably thought was the same fissure 

that Leonard had photographed. In fact, it was not. The 

mountain skyline on Douglas's photographs lines up from a 

viewpoint about 1 mile to the southwest of Leonard's viewpoint. Leonard and Douglas discovered two separate earth 

fissures, and it was Douglas's photo that captured the precursor to the present-day Picacho earth fissure (Carpenter, 1993). 

These early discoveries of multiple earth fissures at a time 

when ground-water withdrawals were just beginning raise 

some doubts about their origin. Although there is little doubt 

that ground-water-level declines since the 1940s have caused 

earth fissures, the cause of the Leonard and Douglas fissures 

remains a mystery. 

Douglas's view 

Mining Ground Water 

Leonard's fissure 

(University of Arizona Tree Ring Laboratory 

photographs GEOL 27-2) 

A fissure moves with the seasonal fluctuation of water levels 

(data from the Picacho Basin). 

can progressively lengthen to thousands of feet. Apparent depths of 

fissures range from a few feet to more than 30 feet; the greatest recorded depth is 82 feet for a fissure on the northwest flank of 

Picacho Peak (Johnson, 1980). Fissure depths of more than 300 feet 

have been speculated based on various indirect measurements in198 1 

Lower water-level fiuctuations 

induce fissure movement. 

\ 

( 

1982 1983 1984 

He z.on t1l 

1itKI>es 

008 

cluding horizontal movement, volume-balance calculations based on the volume of air 

space at the surface, and the amount of sediment transported into the fissures. 

Widening of fissures by collapse and erosion results in fissure gullies (Laney and 

others, 1978) that may be 30-feet wide and 

20-feet deep. No horizontal shear 

(strike-slip movement) has been detected at 

earth fissures, and very few fissures show 

any obvious vertical offset. However, fissures monitored by repeated leveling surveys commonly exhibit a vertical offset of a 
South-Central Arizona 

This fissure near the Picacho 

Mountains is undergoing erosional widening to become a 

fissure gully. 

This aerial view taken in 

October 1967 shows 

the Picacho earth fissure 

as a single crack. A citrus 

grove is visible in the 

upper left. 

73 

By June 1989 the fissure had 

developed into a system of 

multiple parallel cracks. A fissure scarp developed as much 

as 2 feet of vertical offset, with 

the west or left side of the fissure (as pictured) downdropped. 

A lateral canal in the upper left 

skirts a citrus grove. This canal 

originates from the Central 

Arizona Project Aqueduct (not 

visible) at the base of the 

mountains in the background 

and crosses the fissure north of 

the citrus grove. 

few inches. Two notable exceptions are the Picacho earth fissure, 

which has more than 2 feet of vertical offset at many places along its 

10-mile length, and a fissure near Chandler Heights, which has about 

1 foot of vertical offset. 

The Picacho fissure is Arizona's most studied 

The Picacho earth fissure, perhaps the most thoroughly investigated 

earth fissure (Holzer and others, 1979; Carpenter, 1993), began to 

creep vertically in 1961, forming a scarp. The scarp initially grew at a 

rate of more than 2 inches per year, before progressively slowing to 

about one-third inch per year by 1980 (Holzer, 1984). The observed 

opening and closing correlated with seasonal ground-water-level 

fluctuations from 1980 to 1984 (Carpenter, 1993). Surface deformation near the fissure indicated that formation of the vertical scarp 

was preceded by differential land subsidence and the formation of 

other earth fissures distributed over an approximately 1,000-footwide zone. Local geophysical and geologic surveys indicated that the 

Picacho earth fissure is associated with a preexisting high-angle, 

normal fault. 

In the early 1950s Feth (1951) attributed formation of earth fissures 

west of the Picacho Mountains to differential compaction caused by 

ground-water-level decline in unconsolidated alluvium over the 

edge of a buried pediment or bedrock bench. He observed that fissures typically open during and after storms and potentially intercept large quantities of surface runoff. A decade later, the occurrence 

of subsidence-related fissures near Picacho, Chandler Heights, Luke 

Air Force Base, and Bowie was well known (Robinson and Peterson, 
74 

Another area experiencing subsidence-related earth fissures is near 

CasaGrande. This series of photographs shows how irrigation and 

pumping over a period of 22 

years resulted in subsidence, surface depressions, and fissures possibly related to hydrocompaction. 

A hydrograph from the well shown 

in the 1970 photo shows a sudden 

drop in water level after 1940. 

~--===----------

Water level "'\ 

(feet below 20 -----~".--­ land surface) \. 

40 .--.---,----,.-.---. 

1920 1940 1960 

The Central Main Lateral Canal 

of the CAP was damaged where 

it crosses the Picacho earth fissure. Opening of the fissure is 

evident as a dark line in the 

lower middle of the photograph. 

1980 

Mining Ground Water 

0 0.5 Mile 

1962). Subsidence-related earth fissures also have occurred in 

McMullen Valley (northwest of the Harquahala Plain),Avra Valley, 

the east Salt River Valley near Apache Junction, Willcox Basin 

(Schumann and Genauldi, 1986) and, as recently as 1997, in the 

Harquahala Plain (Al Ramsey, Arizona Department of Water Resources, written communication, 1998). Subsurface conditions beneath many subsidence-related earth fissures have been inferred 

principally from geophysical surveys and indicate that most occur 

above ridges or "steps" in the bedrock surface (Peterson, 1962; 

Holzer, 1984). In recent years, with introduction of CAP irrigation 

water, retirement of some farm lands, and the consequent recovery 

of water levels, earth fissures have apparently ceased to be active in 

some areas. 

FISSURES CAN UNDERCUT AND DAMAGE INFRASTRUCTURE 

Structures damaged by fissures include highways, railroads, sewers, 

canals, aqueducts, buildings, and flood-control dikes. The threat of 

damage from earth fissures forced a change in the proposed route of 

the CAP aqueduct. Erosionally enlarged fissure gullies present hazards to grazing livestock, farm workers, vehicles, hikers, and wildlife. 

Aquifer contamination may also occur as a result of ruptured pipelines, dumping of hazardous waste into fissures, and capture of surface runoff containing agricultural chemicals and other contaminants. 

Where Interstate 10 crosses the Picacho earth fissure, more than 2 

feet of vertical offset and several inches of horizontal opening have 

damaged the highway, requiring repeated pavement repairs. Where a 

natural gas pipeline crosses a fissure near the Picacho Mountains, 

erosional enlargement of the fissure left the pipeline exposed. The 
South-Central Arizona 

A natural-gas pipeline undercut by an earth fissure was 

exposed through erosional 

widening of the fissure. The 

pipeline was evacuated and 

cut to determine the stresses 

on it. Tension was evident, 

but no shear. 

Part of this fissure south of 

Apache Junction has been 

trenched and backfilled for 

a land bridge. 

75 

30-foot-wide hole was simply backfilled, but was repeatedly eroded 

for several years thereafter during summer and winter rains and had 

to be repeatedly refilled. 

The CAP aqueduct and associated canals have been affected by 

earth fissures at several localities. Near Apache Junction, the U.S. 

Bureau of Reclamation installed vertical sheet piles on both sides of 

the CAP aqueduct in a fissure that undercuts the aqueduct. Soil 

beneath the aqueduct was compacted to reduce erosion. Erosional 

damage at this site and at another similarly treated site south of the 

Casa Grande Mountains has been minimal (Cathy Wellendorf, U.S. 

Bureau of Reclamation, written communication, 1988). 

Engineering measures can also mitigate damage where fissures undercut roads. At Apache Junction, a trench was dug to a depth of 

about 30 feet, backfilled by about 10 feet of compacted fill, and then 

draped by a reinforced plastic grid, geotextile felt, and an impermeable membrane. The membrane was buried by additional compacted fill. This treatment protects the road from subsurface erosion 

by enhancing its structural strength and by restricting the upward 

flow of water from the fissure into the land bridge during flooding. 

ARIZONA ACTS TO PROTECT THE AQUIFER SYSTEM 

To ensure the future viability of the State's critical ground-water 

resources, the Arizona Groundwater Management Act was passed in 

1980. This innovative law has three primary goals: (1) to control the 

severe overdraft of depleted aquifer systems, (2) to provide a means 

for allocating the limited ground-water resources among competing 

demands and effectively meet the changing needs of the State, and 

(3) to augment Arizona's ground-water resource through development of additional water supplies. The Act recognized ground water 

in Arizona as a public resource that must be managed for the benefit 

of everyone, and in 1986 was named one of the Nation's ten most 

innovative programs in State and local government by the Ford 

Foundation. 

Based upon recommendations of the Groundwater Management 

Study Commission, which included representatives from cities and 
76 

L 

._ , 

-~.\ '\ Lake Havasu 

Parker \. Central Arizona 

Dam/' Project aqueduct 

/ 

Phoenix • 

,J 

--

Mining Ground Water 

' 

d Active Management Areas (AMAs) 

contain more than 80 percent of 

the State's population and account 

for about 70 percent of the estimated 

annual ground-water overdraft. 

Gila ~- I 

<t·e_... i 

I 

Tucson 

0 100 Mi ...._ ~ In Irrigation Non-Expansion 

Areas (INAs), the overdraft 

problems are less severe. 

0 100 Km 

For more information concerning the Arizona Groundwater Management Act, visit 

the Arizona Department of Water Resources 

web site at http:/ /www.adwr.state.az.us/ 

-- ' - ...._'_-- _ l_ ------ _j 

towns, Native American communities, and mining, agricultural, 

and electric utilities industries, the Act focuses on limiting groundwater-level declines. Although it specifically mentions subsidence 

only three times, measures that limit ground-water-level declines 

will ultimately help to control compaction of the aquifer system 

and land subsidence. The Act provides for two levels of water management to respond to geographic regions where ground-water 

overdraft is a problem. Active Management Areas (AMAs) are designated where problems are most severe and Irrigation 

Non-Expansion Areas (INAs) are designated where problems are 

least severe. The Act established the Arizona Department of Water 

Resources (ADWR) to administer the Act. The State Director of the 

ADWR can designate additional AMAs for several reasons, including land subsidence or fissuring that is endangering property or 

potential ground-water-storage capacity (Carpenter and Bradley, 

1986). The Act includes these six key provisions: 

1. A program of ground-water rights and permits. 

2. Restriction on new agricultural irrigation within AMAs. 

3. Water conservation and management plans for AMAs that constitute 5 consecutive and progressively more stringent phases 

implemented during the periods 1980-1990, 1990-2000,2000-

2010,2010-2020, and 2020-2025. 

4. Assured water supply for new growth in AMAs before land may 

be marketed to the public. 

5. Metering of ground-water pumpage for designated wells in AMAs. 

6. Annual reporting of ground-water pump age and assessment of 

withdrawal fees for designated wells in AMAs. 

The original four AMAs were Phoenix, Pinal, Prescott, and Tucson. 

Subsequently, the Santa Cruz AMA was created by separation from 

the Tucson AMA in 1994. The two original IN As were Douglas and 

Joseph City, followed by Harquahala in 1982. The AMAs contain 
South-Central Arizona 

A section of the Central Arizona Project passes through 

Apache Junction . 

(U.S. Bureau of Reclamation) 

77 

more than 80 percent of the State's population and account for 

about 70 percent of the estimated annual ground-water overdraft in 

the State. 

In the Tucson and Phoenix AMAs, which include the large urban 

areas of the State, and in the Prescott AMA, the primary management goal is to achieve safe yield by January 1, 2025. The goal in the 

Pinal AMA, where a predominantly agricultural economy exists, is 

to extend the life of the agricultural economy for as long as feasible 

and to preserve water supplies for future nonagricultural uses. In 

the Santa CruzAMA, where significant ground-water/surface-water, international, and riparian water issues exist, the goal is to 

maintain safe yield and prevent the long-term decline oflocal unconfined aquifers. 

Increasingly stringent conservation measures are being implemented in each of the AMAs during the five management periods. 

Municipal conservation measures include reductions in per capita 

water use measured in gallons per capita per day (GPCD). Therequirements apply to the water providers, who must achieve target 

GPCDs through water-use restrictions or incentive-based conservation programs. Conservation for irrigated agriculture is being 

achieved by prohibiting new ground-water-irrigated acreage and by 

reductions in ground-water allotment, based on the quantity of 

water needed to irrigate the crops historically grown in the particular farm unit. There are also programs for augmenting water supplies, including incentives for artificial recharge, for purchase and 

retirement of irrigation rights, and for levying fees of up to $2.00 

per acre-foot (Carpenter and Bradley, 1986). 

A SUBSIDENCE-MONITORING PLAN WAS ESTABLISHED 

In 1983, the National Geodetic Survey, with advice from an interagency Land Subsidence Committee, created a subsidence -monitoring plan for the Governor of Arizona. The plan summarized 

known subsidence and recognized hazards caused by subsidence, 

differential subsidence, and earth fissures in Arizona. The objectives 

of the plan were ( 1) "Documentation of the location and magnitude 

of existing subsidence and subsidence-induced earth fissures;" and 

(2) "Development of procedures for estimating future subsidence as 

a function of water-level decline and defining probable areas of 

future fissure development." The plan proposed a central facility at a 

State agency for compilation and organization of leveling, compaction, gravity, and other geophysical and stratigraphic information. 

There were plans to coordinate the analysis of existing data, to produce estimates of future subsidence and earth-fissure development, 

and to identify observation requirements. Other provisions included ( 1) " [a] n initial observation program designed to obtain a 

limited amount of additional leveling data, gravity observations, 

compaction measurements, and horizontal strain determinations;" 

and (2) "[a] cooperative effort between State and Federal agencies 

to evaluate new measurement technologies which offer the potential 
78 

(U.S. Bureau of Reclamation) 

Mining Ground Water 

of being faster and more cost effective than current methods of 

subsidence monitoring:' Also included were proposals for directions in research, some initial monitoring plans, and an advisory 

committee to oversee the formation of the central data facility and 

provide continuing guidance. (Strange, 1983). The recommendations have been only partially implemented. The Arizona Geological Survey has a Center for Land Subsidence and Earth Fissure 

Information. The USGS, the Arizona Department of Water Resources, the City of Tucson, and Pima County maintain cooperative 

programs for monitoring subsidence using global positioning system ( GPS) surveying, microgravity surveys, and borehole extensometers. The ADWR has also started its own program of GPS 

surveying and microgravity surveys in the Phoenix metropolitan 

area. 

In 1997, 19 of 29 borehole extensometers installed in south -central 

Arizona to measure aquifer-system compaction were still in operation. In the early 1990s, water levels in the Tucson basin continued 

to decline by as much as 3 to 6 feet per year, and a small amount of 

subsidence, generally less than 0.2 inch per year, was occurring in 

some areas. During the same period, water levels in Avra Valley 

continued to decline by 3 feet per year, and some subsidence, generally less than 0.1 inch per year, was occurring in some areas (City of 

Tucson Water Department, 1995). In the Picacho Basin, despite 

water-level recoveries of as much as 150 feet, some areas continue to 

subside at rates of up to 0.3 inches per year, most likely due to residual compaction of slowly equilibrating aquitards. 

RISING WATER LEVELS OFFER SOME HOPE FOR THE FUTURE 

Importation of CAP water for consumptive use and ground-water 

recharge, retirement of some farmlands, and water-conservation 

measures have resulted in cessation of water-level declines in many 

areas and the recovery of water levels in some areas. However, some 

basins are still experiencing subsidence, because much of the a quifer-system compaction has occurred in relatively thick aquitards. It 

can take decades or longer for fluid pressures to equilibrate between 

the aquifers and the full thickness of many of these thick aquitards. 

For this reason, both subsidence and its abatement have lagged 

pumping and recharge. A glimmer of hope exists from data at the 

borehole extensometer near Eloy, where water levels have recovered 

more than 1 SO feet and compaction has decreased markedly. 
PART II 

Drainage of Organic Soils 

Cultivated peat soils in the 

Sacramento-San Joaquin Delta 

(California Department ofWater Resources) 

Most organic soils occur in 

the northern contiguous 

48 States and Alaska. 

Sacramento-San Joaquin Delta 

Florida Everglades 

I n the U.S. system of soil taxonomy, organic soils or histosols 

are one of 10 soil orders. They are formally defined as having 

more than 50 percent organic matter in the upper 30 inches, but 

may be of lesser thickness if they overlie fragmental rock permeated 

by organic remains. Organic soil is commonly termed "peat;' if 

fibrous plant remains are still visible, or "muck" where plant remains 

are more fully decomposed. Other common names for accumulations of organic soil include "bog;' "fen;' "moor:' and "muskeg:' 

Organic soils generally form in wetland areas where plant litter 

(roots, stems, leaves) accumulates faster than it can fully decompose. 

Fibrous peats typically include the remains of sedges and reeds that 

grew in shallow water. "Woody" peats form in swamp forests. In 

northerly latitudes with cool, moist climates, many peats are composed mainly of sphagnum moss and associated species. The total 

area of organic soils in the United States is about 80,000 square 

miles, about half of which is "moss peat" located in Alaska (Lucas, 

1982). About 70 percent of the organic-soil area in the contiguous 48 

States occurs in northerly, formerly glaciated areas, where moss 

peats are also common (Stephens and others, 1984). 
80 

Evidence of subsidence in 

the Everglades is shown on a 

concrete marker that has 

been driven through the organic soil into the underlying 

, limestone substrate. 

Sub

! 

siding 

ground 

surface 

1954 

1968 

Roman numerals on marker 

indicate feet above the 

limestone substrate. 

Drainage of Organic Soils 

Land subsidence invariably occurs when organic soils are drained 

for agriculture or other purposes. There are a number of causes, 

including compaction, desiccation, erosion by wind and water, and, 

in some cases, prescribed or accidental burning. The effects of 

compaction and desiccation after initial draining can be dramatic, 

because organic soils have extremely low density and high porosity 

or saturated water content (up to 80 to 90 percent). 

DRAINED ORGANIC SOILS WILL LITERALLY DISAPPEAR 

The most important cause of organic-soil subsidence, however, is a 

process commonly termed "oxidation:' The balance between 

accumulation and decomposition of organic material shifts dramatically when peat wetlands are drained. Under undrained conditions, anaerobic microbial decomposition of plant litter-that is, 

decomposition in the absence of free oxygen-cannot keep pace 

with the rate of accumulation. One reason is that lignin, an important cell-wall component of all vascular plants, is much more 

vulnerable to decomposition under aerobic conditions. Oxidation 

under aerobic conditions converts the organic carbon in the plant 

tissue to carbon dioxide gas and water. Aerobic decomposition 

under drained conditions is much more efficient. 

The biochemical origin of much organic-soil subsidence was 

established by 1930 through laboratory experiments with Florida 

peat that balanced the loss of dry soil weight with rates of carbondioxide production (Waksman and Stevens, 1929; Waksman and 

Purvis, 1932). This early laboratory work also suggested optimal 

temperature ranges and moisture contents for microbial decomposition. Later field studies and observations have confirmed "oxidation" as the dominant subsidence process in many instances. For 

example, in the Florida Everglades, sod fields and residential 

areas-where causal mechanisms such as erosion, burning, and 

compaction are minimized or absent-have sunk as rapidly as the 

cultivated land (Stephens and others, 1984).It is believed that 

oxidation-related soil loss can be halted only by complete resaturation of the soil or complete consumption of its organic carbon 

content (Wosten and others, 1997). 

Whereas natural rates of accumulation of organic soil are on the 

order of a few inches per 100 years, the rate of loss of drained 

organic soil can be 100 times greater, up to a few inches per year in 

extreme cases. Thus, deposits that have accumulated over many 

millennia can disappear over time scales that are very relevant to 

human activity. 

SOME ORGANIC SOILS CAN BE CULTIVATED FOR CENTURIES 

Human experience with subsiding organic soils dates back nearly 

1,000 years in The Netherlands and several hundred years in the 

English fen country. The old polders in the western Netherlands 

were reclaimed for agriculture between the 9th and 14th centuries, 
Introduction 

Long-term subsidence rates in 

the Everglades show cycles. 

Subsidence slows during periods of poor drainage and accelerates when pumps are 

installed to improve drainage. 

Land-surface 

altitude 

(feet above 12 -----~'<:-..:::....,----..::.-

sea level) 

Average bedrock elevation (7.94 ft) 

-.-.-.--.-.--.-.-~ 

1910 1930 1950 1970 

(Stephens and others, 1984) 

81 

and by the 16th century the land had subsided to such an extent that 

windmills were needed to discharge water artificially to the sea 

(Shothorst, 1977). Because ground-water levels beneath the polders 

were still relatively high, the rate of subsidence was relatively lowless than 5 feet total, or 0.06 inches per year, over a roughly 1,000-

year period in which progressively more sophisticated drainage 

systems were developed (Nieuwenhuis and Schokking, 1997). 

Greatly improved drainage in the 20th century increased the thickness of the drained zone above the water table. As a result, subsidence rates rose to about 0.2 inches per year between the late 1920s 

and late 1960s, and current rates are more than 0.3 inches per year. 

The organic-soil subsidence rates in The Netherlands are still 

unusually low in a global context. This is due in part to the relatively 

cool climate, where temperatures are generally below the optimal 

range for microbial decomposition, and in part to a thin layer of 

marine clay that caps much of the peat. Larger average rates have 

been observed elsewhere: up to 3 inches per year over the last 100 

years in the Sacramento-San Joaquin Delta, California; about 1 inch 

per year over the past 100 years in the English fens; and about 1 inch 

per year for the last 70 years in the Florida Everglades. 

Both in the English fens and the Everglades, long-term subsidence 

rates have been monitored using stone or concrete columns driven 

into the underlying solid substrate. The history of both areas has 

been marked by alternate cycles of improved drainage followed by 

accelerated subsidence and, consequently, inadequate drainage 

(Stephens and others, 1984), so that the achievements of one generation become the problems of the next (Darby, 1956). 

SACRAMENTO-SAN JOAQUIN DELTA 

The sinking heart of the state 

Sacramento • 

1 Sacramento River ~ 

IS Miles 

i===::;---' 15 Kilometers 

S.E. lngebritsen 

U.S. Geological Survey, 

Menlo Park, California 

Marti E. lkehara 

~ 

National Geodetic Survey, 

Sacramento, California 

CALIFORNIA 

s.tockton 

T he Sacramento-San Joaquin Delta of California was once a 

great tidal freshwater marsh. It is blanketed by peat and 

peaty alluvium deposited where streams originating in the 

Sierra Nevada, Coast Ranges, and South Cascade Range enter San 

Francisco Bay. 

In the late 1800s levees were built along the stream channels and the 

land thus protected from flooding was drained, cleared, and 

planted. Although the Delta is now an exceptionally rich agricultural area (over $500 million crop value as of 1993), its unique value 

is as a source of freshwater for the rest of the State. It is the heart of a 

massive north-to-south water-delivery system. Much of this water is 

pumped southward for use in the San Joaquin Valley and elsewhere 

in central and southern California. 

The leveed tracts and islands help to protect water-export facilities 

in the southern Delta from saltwater intrusion by displacing water 

and maintaining favorable freshwater gradients. However, ongoing 

subsidence behind the levees increases stresses on the levee system, 

making it less stable, and thus threatens to degrade water quality in 

the massive north-to-south water-transfer system. Most subsidence 

in the Delta is caused by oxidation of organic carbon in peat soils. 

THE DELTA MARSHES TEEMED WITH WILDLIFE 

When Spanish explorers first viewed the Delta from Mount Diablo 

in 1772, the Sacramento and San Joaquin Rivers were in flood, and 

they mistook it for a great inland 

sea. In fact, the prehistoric Delta 

consisted largely of"tule" (bulrush) and reed marshes that were 

periodically submerged, with narrow bands of riparian forest on the 

natural levees along major stream 

channels. Exceptionally abundant 

fish and game supported a large 

(The Nature Conservancy) 
84 

The tule marshes of the Delta 

once teemed with migratory 

birds and fish. 

(The Nature Conservancy) 

Drainage of Organic Soils 

Native American population. When the Spanish first set foot in the 

Delta, they found the deer and tule elk trails to be so broad and extensive that they first supposed that the area was occupied by cattle. 

Similarly, American soldiers exploring the Delta in the 1840s found 

waterfowl to be so abundant and tame that they were mistaken for 

domestic fowl. The Native Americans were also able to harvest 

abundant local shellfish and the salmon that migrate through the 

Delta en route to spawning grounds in streams of the Sierra Nevada 

and southern Cascades. 

Trappers from the Hudson Bay Company and elsewhere visited the 

Delta periodically between 1827 and 1849, drawn by the initially 

abundant beaver and river otter. By the beginning of the California 

Gold Rush in 1849, the Native American population of the Delta had 

been nearly destroyed by intermittent warfare with the Spanish and 

Mexicans and great epidemics of malaria(?) and cholera (1833) and 

smallpox (1839) (Dillon, 1982). Shortly after the Gold Rush, a great 

effort to control and drain the Delta for agriculture began. Levees 

were built along the stream channels, and the land thus protected 

from flooding was drained, cleared, and planted. The results of such 

reclamation seemed miraculous-in a letter to a friend, early settler 

George McKinney reported cabbages weighing 53 pounds per head 

and potatoes 33 inches in circumference (Dillon, 1982). 

Agriculture and water now dominate the landscape 

Today, the Delta is largely devoted to agriculture, and includes about 

55 islands or tracts that are imperfectly protected from flooding by 

over 1,000 miles of levees. Many of the islands in the central Delta 

are 10 to nearly 25 feet below sea level because ofland subsidence 

associated with drainage for agriculture. There are also numerous 

smaller, unleveed islands that remain near sea level. Remnants of the 

natural tule marsh are found on the unleveed "channel" or "tule" 

islands and along sloughs and rivers. The strips of natural riparian 

forest have nearly vanished, except on some of the larger channel 

islands, but relicts can be viewed at the Nature Conservancy's 

Cosumnes River Preserve in the northeastern Delta. 

Although the Delta is an exceptionally productive agricultural area, 

its unique value to the rest of the State is as a source of freshwater. 

The Delta receives runoff from about 40 percent of the land area of 

California and about 50 percent of California's total streamflow. It is 

the heart of a massive north-to-south water-delivery system whose 

giant engineered arterials transport water southward. State and Federal contracts call for export of up to 7.5 million acre-feet per year 

from two huge pumping stations in the southern Delta near the 

Clifton Court Fore bay (California Department of Water Resources, 

1993). About 83 percent of this water is used for agriculture and the 

remainder for various urban uses in central and southern California. 

Two-thirds of California's population (more than 20 million people) 

gets at least part of its drinking water from the Delta (Delta Protection Commission, 1995). 
Sacramento-San Joaquin Delta 85 

The Delta soils are composed of mineral sediments delivered by the 

rivers and of peat derived from decaying marsh vegetation. The peat 

began accumulating about 7,000 years ago and, prior to settlement, 

accumulated at a rate just sufficient to keep up with the average 

postglacial sea-level rise of about 0.08 inches per year (Atwater, 

1980). The total thickness of peat was as large as 60 feet in the extreme western areas. The mineral sediments are more abundant on 

the periphery of the Delta and near the natural waterways, whereas 

the peat soils are thickest in former backwaters away from the natural 

channels-that is, towards the centers of many of the current islands. 

The waterways of the entire Delta are subject to tidal action-tidal 

surges from San Francisco Bay are observed 5 hours later along the 

Cosumnes River in the eastern Delta. The position of the interface 

Delta waterways pass through fertile farmland. between the saline waters of the Bay and the freshwaters of the Delta 

(Cal1fornia Department ofWater Resources) depends UpOn the tidal cycle and the flOW Of freshwater through the 

Delta. Before major dams were built on rivers in the Delta watershed, the salinity interface migrated as far upstream as Courtland 

along the Sacramento River (California Department of Water Resources, 1993). Today, releases of freshwater from dams far upstream 

help reduce landward migration of the salinity interface during the 

summer months. A complicated formula agreed upon by all relevant 

parties attempts to maintain the two parts per thousand salinity 

interface near Chipps Island at the western edge of the Delta. 

RECLAMATION FOR AGRICULTURE LED TO SUBSIDENCE 

Sustained, large-scale agricultural development in the Delta first 

required levee-building to prevent frequent flooding. The leveesurrounded marshland tracts then had to be drained, cleared of 

tules, and tilled. The labor force for the initial levee-building effort 

consisted mainly of Chinese immigrants who arrived in large numbers upon completion of the Transcontinental Railroad in 1869. 

Between 1860 and 1880, workers using hand tools reclaimed about 

140 square miles of Delta land for agriculture. The Chinese labor 

force was paid about a dollar per day, or at a piecework rate of 13 

cents per cubic yard of material moved. After about 1880 the clamshell dredge, still in use today, became the dominant reclamation tool. 

Chinese laborers built many of 

the early levees in the Delta. 

(Overland Monthly. 1896) 

A clamshell dredge operates near Sherman Island, 

ca. 1907. 

(National Maritime Museum, 

San Francisco) 
86 

"Watch that first step! " 

The land surface has subsided 

beneath a Delta house, 1950. 

Drainage of Organic Soils 

Levees and drainage systems were largely complete by 1930, and the 

Delta had taken on its current appearance, with most of its 1,150-

square-mile area reclaimed for agricultural use (Thompson, 1957). 

Reclamation and agriculture have led to subsidence of the land surface on the developed islands in the central and western Delta at 

long-term average rates of 1 to 3 inches per year (Rojstaczer and 

others, 1991; Rojstaczer and Deverel, 1993). Islands that were originally near sea level are now well below sea level, and large areas of 

many islands are now more than 15 feet below sea level. The landsurface profile of many islands is somewhat saucer-shaped, because 

subsidence is greater in the thick peat soils near their interior than in 

the more mineral-rich soils near their perimeter. As subsidence 

progresses the levees themselves must be regularly maintained and 

periodically raised and strengthened to support the increasing 

stresses on the levees that result when the islands subside. Currently, 

they are maintained to a standard cross section at a height 1 foot 

above the estimated 100-year-flood elevation. 

Water levels in the depressed islands are maintained 3 to 6 feet below the land surface by an extensive network of drainage ditches, 

and the accumulated agricultural drainage is pumped through or 

overthe levees into stream channels. Without this drainage the islands would become waterlogged. 

0 5 Miles 

0 

~ 

5 Kilometers 

• Antioch 

Contra Costa Canal 

Land subsidence 

(feet below sea level) 

OotoiO 

D IOta IS 

D Greater than I 5 

Clifton Court Forebay--------

Harvey 0. Banks 

Delta Pumping Plant----. 

South Bay 

Aqueduct----. 

----- Pumping stations 

/California Aqueduct (State operated) 

~Delta-Mendota Canal 

(Federal operated) (California Department of 

Water Resources) 
Sacramento-San Joaquin Delta 87 

PREDEVELOPMENT 

POSTDEVELOPMENT 

Natural levees were formed by 

sediments deposited during spring 

fioods and stabilized 

by vegetation. 

Main channel 

"Tule" (bulrush 

and reed species) 

\ 

Peat soils were formed 

from decaying vegetation 

over thousands of years. Water table 

Semicontinuous pumps 

remove agricultural 

Levees must be periodically 

reinforced to support increasing 

stresses from stream channels. 

Pumps, such as these on 

Tw itchell Island, remove agricultural drainage while maintaining 

the water table at a level low 

enough to sustain agriculture. 

Saucer-shaped profile refiects 

greatest thickness and subsidence of peat soils near the 

center of islands. 

I 

Decomposing peat soils are the main cause of subsidence 

The dominant cause of land subsidence in the Delta is decomposition of organic carbon in the peat soils. Under natural waterlogged 

conditions, the soil was anaerobic (oxygen-poor), and organic carbon accumulated faster than it could decompose. Drainage for agriculture led to aerobic (oxygen-rich) conditions. Under aerobic 

conditions microbial activity oxidizes the carbon in the peat soil 

quite rapidly. Most of the carbon loss from the soil occurs as a flux 

of carbon-dioxide gas to the atmosphere. 
88 

These transmission towers on 

Sherman Island show evidence 

of subsidence 

In waterlogged conditons, 

decaying tules decompose 

slowly to release carbon 

dioxide (C02) and 

methane (CH4). 

AEROBIC CONDITIONS: Oxygen rich 

Exposure to air accelerates 

the decay of tules and peat 

soil. Organic material is 

converted mainly to C02 

and water. 

Scientists resolve subsidence mechanisms 

Drainage of Organic Soils 

There has been some debate as to the causes and mechanisms of 

subsidence in the Delta. Possible causes include deep-seated compaction related to the removal of subsurface fluids (oil, gas, and 

water) and the near-surface oxidation and mass wasting of organic 

soils. This debate seems to have been resolved in favor of the carbon oxidation/gas flux hypothesis. Extensometer measurements 

have shown that deep-seated subsidence due to natural-gas production and ground-water withdrawal is minimal. Further, pockets 

of unreclaimed marshland on channel islands remain at sea level. 

Age-dating of sediment cores from these islands indicates low sedimentation rates and, by inference, minimal subsidence in unreclaimed areas (Rojstaczer and others, 1991). These studies made it 

clear that Delta subsidence is a near-surface process, but did not 

establish how the carbon loss takes place. Further studies by the 

USGS, in cooperation with the California Department of Water 

Resources, resolved this issue by simultaneously measuring subsidence and carbon fluxes at several sites (Deverel and Rojstaczer, 

1996); The increased gaseous flux of carbon dioxide was sufficient 

to explain most of the carbon loss and measured subsidence, 

whereas the dissolved organic carbon (DOC) pumped from the 

islands in agricultural drainage could account for only about 1 

percent of the carbon loss. 

The USGS experiments also showed that rates of carbon -dioxide 

production increase with increasing temperature and decrease 

with increasing soil moisture. These results are consistent with field 

and laboratory measurements from the Florida Everglades, where 

subsidence is occurring by the same mechanism, albeit at a smaller 

rate of about 1 inch per year. 

The rate of subsidence has decreased 

The best evidence for long-term rates of subsidence comes from 

two sources-measurements of the exposure of transmission-line 
Sacramento-San Joaquin Delta 

How to slow or reverse subsidence 

Scientists look for answers with controlled experiments 

Investigations on various islands 

in the Sacramento-San Joaquin 

Delta have shown that microbial · 

decomposition of organic-rich 

soils is causing the land to"vaporize" and disappear. Ongoing 

experiments at two sites on 

Twitchell Island in the western 

Delta focus on assessing the factors that affect the 

rate and timing of 

carbon-dioxide production. 

At the other site (not shown), which will be 

permanently flooded, the effects of vegetative 

cover on the potential for biomass accumulation will be assessed. 

Tules will be planted on subsets of this site 

and will spread throughout the site. They will 

decompose relatively slowly under flooded 

conditions. It is anticipated that plant-litter 

accumulations will become peat-like material 

over time and eventually increase landsurface elevations measured relative to stable 

markers set in mineral soil beneath the peat. 

At one of the Twitchell Island sites, 

the land surface is subjected to a variety of flooding scenarios in order 

to assess anaerobic and aerobic 

decomposition processes. 

FUTURE STRATEGIES 

Possible long-term management strategies 

for various Delta islands include: 

1 . Shallow flooding to slow peat oxidation and reverse subsidence through biomass accumulation. 

2 . Shallow flooding combined with thin-layer mineral deposition (a possibly beneficial reuse of dredge material). 

3 . Continued agricultural use of areas with shallow peat 

and/or low organic-matter content, under the assumption that the maximum additional subsidence will not 

destabilize the levees. 

4. Blending mineral soil with peat soil to decrease the rate of 

carbon dioxide (C02) release and allow continued agricultural use. 

5 . Addition of thick layers of mineral soil, possibly using 

controlled levee breaches or deposition of dredge material, 

to slow peat oxidation and raise land-surface elevation. 

6 . Deep flooding to create freshwater reservoirs. 

These strategies'may be implemented in a mosaic throughout the Delta that creates a substantial diversity of wildlife 

habitat-uplands, open water, shallow permanent wetlands, 

and seasonal wetlands. 

89 

foundations on Sherman and Jersey Islands in the western Delta 

and repeated leveling surveys on Mildred and Bacon Islands and 

Lower Jones Tract in the southern Delta (Weir, 1950; Rojstaczer 

and others, 1991). The transmission lines in the western Delta were 

installed in 1910 and 1952. They are founded on pylons driven 

down to a solid substrate, so that comparison of the original foundation exposure with the current exposure allows estimates of soil 

loss. The southern Delta transect was surveyed 21 times between 

1922 and 1981; in 1983 further surveys were precluded when 

Mildred Island flooded. Both data sets indicate long-term average 
90 Drainage of Organic Soils 

subsidence rates of 1 to 3 inches per year, but also suggest a decline 

in the rate of subsidence over time, probably due to a decreased 

proportion of readily decomposable organic carbon in the near 

surface (Rojstaczer and Deverel,1993). In fact, rates of elevation 

loss measured at three selected sites in 1990 to 1992 were less than 

0.4 inches per year, consistent with the inferred slowing of subsidence (Deverel and Rojstaczer, 1996). However, all of these sites 

were near island edges, and likely underestimate the average island-wide elevation loss. 

MANY M ANAGEMENT ISSUES ARE RELATED TO SUBSIDENCE 

The management issues raised by land subsidence range in scale 

from those faced by individual farmers to the possible global-scale 

Living with possible levee failure 

Approximately 1,100 miles of levees need to be maintained 

Levee failure has been common in the Sacramento-San 

Joaquin Delta since reclamation began in the 1850s. Each 

of the islands and tracts in the Delta has flooded at least 

once, with several flooding repeatedly. About 100 levee 

failures have occurred since the early 1890s. Initially, most 

of the failures were caused by overtopping during periods 

of spring flooding. Although construction of upstream 

reservoirs since the 1940s has reduced the threat of overtopping, it has not reduced the incidence oflevee failure. 

Tyler Island 

levee was 

breached in 

a 1986 flood. 

(Californ ia Department of Water Resources) 

EARTHQUAKES 

Dredge material is 

used to reinforce 

levees. 

The Delta sits atop a blind fault system on the western 

edge of the Central Valley. Moderate earthquakes in 1892 

near Vacaville and in 1983 near Coalinga demonstrate 

the seismic potential of this structural belt. 

The increasing height of the levee system has prompted 

growing concern about the seismic stability of the levees. 

The concern is based on the proximity of faulting, the 

nature of the levee foundations, and the materials used 

to build the levees. Many levees consist of uncompacted 

weak local soils that may be unstable under seismic loading. 

The presence of sand and silt in the levees and their foundations indicates that liquefaction is also a possibility. 

Although no historic examples of seismically induced levee 

failure are known in the Delta, the modern levee network 

has not been subjected to strong shaking. Levees were 

either smaller or nonexistent in 1906 when the region was 

strongly shaken by the great San Francisco earthquake. 
Sacramento-San Joaquin Delta 

Areas with peat thickness 

over 10 feet have a great 

potential for continued 

subsidence. 

~'i-er 

Grand I. 

Peat thickness greater 

than I 0 feet as of 1978 

91 

Rio Vista • Brannan I. 

The fertile soils of the Delta 

are vulnerable to flooding. 

Sherman 

Island so.<:-

jersey I. 

• Antioch 

0 5 Miles 

,-------, 

0 5 Kilometers 

(California Department ofWater Resources) 

• 

(Peat-thickness estimates are from the 

California Department ofWater Resources, written communication, 1998) 

issue posed by the carbon-dioxide flux, with its possible link to 

climate change. At the most local level, individual farmers or reclamation districts must maintain drainage networks on the islands 

and pump the agricultural drainage back into waterways. These 

costs increase gradually as subsidence progresses. 

As subsidence continues, levees must be enlarged 

The costs of levee construction and maintenance are borne by the 

State of California and the Federal government, as well as by local 

reclamation districts. These costs also increase as subsidence 

progresses, forcing levees to be built higher and stronger. In 1981 to 

1986 the total amount spent on emergency levee repairs related to 

flooding was about $97 million, and in 1981 to 1991 the amount 

spent on routine levee maintenance was about $63 million (California Department ofWater Resources, 1993). Thus the annual cost 

of repair and maintenance of Delta levees in the 1980s averaged 

about $20 million per year. 

Subsidence could affect California's water system 

Much larger costs might be incurred if land subsidence indirectly 

affects the north-to-south water-transfer system, which is predicated on acceptable water quality in the southern Delta. The western Delta islands, in particular, are believed to effectively inhibit the 

inland migration of the salinity interface between Bay and Delta. If 

these are flooded, the water available to the massive pumping facilities near the Clifton Court Fore bay might become too saline to use. 
92 

Sacramento-San Joaquin Delta 

The heart of California's water systems 

An artificial balance is maintained in the water exchanged 

between the Delta and the San Francisco Bay. Freshwater 

inflows regulated by upstream dams and diversions supply 

water to the Delta ecosystems and to farms and cities in 

central and southern California. Subsidence of Delta islands 

threatens the stability of island levees and the quality of Delta water. Delta levee failures would tip the water-exchange 

balance in favor of more saltwater intrusion, which can ruin the water for agriculture and domestic uses. Several 

An amount equivalent to about 25 percent 

of the Delta's outflow is pumped into 

California's massive water system. 

Some of the rest is used locally, 

but most flows into the 

San Francisco Bay. 

Drainage of Organic Soils 

aqueducts would be affected. Any reductions in the supply 

of imported Delta water could force water purveyors in 

many parts of the State to meet water demand with groundwater supplies. And this, in turn, could renew land subsidence in Santa Clara and San Joaquin Valleys and exacerbate subsidence in the Antelope Valley and other areas 

currently reliant on imported Delta water supplies and 

prone to aquifer-system compaction. 

Annual inflow 

The Delta receives runoff from about 

40 percent of the land area of California 

and about 50 percent of California's total 

streamflow. 

Sacramento River 

(21 .2 maf**) 

East side streams (1.4 maf) 

California 

South Bay Aqueducts and 1 \( '\\ \ 

(2.5 maf) 

Salinity 

Salinity intrusions are linked to the 

interactions of tides, water 

management programs, and 

climatic variability. When 

freshwater flows decrease, 

higher salinity water 

can move into 

the Delta. 

• Flow data from 1980-1991 

(California Department of 

Water Resources, 1993) 

• • mat: millions of acre feet 

Dry Dry 

year year 

Wet year , ~ Wet year 

BEFORE UPSTREAM 

DAMS BUILT --

Maximum sal inity 

instrusions, 

1921 to 1943 

Maximum salinity 

instrusions, 

1944 to 1990 

Upstream control structures, such as 

Folsom, Shasta, and Oroville Dams, 

have reduced the extent of salinity 

intrusions by providing freshwater 

releases during the summer and fall. 
Sacramento-San Joaquin Delta 

The Harvey 0. Banks pumping 

plant at the southern edge of 

the Delta lifts water (lower 

right) into the California aqueduct (center left). The white 

towers on the upper left are 

wind turbines that generate 

electricity. 

93 

(California Department ofWater Resources) 

The timing of levee breaks and flooding is critical in this regard. 

Fortunately, most flooding occurs in winter and spring, when major saltwater intrusion is less likely. However, there are occasional 

levee failures under low-flow conditions. These can cause major 

short-term water-quality problems, even if the flooded areas are 

later reclaimed. During one island flooding under low-flow conditions, chloride levels reached 440 parts per million (ppm) at the 

Contra Costa Canal intake, well above the California standard for 

drinking water of 250 ppm (California Department of Water Resources, 1995). 

The statewide water-transfer system in California is so interdependent that decreased water quality in the Delta might lead to accelerated subsidence in areas discussed elsewhere in this Circular. Both 

the Santa Clara and San Joaquin Valleys rely, in part, on imported 

water from the Delta to augment local supplies and thereby reduce 

local ground-water pumpage and arrest or slow subsidence. Degradation of the Delta source water could well lead to increased 

ground-water use, and renewed subsidence, in these and other 

areas in California. 

Peat soil agriculture plays a minor role in climate change 

The fact that most subsidence in the Delta, and in other drained 

wetlands, is caused by carbon oxidation suggests that such subsidence might affect atmospheric carbon-dioxide levels. The worldwide annual production of atmospheric carbon due to agricultural 

drainage of organic soils has been estimated to be as much as 6 

percent of that produced by fossil fuel combustion (Tans and others, 1990). However, current rates of carbon-dioxide production in 

the Delta are likely to be significantly less than those caused by the 

initial agricultural expansion into virgin areas (Rojstaczer and 

Deverel, 1993). The gradual slowing of subsidence is associated 

with a declining rate of carbon-dioxide production. 
94 

This view of the Delta was 

taken looking westward 

with Mount Diablo on the 

horizon. 

Drainage of Organic Soils 

THE FUTURE OF THE DELTA POSES MANY CHALLENGES 

In cases where subsidence is due to aquifer-system compaction, it 

can often be slowed or arrested by careful water-use management. In 

cases where subsidence is due to peat oxidation, such as the Delta, it 

can be controlled only by major changes in land-use practice. In 

standard agricultural practice, the ultimate limiting factor is simply 

the total peat thickness; that is, the availability of organic carbon in 

the soil. In the Florida Everglades, the original peat thickness was 

less than 12 feet, and most of the potential subsidence has already 

been realized. In much of the cultivated area of the Delta, however, 

substantial thicknesses of peat remain, so that there is great potential 

for further subsidence. 

Like the Everglades, the Delta is currently the subject of a major 

Federal-State restoration effort that includes attempts to improve 

wildlife habitat. These attempts have focused on the periphery of the 

Delta, avoiding the central areas with significant amounts of subsidence. As in the Everglades, much of the extensively subsided area is 

impractical to restore and will continue to be intensively managed. 

As subsidence progresses, the levee system will become increasingly 

vulnerable to catastrophic failure during floods and earthquakes. 

The interrelated issues of Delta land subsidence, water quality, and 

wildlife habitat will continue to pose a major dilemma for California 

water managers. 

(California Department ofWater Resources) 
FLORIDA EVERGLADES 

Subsidence threatens agriculture and 

complicates ecosystem restoration 

Orlando • 

Lake 

Okeecho7 

•RM"'' ~ ':, 

. Historic ~ · freshwater -"-

everglades 

0 30 Miles 

r-----1 

0 30 Kilometers 

S.E. lngebritsen 

U.S. Geological Survey, Menlo Park, California 

Christopher McVoy 

South Florida Water Management District, 

West Palm Beach Florida 

B. Glaz 

U.S. Department of Agriculture, Agricultural 

Research Service, Canal Point, Florida 

Winifred Park 

South Florida Water Management District. 

West Palm Beach, Florida 

T he Everglades ecosystem includes Lake Okeechobee and its 

tributary areas, as well as the roughly 40- to 50-mile-wide, 

130-mile-long wetland mosaic that once extended continuously from Lake Okeechobee to the southern tip of the Florida peninsula at Florida Bay. 

Since 1900 much of the Everglades has been drained for agriculture 

and urban development, so that today only 50 percent of the original wetlands remain. Water levels and patterns of water flow are 

largely controlled by an extensive system oflevees and canals. The 

control system was constructed to achieve multiple objectives of 

flood control, land drainage, and water supply. More recently, watermanagement policies have also begun to address issues related to 

ecosystem restoration. Extensive land subsidence that has been 

caused by drainage and oxidation of peat soils will greatly complicate ecosystem restoration and also threatens the future of agriculture in the Everglades. 
96 

The Everglades were formed in 

a limestone basin, which accumulated layers of peat and 

mud bathed by freshwater 

flows from Lake Okeechobee. 

"The outline of this Florida end-of-land, within the 

Gulf of Mexico, the shallows of the Bay of Florida 

and the Gulf Stream, is like a long pointed spoon. 

That is the visible shape of the rock that holds up 

out of the surrounding sea water the long channel 

of the Everglades and their borders. The rock 

holds all the fresh water and the grass and all 

those other shapes and forms of air-loving life 

only a little way out of the salt water, as a full 

spoon lowered into a full cup holds two liquids 

separate, within that thread of rim. " 

-Marjorie Stoneman Douglas, 1947 

Everglades National Park 

was created in 1947. 

NATURAL FLOW PATIERNS (c.l900) 

Lake 

Okeechobee \ \ 0 sahatchee 1,<· ca 0 -

This central anea was 

underlain by peat and 

nearly always fiooded ~ 

Lake Okeechobee and 

local rainfall once drove ~ slow-moving sheet 

fiow through the 

Everglades. 

HISTORIC 

FRESHWATER 

EVERGLADES 

Occasional ;:: 

fiooding sent "' 

Outer areas underlain water into and -.: 

by mud, sand, and some through the 0 

peat fiooded se\lly . .-----co, astal ridge. u 

Drainage of Organic Soils 

0 20 Mi 

Current boundary of 

Everglades National Park 

The Everglades ecosystem has, in fact, been badly degraded, despite 

the establishment of Everglades National Park in the southern Everglades in 1947. Prominent symptoms of the ecosystem decline include an 80 percent reduction in wading bird populations since the 

1930s (Ogden, 1994), the near-extinction of the Florida panther 

(Smith and Bass, 1994), invasions of exotic species (Bodle and others, 1994), and declining water quality in Florida Bay, which likely is 

due, at least in part, to decreased freshwater inflow (Mcivor and 

others, 1994). 

HISTORIC FLOWS WERE SEVERED 

A thin rim of bedrock protects south Florida from the ocean. The 

limestone bedrock ridge that separates the Everglades from the 

Atlantic coast extends 20 feet or less above sea level. Under natural 

conditions all of southeast Florida, except for a 5- to 15-mile-wide 

strip along this bedrock ridge, was subject to annual floods. Much 

of the area was perennially inundated with freshwater. Water levels 

in Lake Okeechobee and local rainfall drove slow-moving sheet 

flow through the Everglades under topographic and hydraulic gra-
Florida Everglades 

Hoover dike (center) was 

built with digging spoils obtained from a navigable 

channel (foreground). Lake 

Okeechobee is at the top of 

photo. 

A network of dikes and canals controls water movement, providing optimum 

irrigation and drainage for 

sugar cane (left). 

97 

clients of only about 2 inches per mile. Lake Okeechobee, which once overflowed its southern bank at 

water levels in the range of20 to 21 feet above sea 

level, today is artificially maintained at about 13 to 16 

feet above sea level by a dike system and canals to the 

Atlantic and Gulf coasts. 

Early agriculturalists began the drying process 

The first successful farming ventures in the Everglades began in about 1913, not on the sawgrass plain 

itself but on the slightly elevated natural levee south 

of Lake Okeechobee (Snyder and Davidson, 1994). Early efforts to 

clear, farm, and colonize the sawgrass area had little success, being 

plagued by flooding, winter freezes, and trace-nutrient deficiencies. 

(The soil beneath the sawgrass was later shown to be too low in copper to support most crops and livestock.) 

In the 1920s the State of Florida established an Everglades Experiment Station in Belle Glade, and the U.S. Department of Agriculture 

established a Sugarcane Field Station in Canal Point. The combined 

efforts of these units gradually solved the plant- and livestock-pathology problems experienced by early farmers. However, the land 

was still subject to frequent, sometimes catastrophic inundation. The 

great hurricane of 1928 caused at least 2,000 fatalities and flooded 

the Everglades Experiment Station for several months. 

The damage caused by the 1928 hurricane convinced the Federal 

government to fund construction of a permanent dike around the 

southern perimeter of Lake Okeechobee. This more secure protection from flooding cleared the way for intensive settlement of the 

Everglades. It also permanently severed the natural connection between the Everglades proper and its headwaters. For millennia, the 

Everglades had been fed by intermittent, diffuse overflow of the imperfect natural levee south of the Lake. Now, its primary water 

source, other than local rainfall, would be a system of artificial canals. 
98 

Water-control projects in the 

Everglades began in the early 

1900s. After the fires and 

floods of the 1940s, much 

larger water-management 

projects were implemented . 

Water conservation area 2A is 

mostly covered with sawgrass. 

Drainage of Organic Soils 

Further water-management efforts accelerated development 

A comprehensive Federal-State water-management effort in the 

1950s and 1960s was prompted by drought and widespread fires in 

1944 to 1945 and renewed flooding in 1947 to 1948. The primary 

motivation was flood control and water supply for the growing 

urban areas along the Atlantic coast. The drying of the Everglades 

had clearly contributed to rapid saltwater intrusion in these urbanizing areas during the drought. 

A regional flood-control district, the predecessor of today's South 

Florida Water Management District, was created by the State of 

Florida in 1949 to manage a coordinated water system. The urbanizing areas that extended west of the natural bedrock ridge were 

protected from flooding by a high levee known as the "eastern perimeter levee:' Although it was originally built to protect and promote development of urbanizing areas along the southeastern 

Historic 

freshwater 

everglades 

0 

~ 

I 

'---..._ 
Florida Everglades 

NATURAL FLOW PATIERNS (ca. 1900) 

[

i--' 

' Lake 

'.1 Okeechobee 

I 

Mean water 

depth 

0 to 3 feet 

0 30 Miles 

0 30 Kilometers 

Sizes of fl ow 

arrows are proportional to 

fl ow velocity 

CURRENT FLOW PATIERNS (ca. 1990) 

Lake 

Okeechobee 

(Modified from simulation models developed and maintained by the South 

Florida Water Management District) 

99 

Water management has brought 

significant changes to natural 

overland flow patterns. 

Under natural conditions surface 

water moved from Lake Okeechobee southward, then turned 

southwest through a constricted 

area called Shark Slough. 

After canals and dikes were constructed for the agricultural and 

water-conseNation areas, sheet 

flow practically disappeared 

from the northern Everglades 

and diminished to the south. 

coast, this levee has, ironically, become the only effective barrier to 

more extensive urban development of the Everglades (Light and 

Dineen, 1994). 

An area of thick peat soil south of Lake Okeechobee was designated the "Everglades agricultural area:' Farther south, other areas 

of peat soils less suitable for agriculture were designated as "waterconservation areas." These areas are maintained in an undeveloped 

state, but a system of dikes and canals allows water levels to bemanipulated to achieve management objectives that include flood 

control, water supply, and wildlife habitat. 

During dry periods, the level of Lake Okeechobee drops as water is 

released to provide water to the agricultural area, to canals that 

maintain ground-water levels in urban areas along the Atlantic 

coast, and to Everglades National Park. At other times, drainage 

water pumped from the agricultural area is released into the waterconservation areas, providing needed water but also undesirable 

amounts of the nutrient phosphorus. In recent years, "best management practices" have helped reduce phosphorus loads from the 

agricultural area. The managed part of the remaining Evergladesapproximately the northern two-thirds-now consists of a series of 

linked, impounded systems that are managed individually. 
100 

HISTORIC EVERGLADES VEGETATION (ca.l900) 

0 30 Kilometers 

(Vegetation from Davis and Odge n. 1994) 

Drainage of Organic Soils 

Water management has also 

changed vegetation patterns. 

The construction of canals and 

levees and subsequent draining 

and development of the land 

has all but eliminated natural 

vegetation in the agricultural 

area and the region east of the 

eastern perimeter levee. 

Land subsidence followed in the wake of development 

With the addition of trace nutrients, the peat soil or "muck" beneath 

the sawgrass proved extremely productive. But the farmers also saw 

" ... the cushiony layer of dark muck shrink and oxidize under the 

burning sun as if it was consumed in thin, airy flames. As the canals 

and ditches were extended by the local drainage boards, and the 

peaty muck was dried out and cultivated, it shrank ... It is still 

shrinking. Every canal and ditch that drained it made a long deepening valley in the surrounding area. On the east and south the 

subsidence was so great that half that land [drains towards] the 

lowered lake." -Marjorie Stoneman Douglas, 1947 

In today's Everglades agricultural area, evidence of substantial land 

subsidence can readily be discerned from the relative elevations of 

the land surface, the drainage-canal system, and the lake, and from 

the elevation of older buildings that were built on piles extending to 

bedrock. Precise measurements are relatively rare, except at particular points or along a few infrequently revisited transects. However, 

the long-term average rate of subsidence is generally considered to 

have been between 1 and 1.2 inches per year (Stephens and 

Johnson, 1951; Shih and others, 1979; Stephens and others, 1984). 
Florida Everglades 

Line of approximately equal land 

subsidence (feet) 

relative to predevelopment 

conditions 

Historic 

freshwater 

everglades 

30 Kilometers 

(Subsidence data compiled 

by South Florida Water 

Management District) 

Subsidence is greater in areas 

that were intentionally drained 

for urban and agricultural uses. 

101 

In uncultivated areas of the Everglades, subsidence is less obvious 

but probably widespread. Subsidence is not caused by cultivation, 

but occurs wherever drainage desaturates peat soil. Early engineering efforts focused on drainage alone, and, as a result, much of the 

area became excessively drained during drought years. The "river 

of grass" often became a string of drying pools, and great fires 

swept the Everglades. The drying triggered subsidence, which was 

then exacerbated by widespread fires. The persistent peat fires 

sometimes continued smoldering for months before being extinguished by the next rainy season. 

Conventional surveying has always been extremely difficult in the 

Everglades. Stable bedrock bench marks are nonexistent or very 

distant, the surficial material is soft and yielding, and access is difficult. Current best estimates suggest that there have been 3 to 9 feet 

of subsidence in the current Everglades agricultural area and that 

an equally large uncultivated area has experienced up to 3 feet of 

subsidence. Such elevation changes are tremendously significant to 

a near-sea-level wetlands system in which flow is driven by less 

than 20 feet of total relief. 

The current management infrastructure and policies have abated 

land subsidence in undrained areas of the historic Everglades to 

some extent, although comparison of recent soil-depth measureThis building at the Everglades 

Experiment Station was originally constructed at the land 

surface; latticework and stairs 

were added after substantial 

land subsidence. 
102 

A sugar mill outside Belle Glade 

is surrounded by sugar cane 

fields. Note the dark peat soils 

in the lower photograph. 

Drainage of Organic Soils 

ments by the U.S. Environmental Protection Agency (Scheidt, US 

EPA, written communication 1997) with 1940s estimates of peat 

thickness (Davis, 1946; Jones and others, 1948) suggest that there 

has been widespread subsidence in the water-conservation areas 

over the past 50 years. The northern parts of individual water-conservation areas may still experience some minor subsidence. The 

southern or downstream parts of the impoundments are generally 

wetter and may be accumulating peat (Craft and Richardson, 

1993a, 1993b ), very gradually increasing in elevation. In the 

drained agricultural and urban areas, subsidence is an ongoing 

process, except where the peat has already disappeared entirely. 

SUBSIDENCE CLOUDS THE FUTURE OF AGRICULTURE 

The Everglades agricultural area is now mainly devoted to sugarcane, with considerably smaller areas used for vegetables, sodgrass, 

and rice. The value of all agricultural crops is currently about $750 

million (Snyder and Davidson, 1994). 

The eventual demise of agriculture in the Everglades has been predicted for some time (Douglas, 1947; Stephens and Johnson, 1951). 

The agriculture depends upon a relatively thin, continually shrinking layer of peat soil that directly overlies limestone bedrock. 

Agronomists have known for many decades that peat-rich soils 

(histosols), which form in undrained or poorly drained areas, will 

subside when drained and cultivated. The causes include mechanical compaction, burning, shrinkage due to dehydration, and most 

importantly, oxidation of organic matter. Oxidation is a microbially 

mediated process that converts organic carbon in the soil to 

(mainly) carbon dioxide gas and water. 
Florida Everglades 

Two cross sections through the 

agricultural area show the drop 

in land-surface elevation. 

Lake 

Okeechobee 

Agricultural area I Levees 

~ I ~">',., A ' .,~ 

<% "('-5 

\ 

1, 'h A ~ o '0 

B~ 

'5 0 B' 

I 13.c t'J., 

Canals 

0 IOMi L___j 

,--., 

0 10 Km 

Altitude of 

land surface 

A 

20 

(feet above 10 

sea level) 

0 

103 

Through photosynthesis, vegetation converts carbon dioxide and 

water into carbohydrates. Under natural conditions, aerobic microorganisms converted dead plant material (mostly sawgrass root) to 

peat during brief periods of moderate drainage. Vegetative debris 

was deposited faster than it could fully decompose, causing a 

gradual increase in peat thickness. In what is now the Everglades 

agricultural area, a delicate balance of 9 to 12 months flood and 0 to 

3 months slight (0 to 12 inches) drainage for about 5,000 years, with 

sawgrass the dominant species, led to a peat accretion rate of about 

0.03 inches per year. Drainage disrupted this balance so that, instead of accretion, there has been subsidence at a rate of about 1 

inch per year. 

Peat soils may virtually disappear 

Rates of subsidence in the Everglades are slower than those in the 

Sacramento-San Joaquin Delta of California, the other major area 

of peat-oxidation subsidence in the United States; in the Delta, average subsidence rates have been up to 3 inches per year. However, the 

pre agricultural peat thickness was much greater in the Delta (up to 

60 feet) than in the Everglades, where initial thicknesses were less 

than 12 feet. The subsidence rates observed in the Everglades are 

similar to those observed in the deep peat soils of the English fens 

during the past 100 years (Lucas, 1982; Stephens and others, 1984). 

In the Everglades agricultural area, the initial peat thickness tapered 

southward from approximately 12 feet near Lake Okeechobee to 

about 5 feet near the southern boundary. In 1951, Stephens and 

Canals (built between 1912 and 1917) 

\ Land surface 19 12 

• • 

A' 

0 10 20 30 40 

Distance (miles) 

B' 

Altitude of 

land surface 

(feet above 1 0 

sea level) 

0 *estimated 

20 30 

Distance (miles) 

(Stephens and johnson, 19S I) 
104 

A ditch excavation east of Belle 

Glade shows peat soil overlying 

limestone bedrock. 

Canals and a levee separate 

constructed wetland from the 

agricultural area to the right. 

Drainage of Organic Soils 

Johnson extrapolated contemporary subsidence trends to predict 

that by the year 2000 the peat soil would be less than 1 foot thick in 

about half of the area. They further inferred that much of the area 

will by then have gone out of agricultural production, assuming 

that cultivation would not be possible with less than 1 foot of soil 

over limestone bedrock. 

Although the extrapolation of peat thickness done by Stephens and 

Johnson (1951) appears consistent with measurements made in 

1969 (Johnson, 1974), 1978 (Shih and others, 1979), and 1988 

(Smith, 1990), little land has yet been retired from sugarcane. One 

reason is that farmers have managed to successfully produce cane 

from only 6 inches of peat, by first piling it in windrows to allow 

successful germination. It also appears possible that the rate of 

subsidence has slowed somewhat (Shih and others, 1997), due to 

the combined effects of an increasing nonorganic (mineral) content in the remaining soil, a thinner unsaturated zone dictated by 

the decreasing soil depth and, perhaps, an increasing abundance of 

more recalcitrant forms of organic carbon. 

The soil-depth predictions of Stephens and Johnson (1951) may 

prove to have been somewhat pessimistic, but it is clear that agriculture as currently practiced in the Everglades has a finite life expectancy, likely on the order of decades. Extending that life 

expectancy would require development of an agriculture based on 

water-tolerant crops that accumulate rather than lose peat (Porter 

and others, 1991; Glaz, 1995). 

SUBSIDENCE COMPLICATES ECOSYSTEM RESTORATION 

In a wetland area where natural hydraulic gradients were on the order 

of inches per mile, and one half-foot land-surface altitude differences are ecologically significant, the fact of several feet of land 

subsidence substantially complicates ecosystem-restoration efforts. 

Subsidence makes true restoration of the Everglades agricultural 

area itself technically impossible, even in the event that it were po-
Florida Everglades 

A tree island in the Everglades 

105 

litically and economically feasible. Land there that once had a mean 

elevation less than 20 feet above sea level has been reduced in elevation by an average of about 5 feet. Differential subsidence has significantly altered the slope of the land, precluding restoration of the 

natural, shallow sheet-flow patterns. If artificial water management 

and conveyance were now to cease, nature would likely reclaim the 

land as a lake, rather than the predevelopment sawgrass plains. With 

removal of the "sponge" of peat and native vegetation, the agricultural area has also lost most of its ability to naturally filter, dampen, 

and retard storm flows. Other strong impediments to restoration of 

the Everglades agricultural area include loss of the native seed bank, 

accumulations of agricultural chemicals in the soil, and the potential for invasion by aggressive exotic species. 

Subsidence will also complicate efforts to manage the water-conservation areas to the east and south in a more natural condition. For 

example, the wetlands in these areas are speckled with tree islands, 

which are an important ecosystem component. Though definitive 

data are lacking, these tree islands likely have subsided, possibly 

more than the surrounding area. Thus, restoration of the waterconservation areas will require careful management of water levels 

in a depth range sufficient to promote appropriate wetland species 

without further damaging tree islands. 

CAREFUL WATER MANAGEMENT IS A KEY TO THE FUTURE 

Because of peat loss, agriculture as currently practiced in the Everglades will gradually diminish over the next decades. R. V. Allison, 

the first head of the Everglades Research Station, likened the peat 

soil to "the cake which we cannot eat and keep at the same time:' His 

confident prediction that 

"As the use of Everglades lands for agricultural purposes approaches 

the sunset of ... production, there is little doubt that transition into a 

wildlife area of world fame will follow, perhaps in an easy and natural manner:' -Allison, 1956 

now seems overly optimistic. This is still a possible scenario but, as 

we have noted, the result would be very different from the natural 

system, due to subsidence. There are also alternative possibilities, including urban development or invention of a sustainable agriculture. 

A sustainable agriculture in the Everglades would require at least 

zero subsidence and, optimally, some peat accretion. Glaz (1995) 

discussed a program of genetic, agronomic, and hydrologic research 

aimed at gradually (over a period of20 to 40 years) making a currently used sugarcane-rice rotation sustainable. Achievement of this 

goal may prove difficult. However, documented water tolerance of 

sugarcane (Gascho and Shih, 1979; Kang and others, 1986; Deren 

and others, 1991), a recently discovered explanation for this water 

tolerance (Ray and others, 1996), and rapid gains in molecular genetics combine to suggest that substantial reductions in subsidence 

might be attainable. 
106 Drainage of Organic Soils 

Even in the complete absence of agriculture in the Everglades, the 

existing pattern of urban development and land subsidence would 

prevent restoration of the natural flow system. Engineered water 

management and conveyance will be required indefinitely. Land 

subsidence over a large area south of Lake Okeechobee has created 

a significant trough within the natural north-south flow system, 

thereby preventing restoration of natural sheet-flow and vegetation 

patterns. 

The Everglades are currently the subject of a major Federal-State 

ecosystem restoration effort. "Restoration" is perhaps a misnomer, 

as the focus of this effort is on more natural management of the 

remaining 50 percent of the Everglades wetlands, not on regaining 

the 50 percent that has been converted to urban and agricultural 

use. Even improving the natural functioning of the remaining wetlands will be a complex problem, due to the lost spatial extent, the 

hydrologic separation from Lake Okeechobee, and land subsidence. 

The Everglades will likely continue to be an intensively managed 

system. However, much as the major engineering effort in the 1950s 

and 1960s halted the destructive fires and saltwater intrusion of 

preceding decades, the current restoration effort has the potential 

to halt and reverse more recent environmental degradation. A rnajor challenge will be to deliver water from Lake Okeechobee 

through the extensive subsided areas so that it arrives in the undeveloped southern Everglades at similar times, in similar quantities, 

and with similar quality, as it did prior to drainage and subsidence. 
PART III 

Collapsing Cavities 

This sinkhole in Kansas was formed 

by collapsed evaporite rocks. 

The Retsof Salt M ine Collapse 

Sinkholes, West-Central Florida 

S udden and unexpected collapse of the land surface into 

subsurface cavities is arguably the most hazardous type of 

subsidence. Such catastrophic subsidence is most commonly 

triggered by ground-water-level declines caused by pumping, or by 

diversion of surface runoff or ground-water flow through susceptible rocks. Though the collapse features tend to be highly localized, 

they can introduce contaminants to the aquifer system and, thereby, 

have lasting regional impacts. Collapse features tend to be associated 

with specific rock types having hydrogeologic properties that render 

them susceptible to the formation of cavities. Human activities can 

facilitate the formation of subsurface cavities in these susceptible 

materials and trigger their collapse, as well as the collapse of preexisting subsurface cavities. 

In terms ofland area affected, underground mining accounts for 

about 20 percent of the total land subsidence in the United States, 

and most of this fraction is associated with underground mining for 

coal. Subsidence over underground coal workings develops as a 

gradual downwarping of the overburden into mine voids and is generally unrelated to subsurface water conditions. Underground salt 

and gypsum mines are also subject to downwarping of the overburden, but these evaporite minerals are also susceptible to rapid and 

extensive dissolution by water. Salt and gypsum are, respectively, almost 7,500 and 150 times more soluble than limestone, the rocktype often associated with catastrophic sinkhole formation and the 

distinctively weathered landscapes collectively known as karst. Here, 

we consider only the collapse of cavities that form in soluble rocks 

such as salt, gypsum, and limestone. 

Formation of subsurface cavities by dissolution requires: 1) bedrock 

composed in large part of soluble minerals; 2) a water source that is 

unsaturated with respect to these minerals and, therefore, can dissolve them; 3) an energy source in the form of a hydraulic gradient 

to move the water through the rock; and 4) an outlet for the escaping, mineralized water. Once a through-flowing passage develops in 

the soluble rock, erosion and further dissolution enlarges the pas-
108 

In western Kansas dissolution 

of gypsum and salt beds several hundred feet below the 

surface caused the sudden 

formation of the Meade Sink 

in March 1879. The hole was 

about 60 feet deep and 610 

feet in diameter and filled 

with saltwater. Today the sink 

has partly filled with sediment and is usually dry. 

Salt and gypsum underlie about 

40 percent of the contiguous 

United States. 

Collapsing Cavities 

(Kansas Geological Survey) 

sage, further enhancing the throughflow. Once established, subsurface cavities may provide habitat for populations of species specially 

adapted to cave environments-a cave ecosystem. The interaction 

between these biological communities and the mineral substrate of 

the host cavities may further enhance mineral dissolution and cavity enlargement through the production of acid metabolites. 

EVAPORITE ROCKS CAN FORM CAVITIES WITHIN DAYS 

Evaporites are sediments deposited from natural waters that have 

been concentrated as a result of evaporation. Evaporite rocks such 

as salt and gypsum underlie about 35 to 40 percent of the contiguous United States. Natural solution-related subsidence has occurred 

in each of the major salt basins (Ege, 1984), perhaps most notably 

in the Permian basin of Texas, New Mexico, Oklahoma, and Kansas 

and the smaller Holbrook basin of northeast Arizona. Although 

evaporites underlie most of the Michigan-Appalachian and Gulf 

Coast basins, naturally forming collapse features are much less 

common in these areas. Human-induced collapse cavities are relatively uncommon in gypsum deposits, and more likely to develop 

above salt deposits, where they are associated with both purposeful 

and accidental dissolution of salt. 

(Martinez and others, 1998) 
Introduction 

Carbonate karst landscapes 

comprise about 40 percent of 

the contiguous United States 

east of Tulsa, Oklahoma. 

These karst towers in Puerto 

Rico are hills of limestone 

surrounded by nearly flat alluvial plains cultivated with 

pineapple. 

D Karst from evaporite rock 

c:::J Karst from carbonate rock 

109 

(Davies and Legrand. 1972) 

CARBONATE ROCKS FORM CAVITIES OVER CENTURIES 

Natural cavities in carbonates (limestone and dolomite) develop by 

the same processes that form cavities in evaporite rocks, albeit much 

more slowly. The high solubilities of salt and gypsum permit cavities 

to form in days to years, whereas cavity formation in carbonate bedrock is a very slow process that generally occurs over centuries to 

millennia. The slow dissolution of carbonate rocks favors the stability and persistence of the distinctively weathered landforms known 

as karst. 

Both dissolution and erosional processes play roles in the maturation of karst in carbonates; if not for a balance between mechanical 

erosion and dissolution, the distinctive landscapes could not persist. 

The high strength of carbonate rocks confers resistance to mechanical failure despite progressive weakening by chemical dissolution. 

The potential for dissolution is controlled by the amount of water 

available and also by the level of saturation of that water with respect to calcium carbonate. Where the potential for dissolution is 

low, mechanical erosion dominates the morphology of carbonates. 

For example, in the arid Southwest, limestone exposures tend to 

erode as cliffs rather than form karst. · 

Carbonate karst landscapes comprise about 40 percent of the contiguous United States east of the longitude of Tulsa, Oklahoma 

(White and others, 1995). In these more humid landscapes, surface 

and subsurface drainage pathways converge in discrete conduits 

formed in the carbonate bedrock. Sinkholes, swallows (where 

streams disappear into the subsurface), and springs are linked to 

form an interconnected surface and subsurface drainage network. 

Thus, karst aquifer systems are directly affected by variabilities in 

timing and magnitude of surface runoff. Surface runoff carries all 

the components of streamflow into the conduit flow system, including suspended sediment, dissolved contaminants, immiscible fluids, 

and micro- and macro biological agents. The slower infiltration of 

surface water through porous soil and rock to the water table, which 

helps to protect ground water from surficial contamination in most 

areas, is short -circuited in karst landscapes. 
110 

An accidental sinkhole: Freshwater from shallow aquifers 

flowed down an abandoned 

oil well and dissolved the salt 

in an underlying formation . A 

large brine-filled cavity formed 

and eventually the roof collapsed. Through successive 

collapses the cavity migrated 

upward until it formed a sinkhole. 

Oil well 

Sands 

Shales 

Salt 

Dolomite 

Abandoned 

well 

Freshwater 

\)Brinefilled 

cavity 

~ 

o/ 

Collapsing Cavities 

Sinkhole 

Brine 

HUMAN ACTIVITY EXACERBATES FORMATION AND 

OCCURRENCE OF SINKHOLES 

The intimate connection between surface hydrologic processes and 

karst aquifer systems underscores problems related to our land- and 

water-resources practices in karst landscapes. Human activities tend 

to accelerate the progress of karstification. Our practice of redirecting surface drainage away from engineered structures and developed lands refocuses higher intensity runoff onto other, generally 

undeveloped, land surfaces. In karst terrane this increases both mechanical and chemical erosion of susceptible carbonates and evaporites and often accelerates the formation of new sinkholes and the 

failure of preexisting sinkholes. Exploitation of ground water causes 

long-term lowering and seasonal and daily cycling of ground-water 

levels that may destabilize cavities. Lower water tables reduce fluidpressure support of cavities, sometimes causing drying and ravelling 

of loose, unconsolidated overburden deposits through preexisting 

sinkholes and sometimes causing their catastrophic collapse. 

Here we will consider two examples where humans have helped to 

create collapse features in soluble rocks-the Retsof Salt Mine in 

Genesee Valley, New York and the mantled karst of west -central 

Florida. In the Genesee Valley the catastrophic collapse and eventual 

flooding of an underground salt mine threatened the water resources and economic future of a rural New York community. In 

west-central Florida, where sinkholes naturally dot the landscape, 

new sinkholes related to land and water-resources development 

threaten public safety and one of the most productive aquifers in the 

world, the Floridan Aquifer System. 
THE RETSOF SALT MINE CoLLAPSE 

Widespread subsidence occurred after a mine 

collapse in the Genesee Valley, New York 

NEW YORK 

.J . 1 

I Rochester 

RETSOF 

~ I S~L .... M 1'-'fil 

0 10 Miles 

0 I 0 Kilometers 

Wil liam M. Kappel, Richard M. Yager, and 

Todd 5. Miller 

US. Geological Survey, Ithaca, New York 

0 n March 12, 1994, at 5:43a.m. (local time), an apparent 

earthquake of magnitude 3.6 centered near Cuylerville, 

New York, woke residents and registered on seismographs 

300 miles away. Prompted by a call placed from a local resident, the 

USGS National Earthquake Information Center confirmed that a 

seismic event had occurred near Cuylerville and immediately notified State emergency services offices in New York who, in turn, notified the Livingston County Sheriff's Department. The Sheriff's 

Department contacted the Retsof Mine, which, except for some limited subsurface maintenance activity, had suspended active mining 

that weekend. 

Mine officials discovered that a 500- by 500-foot section of shale 

roof rock some 1,200 feet below land surface had collapsed in a 

part of the mine known as room 2-Yard South. Mine officials detected methane and hydrogen sulfide gases, and ground water was 

flowing into the mine from the roof collapse area at nearly 5,000 

gallons per minute. 

This collapse began a series of events that would eventually lead to 

the further collapse and complete flooding of the mine, large declines in local ground-water levels, degradation of potable groundwater supplies, land subsidence, release of natural gases (methane 

and hydrogen sulfide) to the atmosphere, and other detrimental 

effects on the cultural resources and infrastructure in this part of the Genesee Valley. 

Seismogram recorded at 

Cuylerville, New York 

March 12, 1994 
112 

t 

N 

0 2 Miles 

~==----' 

0 2 Kilometers 

Land-surface attitude 

(feet above sea level) 

.-..-:§:J 

../ 

~\0~ 

"<:> '<i 

Fowlerville 

Mo r aine 

This cross-sectional schematic shows how water 

from the basal aquifer entered the mine through 

the collapsed area. After 

21 months the salt mine 

was completely flooded. 1200 

feet 

Collapsing Cavities 

SALT MINING HAS A LONG HISTORY IN THE GENESEE VALLEY 

Salt mining (both salt-solution and rock-salt mining) began in the 

Genesee Valley in the early 1880s, and in 1884 the Empire Salt Company excavated a shaft to extract rock salt from seams 900 feet below 

land surface. In 1885 the Empire Salt Company was renamed the 

Retsof Mine Company and the Village of Retsof was founded near 

the mine shaft. During the next 110 years, the mine grew to become 

the largest salt-producing mine in the United States and the second 

largest in the world. Before the initial collapse in March 1994, the 

mine encompassed an underground area of more than 6,000 acres, 

and the mine footprint (outer edge of mined area) extended over an 

area of nearly 10 square miles. 

At the time of the collapse, the Retsof Mine was owned by AkzoNobel Salt Incorporated (ANSI), and, during the winter of 1993-94, 

operated at full capacity to meet demands for road salt throughout 

the northeastern United States. Prior to its closure, the Retsof Mine 

played a major role in the Livingston County economy, providing 

more than 325 jobs with an annual payroll in excess of $11 million 

and estimated annual gross sales of more than $70 million 

(NYSDEC, 1997). During the 17 months following the collapse, 

mining operations shifted to the northern, high end of the mine in a 

race to salvage mineable salt before the mine flooded. The Retsof 

Mine ceased operations on September 2, 1995, and by December, 21 

months after the initial collapse, the mine was completely flooded. 

THE COLLAPSE TRIGGERED A SERIES OF LOCAL EVENTS 

Four months before the collapse, in November 1993, room 2-Yard 

South was abandoned because of concerns over large and increasing 

rates of "convergence" or reduction of the opening between the 

floor and ceiling of the room. (A new mining technique, "yielding 

pillar;' was used in this area in response to floor buckling and roof 

collapse, which was occurring with greater frequency in the southSouth North 

Collapse area Land surface 

Alluvium, lacustrine, glacial drift undifferentiated 

Basal aquifer - Retsof Mine Salt layer 

Support pillars I 

I -

1994 1995 
The Retsof Salt Mine Collapse 

This northwest aerial view 

shows sinkholes above room 

11-Yard West (left foreground) 

and room 2-Yard South (right 

center, partially obscured by 

trees). Circle indicates location 

of the Retsof main plant area, 

some 4 miles northwest of the 

sinkholes. 

A sinkhole developed above 

room 11-Yard West and filled 

with water from Beards Creek. 

The roadbed of Route 20A 

was fractured on the east side 

of the collapsed bridge over 

Beards Creek. This view is 

above the room 2-Yard South 

collapse area looking west, toward the former Hamilton 

farm house (subsequently purchased by ANSI). 

(Richard Young. Geological Sciences, 

SUNY Geneseo, April 12. 1994) 

113 

(Ron Pretzec LUXE, May 1994) 

ern end of the mine.) After the March 12, 1994, collapse of room 2-

Yard South, ground water flowed into the previously dry mine at a 

rate of about 7 million gallons per day, dissolving residual rock salt 

and filling the lowest, downdip levels of the mine with saturated 

brine. ANSI monitored the concentration of hazardous gases and 

the encroaching water level in the mine as the shoreline in the mine 

steadily moved northward. 

Local governmental officials had posted warning signs at the Route 

20A bridge over Beards Creek the day before the collapse because of 

small bumps in the pavement on the bridge approach sections. 

There is some anecdotal evidence that, several days earlier, local 

travelers had noticed a change in the smoothness of the roadbed 

near the bridge, suggesting a surface expression of the underground 

convergence that led to abandonment of room 2-Yard South several 

months earlier. 

Within days of the collapse, impacts on the glacial and bedrock 

aquifer systems and on the land surface were reported on an expanding scale. Some homes had sustained structural damage due to 

the initial earth tremors and, within 1 week of the collapse, residents 

along Wheelock Road, south of Cuylerville and southwest of the 

mine, reported that several water wells had gone dry (NYSDEC, 

1997). The USGS and the Livingston County Health Department 

began monitoring ground-water levels and streamflow in the area. 

On April6, a 200-foot diameter by 20-foot deep, cone-shaped sinkhole appeared along the channel of Beards Creek, immediately 

above the room 2-Yard South collapse zone, just south of the Route 

20A bridge. And 2 weeks later, accompanied by additional earth 

tremors, this sinkhole expanded to about 600 feet in diameter. 

On AprilS, seismic events indicated a roof collapse in mine room 

11-Yard West, south of and adjacent to room 2-Yard South. Following this collapse, ground-water inflow to the mine increased to 

about 22 million gallons per day. An expanding sinkhole developed 

over 11-Yard West on May 25, 1994. The sinkhole was about 50 feet 
114 

BEDROCK 

Shale and limestone 

1---------~ Onondaga Limestone t::::========:J Salt layer 

The ancient Genesee River crossed sedimentary rocks. 

Glaciers scoured the bedrock, leaving a wide, deep valley 

that did not always follow the course of the Genesee 

River. At times the glaciers covered the entire area. 

During periods of glacial retreat. subglacial lakes formed 

and sediment, was deposited. 

Sand 

"o::----:1-----r Sand, gravel and silt 

The periodic retreat and advance of glaciers left behind 

mounds of debris (moraines) and thick glacial deposits (drift). 

During deglaciation a series of preglacial lakes formed that 

deposited lake (lacustrine) sediments on the valley fioor. 

After glaciation alluvial (fioodplain) gravel, sand, and silt 

were deposited on top of the glacial sediments. 

Collapsing Cavities 

deep, about 200 feet in diameter, and immediately filled with water 

captured from Beards Creek. Over time this sinkhole grew to about 

800 feet in diameter. 

THE NATURAL HISTORY OF THE GENESEE VALLEY SET THE 

STAGE FOR WIDESPREAD DAMAGE AFTER THE COLLAPSE 

Current knowledge of the occurrence and flow of ground water and 

the complex stratigraphy of the glacial aquifer system in Genesee 

Valley is sparse. Prior to the collapse, the hydrogeologic framework 

of the valley-fill materials had not been investigated in detail. Since 

the mine collapse, several studies have addressed the hydrogeologic 

framework (Nittany Geoscience, 1995; Alpha Geoscience, 1996), 

but insufficient data exist to thoroughly characterize the interconnections among glacial units and bedrock aquifer zones. 

The Genesee Valley from Dansville to Avon, New York, includes the 

Canaseraga Creek Valley and, from Mt. Morris northward, the 

Genesee River Valley. The valley formed as a result of several geologic processes including the ancestral uplift and stream erosion of 

gently dipping Paleozoic sedimentary rocks, followed by periods of 

glaciation in which ice scoured and modified the bedrock topography, leaving behind unconsolidated sediments. Recently, stream 

erosion and deposition added about 50 feet of alluvium (gravel, 

sand, and silt) to the glacial sediments. 

The unconsolidated glacial sediments that fill the Genesee Valley 

were deposited during cycles of glacial advances and retreats. Glaciers several thousands of feet thick deepened and widened the 

valley. About 12,000 years ago the most recent glacier retreated 

from the valley, leaving behind thick glacial deposits. Where the 

glaciers paused and the ice melted, mounds of glacial debris, called 

end moraines, were deposited at the frontal (southern) ice margin. 

The melting ice produced large volumes of water that transported, 

sorted, and deposited boulders, gravel, cobbles, sand, silt, and clay 

and carried these sediments in meltwater streams to the south. Proglacial lakes existed in the glacially-deepened valley between the 

valley walls and the receding glacier during most of the glacial period. 

During deglaciation, outlets low enough to drain the proglacial 

lakes did not exist until the ice margin was 10 to 12 miles north of 

Geneseo. During this period, the present Genesee River and 

Canaseraga Creek watersheds drained to the north, toward the glacier, into a series of progressively lower pro glacial lakes. The final 

and lowest proglaciallake formed when the ice deposited the Fowlerville moraine, which extends from about 4.5- to 8-miles north of 

the collapse area. Water ponded in the Genesee Valley south of the 

Fowlerville moraine, depositing lake sediments on the valley floor. 

Eventually the lake drained as the Genesee River cut a channel in 

the Fowlerville moraine (Young, 1975). As much as 700 feet of glacially derived gravel, sand, silt, and clay were deposited in a subglacial and glaciolacustrine (glacial lake) environment. 
The Retsof Salt Mine Collapse 

This view shows the Upper 

Genesee Valley looking east. 

Water from the basal aquifer 

entered the mine through the 

collapsed area. 

(Richard Young, Geological Sciences. 

SUNY Geneseo) 

llS 

The buried bedrock surface follows the slope of the resistant sedimentary carbonate beds of the Onondaga Limestone, dipping approximately 42 feet per mile to the south. Overlying the bedrock 

surface is a thickening wedge of glacial valley-fill sediments that 

ranges from a few hundred feet thick on the north, near the Fowlerville Moraine, to about 750 feet thick in the deepest part of the valley near Sonyea. South of Sonyea, the valley fill thins. Ground water 

in the glacial deposits and portions of the underlying carbonate 

bedrock has been the primary source for the inflows to the flooded 

Retsof Mine. The fine-grained lake silt and clay closer to the land 

surface form a barrier between the alluvial and deeper glacial aquifers. 

Water-bearing zones are found within the fractures and bedding 

planes near the top of the Onondaga Limestone at the base of the 

valley fill. Another water-bearing zone is found at the contact between the Onondaga Limestone and the underlying Bertie Limestone. Few valley wells tap bedrock, and the most productive wells 

completed in the Onondaga and Bertie Limestones seldom produce 

more than several tens of gallons per minute (Dunn, 1992). The 

Bertie Limestone subcrops beneath the valley floor north of the 

Fowlerville moraine, under several hundred feet of glacial sediment, 

and is generally considered a divide between fresher water above 

and a more mineralized water below. 

The principal aquifer in the valley appears to occur at the base of 

the valley fill. The relatively thin basal aquifer is composed of sand 

and gravel deposited on top of the Onondaga Limestone in the central and northern parts of the valley and on top of the low-permeability Devonian shales to the south. The hydraulic connection 

between the basal aquifer and the underlying bedrock units 

throughout the valley is poorly understood, but the connection is 

generally assumed to be better in the northern half of the valley, 

where the aquifer is in direct contact with the weathered and fractured top of the Onondaga Limestone. Under natural conditions 

ground water flows upward from the Onondaga to the basal aquifer. 

Though the basal aquifer is generally overlain by lower-permeability 

West East South Shallow aquifer North 

600 

Altitude 

(feet above 

sea level) 

Sea 

level 

Area above collapse 

I 

0 

Retsof salt mine 

I 

Horizontal distance from collapse (miles) 

2 

Dansville 

I 

Basal aquifer Glacial deposits 

Sonyea 

Alluvial (floodplain) 

Lacustrine (lake) 

Till 

Lacustrine (lake) 

Fresh water Mixed 

Devonian shales 

IS 10 5 

Area above collapse 

Geneseo I 

Fowlerville 

moraine Avon 

0 

Shales, siltstones 

and sandstone 

Salt layer 

I 

Re~sof Salt Mine 

5 10 

Horizontal distance from collapse (miles) 
116 

Shallow wells not affected 

by dewatering 

Water level 

altitude 

(feet above 

Footprint of Retsof Mine 

Collapsing Cavities 

Deep wells experiencing 

rapid water decline 

sea level) Floodplain deposits 

560 _~- (feet above 

sea level) Glacial deltaic deposits 540 ..,--.,----.---,----,-

580 • 

Upland bedrock Welldry 720 _'-:::::::: ~ 540 

March July Dec. May 

1994 1995 

The lower aquifer systems are 

relatively isolated from the 

surficial aquifers, and sudden 

changes in the lower aquifer 

had little impact on shallow 

aquifers. 

This view of the Genesee Valley 

floodplain was taken above the 

southern end of the mine looking southwest. 

(Richard Young, Geological Sciences. 

SUNY Geneseo. 1995) 

...... 

0 .- 0 

Selected monitoring wells for the period 

March 12. 1994, to August 30, 1995 

0 2 Miles 

,-----, 

0 2 Kilometers 

Land-surface attitude 

(feet above sea level) 

March July 

1994 

Dec. 

Basal gravel 

May 

1995 

Oct 

glacial drift, in some areas north of the mine more permeable layers 

have been reported within the glacial deposits. 

Some wells in the valley are completed within the glacial deposits, 

and some wells completed in the deeper basal aquifer are also 

screened in the glacial deposits, an indication that there is locally 

enhanced permeability at intermediate depths. There appears to be 

a vertical hydraulic connection between the basal aquifer and the 

permeable zones in the glacial deposits, based upon recent data 

from ground-water monitoring wells, but the areal extent of these 

vertically connected zones is unknown. 

Shallow ground water occurs in the alluvial deposits found to a 

depth of 50 feet below the valley floor. The water table in the alluvium is generally less than 15 feet below land surface, and is in hydraulic connection with the Genesee River, Canaseraga Creek, and 

other tributaries on the valley floor. Other shallow ground water 

occurs in the Fowlerville Moraine deposits. Most recharge and discharge of the Genesee Valley aquifer system occurs between the 

Genesee River, its tributaries, and the shallow water-table aquifer in 

the alluvium (Nittany Geoscience, 1995). Water levels in wells completed in the alluvium were not affected by the mine collapse. 

After the mine collapse, most of the inflows to the mine probably 

came from storage in the basal aquifer and the glacial deposits 

through the collapse areas above rooms 2-Yard South and 11-Yard 
The Retsof Salt Mine Collapse 

0 2 Miles 

,.-------, 

0 2 Kilometers 

Land~surface altitude 

(feet above sea level) 

•• 

"(;)t> 

Footprint of 

Retsof Mine 

• • 

... 

.. Collapse ~ 

'"'" ~~ '"" 

• 

Dewatered well 

• Damage to structure 

• Damage to road or utility 

• Well-water loss with visible 

damage to structures 

• Gas fiares 

• Pumping wells 

,. 

• • 

,, . 

Fow lervil le 

Morain e 

• 

117 

West. Water levels in wells began declining almost immediately near 

the collapse zones. Water levels continued to decline rapidly through 

1994, and more slowly in 1995, until the mine was completely 

flooded in January 1996. By then, water levels had fallen more than 

350 feet in some wells near the collapse zones. In total, an estimated 

42,000 acre-feet of ground water invaded the mine. 

The basal aquifer is relatively isolated from surficial sources of recharge and discharge, and changes in the lower part of the aquifer 

system are not likely to have immediate or significant impact on the 

shallow sources. However, the rate of ground-water drainage into the 

mine far exceeded the estimated rate of recharge to the deeper subsurface aquifers, and it is expected that it will take a decade or longer 

for ground-water levels to recover throughout the aquifer system. 

IMPACTS OF THE COLLAPSE WERE OBSERVED MILES AWAY 

The effects of the collapse include, but are not limited, to the following: 

• Reduced air quality and public-safety issues resulting from the 

emanation of methane and hydrogen-sulfide gases 

• The loss of potable water supplies-both a reduction of quantity 

and degradation in quality and 

\:><:> • Short- and long-term land subsidence '<5 

Natural gas was vented into the environment 

Soon after the mine began to flood and water levels in the basal 

aquifer were lowered, natural gas in the form of hydrogen sulfide 

(odor of rotten eggs ) and methane (odorless, combustible) began 

exsolving from ground water-just as carbon dioxide comes out of 

solution after a bottle of soda is opened. In the area of the collapse, 

lowered water levels allowed natural gas to escape through test wells 

drilled near the collapse area and preexisting domestic water-supply 

wells several miles farther to the southeast. In September 1994 the 

State Department of Environmental Conservation ordered ANSI to 

develop a natural-gas monitoring and response plan. By May 1995 

the County and State Health Departments required ANSI to flareoff (burn) gas from several collapse-area wells to reduce the odor 

and protect the health and safety of residents living in Cuylerville 

and the surrounding area. 

Potable water supplies were diminished 

Although some shallow alluvial wells near the mine were unaffected, 

some domestic wells along the margins of the valley and in the 

deeper zones of the Genesee Valley aquifer system experienced lowered water levels, and some wells went dry. The rate of water-level 

decline varied: water levels declined 20 feet or more along Wheelock 

Road (about 1 mile southwest of the mine) within days of the collapse, whereas water levels gradually declined 50 feet or more in the 
118 

Local subsidence occurred due to dissolution of 

salt pillars by freshwater inflow March 1994 to 

March 1996. 

Measured subsidence 

0 I Mi L_________J 

,----, 

0 I Km 

A well flares methane and hydrogen sulfide gases from a 

fracture zone on the eastern 

margin of the room 2-Yard 

South sinkhole. 

(Richard Young, Geological Sciences. 

SUNY Geneseo. 1995) 

More extensive subsidence due to closure of the 

salt cavity is projected. 

Projected subsidence 

Subsidence (feet) 

02to4 

04to6 

06to8 

D Greater than 8 

(Moran. Scovazzo,and Streib, 1995) 

Collapsing Cavities 

Fowlerville area (about 6 miles north of the mine) and in Mt. Morris (about 4 miles south of the mine) for 2 years following the collapse. Pursuant to an agreement between ANSI, Livingston County, 

and the State of New York, ANSI has been supplying water to residents whose wells have gone dry and where water quality has deteriorated. 

The effects of ground-water flow to the mine extenCl more than 10 

miles north and south of the collapse area. Following the mine collapse and lowering of ground-water levels, highly mineralized 

ground water has apparently migrated into freshwater supplies. 

There are two potential sources: a deep-basin brine that migrates 

upward along bedding-plane fractures within the Bertie Limestone 

to the intersection of the Bertie outcrop and the basal aquifer, and a 

halite (rock salt) component, which may be introduced through 

older natural-gas or salt-solution wells within the Fowlerville moraine. The mineralized ground water flows downdip (to the south) 

through the basal aquifer toward the mine collapse area, an apparent reversal of the pre-collapse hydraulic gradient. Presently, salinity 

is increasing in Fowlerville Moraine wells, south of where the Bertie 

outcrops and is in contact with the basal aquifer. 

Several types of subsidence were observed 

Subsidence damage related to the mine collapse includes: 

• The creation of 2 large sinkholes 

• The temporary loss of State Route 20A through Cuylerville 

• Structural damage to homes and businesses and 

• Damage to agricultural lands, public utilities, and cultural resources. 

Besides the catastrophic formation of sinkholes over rooms 2-Yard 

South and 11-Yard West, the damage involves three other types of 

subsidence, which are important at different scales. 
The Retsof Salt Mine Collapse 

An earth fissure ruptures a field 

above the western edge of the 

mine, northwest of 2-Yard 

South sinkhole. The field surrounding the fissure subsided 

almost 1 foot. 

The lowering of water levels 

in areas beyond the mine 

footprint may eventually 

cause aquifer-system compaction and land subsidence. 

"'---~------1 Lines of equal altitude of poten-

~ 

2 Miles 

0 2 Kilometers 

Land-surface attitude 

(feet above sea level) 

tiometric surface, December 

1995 (feet above sea level) 

119 

(Richard Young, Geological 

Sciences, SUNY-Geneseo, 

March 1995) 

The first type of subsidence that normally occurs over any minedout area is due to the slow closure of the mine opening. Mining 

engineers expect the land overlying the Retsof Mine footprint to 

subside about 8 to 9 feet over the next 100 to 200 years (Van 

Sambeek, 1994). Most of the estimated subsidence is expected to be 

realized during the next 100 years (Shannon and Wilson, 1997). 

Differential subsidence is expected along the margins of the mine, 

where adjacent areas will subside nonuniformly. This creates 

stresses within the land mass, which may rupture the surface or 

subsurface. Some horizontal movement ofland surface in these 

areas is expected, as well as some tilting of the land surface toward 

the mine. Structures located in these regions may continue to be 

prone to damage as the mine subsidence evolves. 

A second type of subsidence seen near the collapse area and farther 

to the north and east was caused by the flow of ground water into 

the mine and resultant dissolution of unmined salt. Fresh ground 

water, less dense than saltwater, entered the mine cavity quickly, and 

preferentially dissolved the salt along the mine roof. As the mine 

roof collapsed, it allowed the freshwater to dissolve more salt in the 

supporting salt pillars and, over time, left large areas without roof 

support. This type of subsidence evolved rapidly as many salt pillars were quickly dissolved by the large inflow of freshwater, and 

subsidence in this area was greater and occurred sooner than would 

be expected for a dry mine situation (Van Sambeek, 1996). When 

the mine ftlled with saturated brine, this type of subsidence ceased. 

The third type of subsidence to occur in the Genesee Valley is due 

to the dramatic lowering and anticipated slow recovery of groundwater levels in the confined-aquifer system. This type of subsidence 

is due to aquifer-system compaction that typically accompanies the 

depletion of alluvial aquifer systems. The ground-water level declines experienced after the mine-roof collapse-more than 350 

feet near the collapse and as much as 50 feet as far as 8 miles 

away-is sufficient to cause measurable dastic compression of the 

glacial sediments of the Genesee Valley aquifer system. It is possible 

that the large stresses imposed on the aquifer-system skeleton by 

the large drawdowns may have caused some inelastic, and largely 

irreversible, compaction of aquitards in the Genesee Valley, but 
120 Collapsing Cavities 

currently this effect is presumed to be small. Aquifer-system compaction may contribute to land subsidence on a spatial scale larger 

than the mine footprint, especially in regions where large changes 

in ground-water levels persist and where the glacial deposits contain an appreciable thickness of fine-grained, more compressible 

sediments. It is possible that the valley floor may continue to be 

affected by residual compaction long after ground-water levels 

have fully recovered in the aquifers (Riley, 1969).An accurate 

evaluation of the magnitude, timing, and areal extent of land subsidence due to aquifer-system compaction will depend on more 

detailed knowledge of the hydrogeology of the Genesee Valley. 

CONTINUING STUDIES WI LL ASSESS FUTURE IMPACTS 

The long-term lowering of aquifer hydraulic heads creates the potential for permanent compaction of the aquifer system and additionalland subsidence. The distribution of compressible sediments 

and their mechanical behavior need to be better understood in 

order to predict potential impacts. The sources of poor-quality 

water and potential paths of migration in the aquifers also need to 

be assessed in order to evaluate and predict changes in groundwater quality throughout the Genesee Valley. 

The USGS is currently implementing conceptual and numerical 

models of ground-water flow in Genesee Valley to cy;sist in determining the impact of mine flooding on the regional aquifer system. 

Drainage of ground water into the collapse areas is being simulated 

using data collected by ANSI consultants; the State Departments of 

Law, Environmental Conservation, and Health; Livingston County; 

local citizens; the USGS; and others. The models will provide insight into the problems of lowered ground-water levels, land subsidence caused by aquifer-system compaction, and migration of 

mineralized ground water. 
SINKHOLES, WEST-CENTRAL FLORIDA 

A link between surface water and ground water 

PASCO ;. ' 1 ..-~ 

-''r' 

HIL,_SB-~f-' l WI t H , ,. o_over n er ave~ 

• ampa 1 • 

• Pp~K 

0 20 M1les 

,----, 

0 20 Kilometers 

Ann B. Tihansky 

I 

. ~ 

-~ ( 

<:<: 

HAf<Dl 

U.S. Geological Survey, Tampa, Florida 

S inkholes are a common, naturally occurring geologic feature 

and one of the predominant landforms in Florida, where they 

pose hazards to property and the environment. Although 

many new sinkholes develop naturally, in west-central Florida and 

elsewhere, their increasing frequency corresponds to the accelerated 

development of ground-water and land resources. Usually little 

more than a nuisance, new sinkholes can sometimes cause substantial property damage and structural problems for buildings and 

roads. Sinkholes also threaten water and environmental resources by 

draining streams, lakes, and wetlands, and creating pathways for 

transmitting surface waters directly into underlying aquifers. Where 

these pathways are developed, movement of surface contaminants 

into the underlying aquifer systems can persistently degrade 

ground-water resources. In some areas, sinkholes are used as storm 

drains, and because they are a direct link with the underlying aquifer systems it is important that their drainage areas be kept free of 

contaminants. Conversely, when sinkholes become plugged, they 

can cause flooding by capturing surface-water flow and can create 

new wetlands, ponds, and lakes. 

Most of Florida is prone to sinkhole formation because it is underlain by thick carbonate deposits that are susceptible to dissolution 

by circulating ground water. Florida's principal source of freshwater, 

ground water, moves into and out of storage in the carbonate aquifers-some of the most productive in the nation. Development of 

these ground-water resources for municipal, industrial and agricultural water supplies creates regional ground-water-level declines 

that play a role in accelerating sinkhole formation, thereby increasing susceptibility of the aquifers to contamination from surfacewater drainage. Such interactions between surface-water and 

ground-water resources in Florida play a critical and complex role 

in the long-term management of water resources and ecosystems of 

Florida's wetlands (see Florida Everglades in Part II of this Circular). 
122 

There appears to be an increasing frequency of sinkholes, although the statistics 

may be affected by reporting biases. 

Reported sinkholes 

from 1960 to 1991 

Collapsing Cavities 

(Wilson and Shock 1996) 

SINKHOLES ARE A NATURALLY OCCURRING FEATURE 

IN THE FLORIDA LANDSCAPE 

The exposed land mass that constitutes the Florida peninsula is only 

part of a larger, mostly submerged carbonate platform that is partially capped with a sequence of relatively insoluble sand and clay 

deposits. Siliciclastic sediments (sand and clay) were deposited atop 

the irregular carbonate surface, creating a blanket of unconsolidated, relatively insoluble material that varies in composition and 

thickness throughout the State. In west-central Florida, the relation 

between the carbonate surface and the mantling deposits plays an 

important role in the circulation and chemical quality of ground 

water and the development of landforms. Sinkhole development 

depends on limestone dissolution, water movement, and other environmental conditions. Limestone dissolution rates (on the order of 

millimeters per thousand years) are highest in areas where precipitation rates are high. Cavities develop in limestone over geologic 

time and result from chemical and mechanical erosion of material 

(Ford and Williams, 1989). 

Dissolving carbonate rocks create sinkholes and other features 

The soluble limestones and dolomites that constitute the carbonate 

rocks are sculpted by dissolution and weathering processes into a 

Data collected by 

Florida Sinkhole Research Institute 

Data col lection 

100 ------------------------------------ Number of 

reported new 

sinkholes 

Drought years 

(\Nilliam L Wilson, 

Subsurface Evaluations, Inc., 

written communication, 1997) 
Sinkholes, West-Central Florida 

Mining exposed this typical 

karst limestone surface, 

which is riddled with dissolution cavities. 

(William A. VVsner. 1972) 

Changes in sea level have 

alternately submerged 

and exposed the carbonate platform. 

123 

distinct geomorphology known as karst. Features characteristic of 

karst terranes are directly related to limestone dissolution and 

ground-water flow and include sinkholes, springs, caves, disappearing streams, internally drained basins, and subsurface drainage networks. Dissolution cavities can range in size from tiny vugs to 

gigantic caverns. As these enlarging voids coalesce and become hydraulically interconnected, they greatly enhance the movement of 

ground water, which can perpetuate further dissolution and erosion. 

On a local scale, the caverns and cave networks can form extensive 

conduit systems that convey significant ground-water flow at very 

high velocities (Atkinson, 1977; Quinlan and others, 1993). On a 

regional scale, the many interconnected local-scale features can create a vast system of highly transmissive aquifers that constitute a 

highly productive ground-water resource. 

Changes in sea level helped develop karst terranes 

Karst is well-developed in the carbonate rocks throughout the 

Florida carbonate platform. Throughout recent geologic time, fluctuations in sea level have alternately flooded and exposed the platform, weathering and dissolving the carbonate rocks. During the Ice 

Ages, an increased proportion of the Earth's water was frozen in 

polar ice and continental glaciers, lowering sea level along the 

Florida peninsula by 280 to 330 feet as recently as 18,000 years ago. 

The sea-levellow stands exposed the great carbonate platforms of 

the Gulf of Mexico and the Caribbean Sea to karst processes. The 

lower sea-level stands were accompanied by lower ground-water 

levels (Watts, 1980; Watts and Stuiver, 1980; Watts and Hansen, 

1988), which accelerated the development of karst. With the melting 

of the ice, sea levels and ground-water levels rose and many of the 

karst features were submerged. Examples of these flooded features 

include the "blue holes" found in the Bahamas, the cenotes of the 

Yucatan, the springs of Florida, and numerous water-filled cave passages throughout these terranes. Many of the numerous lakes and 

ponds of west -central Florida formed as overburden materials 

settled into cavities in the underlying limestone. 
124 

In mantled karst terrane, the 

buried carbonate rock is furrowed and pitted. When the 

covering deposits subside into 

the underlying depressions, 

sinkholes and a hummocky topography result. 

Overburden 

(mantle) 

Carbonate 

bedrock 

(karst) 

Collapsing Cavities 

Karst is an important part of the ground-water plumbing 

At present, in west -central Florida, most of the soluble bedrock is 

below the water table. As ground water flows through the rock, 

geochemical processes continually modify both the rock and the 

chemical composition of the ground water. In many areas within 

the platform, the carbonates continue to dissolve, further enlarging 

cavities and conduits for ground-water flow. Fractures, faults, bedding planes and differences in the mineral composition of the carbonate rocks also play a role in the development, orientation, and 

extent of the internal plumbing system. Lineaments (linear features 

expressed in the regional surface terrain and often remotely sensed 

using aerial photography or satellite imagery) are often associated 

with locations of sinkholes and highly transmissive zones in the 

carbonate platform (Lattman and Parizek, 1964; Littlefield, and 

others, 1984). 

THE MANTLED KARST OF WEST-CENTRAL FLORIDA 

Where karst processes affect rocks that are covered by relatively 

insoluble deposits, the presence of buried karst features forms a 

distinctive type of terrain known as mantled karst. In mantled 

karst regions, the carbonate units are not exposed at land surface, 

but their presence may be indicated by sinkholes and the hummocky topography that results when the covering deposits take the 

shape of the underlying depressions. The mantled karst of westcentral Florida has resulted in a number of distinct geomorphic 

regions (White, 1970; Brooks, 1981 ), including several lake districts 

with numerous lakes created by subsidence of overburden into the 

buried karst surface. In other areas, especially where the mantling 

deposits are thick, the buried karst surface is not reflected in the 

topography. 

Sinkhole formation is related to the thickness and 

composition of the overlying materials 

The mantled karst of west-central Florida has been classified into 

four distinct zones on the basis of the predominant type of sinkholes (Sinclair and Stewart, 1985). The type and frequency of sinkhole-subsidence activity have been correlated to the composition 

and thickness of overburden materials, the degree of dissolution 

within the underlying carbonate rocks, and local hydrologic conditions. Three general types of sinkholes occur: dissolution sinkholes-depressions in the limestone surface caused by chemical 

erosion of limestone; cover-subsidence sinkholes-formed as overburden materials gradually infill subsurface cavities; and covercollapse sinkholes-also formed by movement of cover materials 

into subsurface voids, but characteristically formed more abruptly. 

In the northern part of the region a thin ( 0 to 30 feet thick) mantle 

of highly permeable sediments overlies the carbonate rock. Rain 

water moves rapidly into the subsurface, dissolving the carbonate 
Sinkholes, West -Central Florida 125 

The type, location, and frequency of sinkhole subsidence in the Southwest Florida Management District of 

west-central Florida have been related to the type and 

thickness of overburden materials. Reported sinkholes from 1960 to 1991 

(In general, sinkhole occurrence is 

under-reported in remote areas; 

urban areas often appear to have 

higher sinkhole occurrence due 

TYPE AND THICKNESS 

OF OVERBURDEN 

D Thin; highly permeable 

D 30 to 200 feet thick; 

permeable sands are 

dominant 

D 30 to 200 feet t hick; 

more clayey 

D Greater than 200 feet 

A cover-collapse sinkhole 

formed in an orange grove 

east of Tampa. 

to good reporting.) 

New sinkholes in the coastal 

region are small and numerous. :----- ---'""""'• 

The buried limestone surface is 

intensely karstified, and the 

thin, sandy over-burden materials constantly settle int o the 

buried voids and cavities. Recent urban development in this 

region increases the observat ion and occurrence of sinkhole activit y. 

FREQUENCY TYPE OF 

OF SINKHOLES SINKHOLES 

Southwest Florida Water 

Management District 

Generally Dissolution; coverfew 

Numerous 

Very 

numerous 

Few 

subsidence; covercollapse 

Cover-subsidence-occur 

slowly; cover-collapseusually induced 

Cover-collapse-occur 

abruptly 

Cover-collapse-large 20Miles 

(Sinclair and Stewart. 1985: diameter and deep ,----, 

0 20 Ki lometers W ilson and Shock. 1996) 

rock, and dissolution-type sinkholes tend to develop. The slow dissolution of carbonates in these terranes has little direct impact on 

human activity ( Culshaw and Waltham, 1987). 

To the south, the overburden materials are generally thicker and less 

permeable. Where the overburden is 30 to 200 feet thick, sinkholes 

are numerous and two types are prevalent, cover-subsidence and 

cover-collapse. Where permeable sands are predominant in the 

overburden, cover-subsidence sinkholes may develop gradually as 

the sands move into underlying cavities. Where the overburden 

contains more clay, the greater cohesion of the clay postpones failure, and the ultimate collapse tends to occur more abruptly. 

In the southernmost part of the region, overburden materials typically exceed 200 feet in thickness and consist of cohesive sediments 

interlayered with some carbonate rock units. Although sinkhole 

formation is uncommon under these geologic conditions, where 

sinkholes do occur they are usually large-diameter, deep, covercollapse type. 
126 

Categorizing sinkholes 

Two processes create three types of sinkholes 

Three types of sinkholes are common in Florida: dissolution, coversubsidence and cover-collapse sinkholes. They develop from dissolution 

and "suffosion." Dissolution is the ultimate cause of all sinkholes, but the 

type of sinkhole is also controlled by the thickness and type of overburden 

materials and the local hydrology. 

Although it is convenient to divide sinkholes into three distinct types, sinkholes can be a combination of types or may form in several phases. 

Cover-collapse sinkhole 

near Ocala, Florida 

Collapsing Cavities 

PROCESSES --------------------------------------------------------- Dissolution of soluble carbonate rocks by weakly 

acidic water is ultimately 

responsible for virtually 

all the sinkholes found in 

Florida. 

ATMOSPHERE 

MANTLE or 

COVER SEDIMENT 

CARBONATE 

BEDROCK 

(Limestone and 

dolomite) 

Suffosion occurs when unconsolidated 

overburden sediments infill preexisting 

cavities below them. This downward erosion of unconsolidated material into a 

preexisting cavity is also called raveling 

and describes both the catastrophic 

cover-collapse sinkhole and the more 

gradual cover-subsidence sinkhole. 

Q 

Carbon ; ;, ;, : \ ·, Water 

dioxide \\\\\\ ) (HPJ 

(C02) --....:.. : ·,, \, .._____,' \' \ '. _______ ..., 

Carbon dioxide Y Water 

(COJ) t (H20) 

Carbonic acid 

(H2C03) 

Dolomite ?( V Limestone 

[CaMg(C03lJ] ~ ~ (CaC03) 

Magnesium. Calcium. Bicarbonate 

(Mg••l cca• •l (Hcon 

Water (HPJ falling through the atmosphere 

and percolating the ground dissolves carbon 

dioxide (COJ) gas from the air and soil. 

forming a weak acid-carbonic acid (H2C03 ). 

As the carbonic acid in~ lt rates the ground 

and contacts the bedrock surfaces, it reacts 

readily with limestone (CaC03) and/or 

dolomite [CaMg(C03hJ. 

Cavities and voids develop as limestone or 

dolomite is dissolved into component 

ions of calcium (Ca ++), magnesium (Mg ++), 

and bicarbonat e (Hcon. 

When the ground water becomes supersaturated with dissolved minerals, further 

dissolution is not possible, and carbonate salts of calcium and magnesium may 

precipitate from the water, often forming interesting shapes such as stalactites. 

The reactions are fully reversible, and when precipitates are exposed to undersaturated ground water they may redissolve. The geochemical interactions are 

controlled partly by the rate of circulation of water. 

The erosion begins at the top of the carbonat e 

bedrock and develops upward t hrough the 

overlying sediments toward the land surface. 
Sinkholes, West-Central Florida 

TYPES OF SINKHOLES-------------------------

Dissolution of the limestone or dolomite 

is most intensive where the water first con -

tacts the rock surface. Aggressive dissolution 

also occurs where flow is focussed in preexisting openings in the rock, such as along 

joints, fractures, and bedding planes, and 

in the zone of water-table fluctuation 

where ground water is in contact with the 

atmosphere. 

Thin overburden 

Rainfall and surface water percolate through 

joints in the limestone. Dissolved carbonate 

rock is carried away from the surface and a 

small depression gradually forms. 

On exposed carbonate surfaces, a depression 

may focus surface drainage, accelerating the 

dissolution process. Debris carried into the 

developing sinkhole may plug the outflow, 

ponding water and creating wetlands. 

Gently rolling hills and shallow depressions caused by solution sinkholes 

are common topographic features throughout much of Florida. 

Cover-subsidence sinkholes tend to 

develop gradually where the covering sediments are permeable and contain sand. 

Granular sediments spall 

into secondary openings 

in the underlying carbonate 

rocks. 

A column of overlying sediments settles into the 

vacated spaces (a process 

termed "piping"). 

Dissolution and infilling continue, forming a noticable 

depression in the land 

surface. 

The slow downward erosion 

eventually forms small surface depressions I inch to 

several feet in depth and 

diameter. 

In areas where cover material is thicker or sediments contain more clay, cover-subsidence 

sinkholes are relatively uncommon, are smaller, and may go undetected for long periods. 

Cover-collapse sinkholes may develop 

abruptly (over a period of hours) and cause 

catastrophic damages. They occur where the 

covering sediments contain a significant 

amount of clay. 

Sediments spall into a cavity. As spalling continues, the 

cohesive covering sediments form a structural 

arch. 

The cavity migrates upward by progressive roof 

collapse. 

The cavity eventually 

breaches the ground surface, creating sudden and 

dramatic sinkholes. 

Over time, surface drainage, erosion, and deposition of sediment tran sform the steep-walled 

sinkhole into a shallower bowl-shaped depression. · 

127 
128 

Many of the numerous lakes 

and ponds that dot the 

Florida landscape, such as 

these in central Polk County, 

are actually subsidence depressions that are filled with 

water. 

A sinkhole that breached a 

confining clay layer illustrates 

the interconnectivity of the 

aquifers. The water-level 

drop in the surficial aquifer 

system and the coincident 

rise in the Upper Floridan 

aquifer occurred as the sinkhole drained. 

Sinkhole formed 

120 ~~~--~----~--------­ Surfcial aquifer system 

Water movement 

Water-level 

altitude* 

(feet above 100 

sea level) 

~ Upper Floridan aquifer 

80 

II 12 

July 1991 

*Water levels were recorded at a SWFWMD Regional 

Observation Monitoring Program wellsite that is less 

than I ,000 feet from the sinkhole 

(Southwest Florida Water Management 

District, wrftten communication, 1998) 

Collapsing Cavities 

SINKHOLE DEVELOPMENT IS AFFECTED BY THE 

HYDROGEOLOGIC FRAMEWORK 

The flow of subsurface water through sediments and eroded carbonate rocks affects how, where, and when sinkholes develop. Thus, 

formation of sinkholes is sensitive to changes in hydraulic and 

mechanical stresses that may occur naturally or as the result of 

human activity. Whether the stresses are imposed over geologic 

time scales by changes in sea level or over the time scale of human 

ground-water-resources development, they are expressed as 

changes in ground-water levels (hydraulic heads) and the gradients 

of hydraulic head. The hydraulic properties of the aquifers and the 

extent, composition, and thickness of overburden materials control 

how these stresses are transmitted. The chemistry of the ground 

water determines where dissolution and karst development occurs. 

Together, these hydrogeologic factors control the type and frequency of sinkholes that develop in west-central Florida. 

Just as the hydrogeologic framework influences the development of 

sinkholes, the sinkholes influence the hydrogeologic framework. 

Understanding of the hydrogeologic framework can lead to landand water-resources management strategies that minimize the impact of sinkholes. 

Vast aquifer systems underlie west-central Florida 

The hydrogeologic framework of west -central Florida consists of 

three layered aquifer systems that include both carbonate and 

siliciclastic rocks. The shallowest or "surficial" aquifer system generally occurs within unconsolidated sand, shell, and clay units. The 

surficial aquifer system ranges from less than 10 to more than 100 

feet in thickness throughout west-central Florida. The water table is 

generally close to the land surface, intersecting lowlands, lakes, and 

streams. Recharge is primarily by rainfall. When sinkholes occur, it 

is the surficial aquifer deposits that commonly fail and move to 

infill any underlying cavities. 
Sinkholes, West-Central Florida 

Limestone near or at land 

surface; solution sinkhol 

are prevalent. 

Large volumes of water 

move through the 

Upper Floridan aquifer. 

The type and frequency of sinkholes in westcentral Florida are related to the presence or 

absence of the intermediate aquifer system. 

Ground water is recharged in the 

northern and eastern upland areas. 

When a cover-collapse 

sinkhole breaches the 

confining unit, water can 

move into the Upper 

Floridan aquifer. 

J 

Mantle or 

overburden 

(clay/sand) 

j 

Paleokarst 

carbonate 

bedrock 

(dolomite/ 

limestone) 

129 

In most of west-central Florida the surficial aquifer system is separated from the Upper Floridan aquifer by a hydrogeologic unit 

known as either the "intermediate aquifer system" or "intermediate 

confining unit;' depending upon its local hydraulic properties 

(Southeastern Geological Society, 1986). The intermediate confining unit, delineated as such where fine-grained clastic deposits are 

incapable of yielding significant quantities of water, impedes the 

vertical flow of ground water between the overlying surficial aquifer 

system and the underlying Floridan aquifer system. In northern 

west-central Florida, where this unit is absent, the surficial aquifer 

system lies directly above the Floridan aquifer system. In general, 

the intermediate confining unit consists of heterogenous siliciclastic 

sediments that mantle the carbonate platform. These deposits 

thicken westward and southward, where they include more permeable clastic sediments and interbedded carbonate units. In these 

regions they are referred to as the intermediate aquifer system. The 

lateral extent of permeable units within the intermediate aquifer 

system is limited, and the transmissivities of these units are significantly smaller than those of underlying carbonate rocks of the 

Floridan aquifer system. The type and frequency of sinkholes in 

west -central Florida are correlated to the presence or absence of this 

intermediate layer and, where present, its composition and thickness. 

The thick carbonate units of the Floridan aquifer constitute one of 

the most productive aquifer systems in the world. The Upper 

Floridan aquifer is between 500 and 1,800 feet thick and is the primary source of springflow and ground-water withdrawals in westcentral Florida. Transmissivities commonly range from 50,000 to 

500,000 square feet per day and may be as large as 13,000,000 

square feet per day near large springs (Ryder, 1985). These transmis-
130 Collapsing Cavities 

The presence of a confining 

unit affects the water level 

and the potential for sinkholes. 

4 Discharge Recharge 

Water in the Upper Floridan 

aquifer moves from recharge 

areas in the,northern and eastern upland regions toward discharge areas near the coast. 

(Mularoni. 1993) 

,-----, 

sw Water level 

Surficral [ 

aqurfer system 1------l 

lntermedrate [ 

aqurfer system ~....::..:>-----'-'11 

Water 

movement 

Carl::>onate 

Upper Florrdan [ >-------' bedrock 

aqurfer '----------'------------------' 

20 Mrles 

In confined areas, water levels In upland, unconfined areas, 

in the Upper Floridan aquifer the water level in the Upper 

and intermediate aquifer system Floridan aquifer is lower 

are higher than the water table than the water table in the 

in the surficial aquifer system. surficial system. 

Low Moderate High 

Sinkhole potential 

In discharge areas, upward 

ground-water fiow helps provide bouyant support for overburden materials, and sinkholes 

rarely occur 

In recharge areas, where 

water movement is downward, sinkholes are more 

likely to occur 

sivityvalues far exceed those typical of diffuse ground-water flow in 

porous media such as sand and reflect the influence of karst-dissolution features. 

In upland regions, hydraulic heads in the Upper Floridan aquifer are 

generally lower than heads in the surficial and intermediate aquifer 

systems. In these areas ground water moves downward from the 

surficial aquifer system, recharging the intermediate aquifer system 

and the Upper Floridan aquifer. This downward movement of 

ground water enhances the formation of sinkholes by facilitating 

raveling of unconsolidated sediments into the subterranean cavities. 

Where the intermediate confining unit is present, recharge to the 

Upper Floridan aquifer may be diminished. However, where the clay 

content of the confining unit is low, or the unit has been breached 

by sinkhole collapse or subsidence, downward movement of water 

and sediments from the surficial aquifer system can be greatly accelerated. Vertical shafts and sand-ftlled sinkholes can form highpermeability pathways through otherwise effective confining units 

(Brucker and others, 1972; Stewart and Parker, 1992). 

0 20 Kilometers 

Artesian conditions exist along much of the coast and, where confinement is poor, springs commonly occur. Parts of the northern 

coastal area are highly karstified, and the Upper Floridan aquifer is 

exposed at the land surface except where it is covered by unconsolidated sands. In the southern coastal regions, where the intermediate 

aquifer system and the Upper Floridan aquifer are well confined, 

water levels in those deeper units are higher than those in the surficial aquifer system, and ground water moves upward toward the 

surficial aquifer. Sinkholes rarely occur under these conditions. 
Sinkholes, West-Central Florida 131 

Seasonal changes affect 

ground-water levels 

and sinkhole formation . 

Rainy season 

Long-term ground-water 

pumping near Kissengen 

Spring in central Polk County 

led to a decline in water levels and ultimately caused the 

spring to stop flowing. 

W~t vel 

llttt .. ule 

1feet above 

sc I vel, 

10 --,.-----"' 1:-11-: 

~~ ~~~------~~~~JLJL~-- o 

Jan. Mar. May July Sept. Nov. 

LJ LJ LJ 

Freeze Annual low Annual high 

protection water levels water levels 

pumping 

Reported new 

smkho/es by 

month from 

1948 to 1997 

(William L Wilson, Subsurface Evaluations. Inc., 

written communication. 1997) 

Cyclical changes in water levels often occur in response to seasonal 

conditions in west -central Florida. At the end of the dry season, in 

May, ground-water levels are near their annual lows and, after the 

rainy season, in September, recover to their annual high levels. The 

range between the annual minimum and maximum levels can be 

significant. In some areas, especially during prolonged drought or 

large rainfall events, seasonal change in ground-water levels can lead 

to temporary reversals in the direction of vertical flow. More new 

sinkholes form during periods when ground-water levels are low. 

Temporary reversals in head gradients may also be created by extreme, short-lived pumping. Longer-term ground-water pumping 

can lead to sustained ground-water level declines and gradient reversals, creating new recharge areas within the aquifer system and 

sometimes converting flowing springs to dry sinkholes. After the 

pumping stops, ambient conditions are usually restored, but the 

changes can become semipermanent or permanent if pumping persists over long periods of time, or confining units are compromised. 

Kissengen Spring 

discharge 

._), ~har e l >- 100 -- +-~--~-------==- tr .,.- 1 e...,.le- v-at-,-io_n_f;=-o-r -----

Ul:oiC feet 

per sec~ r-J cWater-level 

altitude 

(feet above 

sea level) 

1930 

Kissengen Spring outflow 83.55 feet 

~~ ----~----------------

Water levels monitored at 

three wells near Kissengen Spring 

1940 1950 1960 1970 1980 1990 

(Lewelling and others. 1998) 
132 

Changes in relative water levels caused by human activity 

can induce sinkholes 

Normal conditions 

Land surface 

Loading 

Loading results when water is 

, applied on the land surface by 

F_;;:,.....,;,jjiiiiiio...:::;;.-=-.!-9 precipitation, irrgation, or 

stream diversion. 

Applied surface water increases 

the load on subsurface cavities. 

tr'-"t'"""~;;or..l;f=:!~"::: ::=-=:"8 Downward drainage of the applied 

__ _,.. ;--....,_ - ...J., water raises the water level in the 

:-c, surlicial aquifer and may enhance 

--;=±=r-~r==r-, erosion of the subsurface structural 

'----'----'--'--- --L__u, support. 

Pumping 

Pumping commonly involves extraction of water from the lower 

F-;----'"-1 aquifer and subsequent discharge 

onto the land surface. 

Loading and pumping 

When loading and pumping occur 

, together. the increased overburden 

F._;:,.tiiill!l!!l!!,...::;....;;::,-,:..j load on subsurface cavities and enhanced downward drainage may 

combine to increase downward 

erosion or collapse cavities. 

Collapsing Cavities 

GROUND-WATER PUMPING, CONSTRUCTION, AND 

DEVELOPMENT PRACTICES INDUCE SINKHOLES 

New sinkholes have been correlated to land-use practices (Newton, 

1986). Induced sinkholes are conceptually divided into two types: 

those resulting from ground-water pumpi!lg (Sinclair, 1982) and 

those related to construction and development practices. Modified 

drainage and diverted surface water commonly accompany construction activities and can lead to focused infiltration of surface 

runoff, flooding, and erosion of sinkhole-prone earth materials. 

Manmade impoundments used to treat or store industrial- process 

water, sewage effluent, or runoff can also create a significant increase 

in the load bearing on the supporting geologic materials, causing 

sinkholes to form. Other construction activities that can induce 

sinkholes include the erection of structures, well drilling, dewatering foundations, and mining. 

The overburden sediments that cover buried cavities in the aquifer 

systems are delicately balanced by ground-water fluid pressure. In 

sinkhole-prone areas, the lowering of ground-water levels, increasing the load at land surface, or some combination of the two may 

contribute to structural failure and cause sinkholes. 

Aggressive pumping induces sinkholes 

Aggressive pumping can induce sinkholes by abruptly changing 

ground-water levels and disturbing the equilibrium between a buried cavity and the overlying earth materials (Newton, 1986). Rapid 

declines in water levels can cause a loss of fluid-pressure support, 

bringing more weight to bear on the soils and rocks spanning buried voids. As the stresses on these supporting materials increase, the 

roof may fail and the cavity may collapse, partially filling with the 

overburden material. 

Prior to water-level declines, incipient sinkholes are in a marginally 

stable stress equilibrium with the aquifer system. In addition to providing support, the presence of water increases the cohesion of sediments. When the water table is lowered, unconsolidated sediments 

may dry out and coarser-grained sediments, in particular, may 

move easily into openings. 

Induced sinkholes are generally cover-collapse type sinkholes and tend 

to occur abruptly. They have been forming at increasing rates during 

the past several decades and pose potential hazards in developed and 

developing areas of west-central Florida. The increasing incidence of 

induced sinkholes is expected to continue as our demand for groundwater and land resources increases. Regional declines of ground-water 

levels increase sinkhole occurrence in sinkhole-prone regions. This 

becomes more apparent during the natural, recurring periods of low 

annual rainfall and drought. 
Sinkholes, West-Central Florida 

Section 21 Well Field 

Ground-water pumping for urban water supply induces new sinkholes 

By the early 1930s, ground-water pumping along the west 

coast of Florida had lowered hydraulic heads in the freshwater aquifers and caused upconing of saline water. Coastal 

municipalities began to abandon coastal ground-water 

sources and develop inland sources. 

The city of St. Petersburg began pumping ground water from 

well fields in a rural area north of Tampa. By 1978, four well 

fields had been established in parts of Hillsborough, Pasco, 

and Pinellas Counties, and were pumping an average of 

69,900 acre-feet per year. Sinkholes occurred in conjunction 

with the development of each of the well fields: Cosme (1930), 

Eldridge-Wilde (1954), Section 21 (1963), and South Pasco (1973). 

SECTION 21 W ELL FIELD 

The effects of pumping on sinkhole development near the 

Section 21 well field illustrate the general relation between 

aggressive pumping, ground-water declines, and sinkhole 

development. 

Water-level 

altitude 

(feet above 

sea level) 

42 --~~------------;r -.) 

Reported 

sinkholes 

( 1960-1991) 

Well fields 

. . ~,. . ;• • • PASCO CO .• 

::~·. '- ~h! :_;_ _ . 

: dridg!f':,il ; ~· • :,; on 21 • ~e vv .. • ~ • (', •• I • , . .• 

;• • sm ~V'~·~ • • ._ •,!c•.-..,:.• • . "'. :'· : ,:, ·'1:: . ... .. . rl.i5E>'t>RO~~ ~0. . . . 

0 

Tampa • .. (:) 

Average f'10r>thly 

ground-water pumpage 

(acre-feet per day) 

Pumping began in 1963 and ground- water levels began to decline. In April 1964 the pumping rate nearly tripled, lowering ground-water levels more than I 0 feet, and within I month, 64 sinkholes had formed. (Sinclair, 1982) 

Within 1 month of increasing the pumping rate, 64 new 

sinkholes formed within a 1-mile radius of the well field. 

Most of the sinkholes were formed in the vicinity of 

well21-10, which was pumping at nearly twice the rate of the 

other wells. Neighboring areas also noticed dramatic declines 

in lake levels and dewatering of wetland areas. 

The Section 21 well field is still in operation and researchers 

continue studying the effects of ground-water 

pumping on lake levels and wetlands. 

• 

Well field boundary 

- ~0 ~ ~ •. • Sinkholes 

· 

• We1121-10 1 • 

• I .. 

• . 

• 

-------.... \~ .. I .,. 0 O.SM1 • ~ • • .. L____j 

. ,...---, 

The sinkholes were appar- ently distributed randomly, except for those south and 

east of well 2 1 -I 0, which were clustered along pre- existing joints. 

.__ ___ • _____ ..:.0 _ _ 0..:.5_Km _ _, (Sinclair, 1982) 

133 
134 Collapsing Cavities 

Crop freeze protection 

Heavy ground-water pumping during winter freezes produces new sinkholes 

The mild winters are an important growing season for westcentral Florida citrus, strawberry and nursery farmers. 

However, occasional freezing temperatures can result in substantial crop losses. To prevent freeze damage,growers pump 

warm (about 73° F) ground water from the Upper Floridan 

aquifer and spray it on plants to form an insulating coat of 

ice. Extended freezes have required intense and prolonged 

ground-water pumping, causing large drawdowns in the 

Upper Floridan aquifer and the abrupt 

appearance of sinkholes. 

• • 

Pumping --...... 

we lls 

/ -........_ Strawberry 

fie lds A thin laye r of ice provides insulatio n 

from freezing temperatures. Si nkhol es 

The relation between freezing weather, • D Dover 

prolonged ground-water withdrawals, 4 

and sinkhole occurrence has been well el 1~ '~ 41 • In january 1977, extended freezes and 

associated ground-water withdrawals 

led to the sudden fonmation of 22 new 

sinkholes. 

documented in the Dover area about .I' ;, 10 miles east of Tampa (Bengtsson, 1987). c__ _ ____ ,_____ __J 

(Metcalf and Hall. 1984) 

During the period of cold weather, 

county officials received numerous 

complaints about new sinkholes. 

Water-level 

altitude 

(feet 1bove 

sea level) 

52 

Air 68 

temperature 

(degrees 

Fahrenheit) 320 

IS 17 

january 1977 

Periods when 

MANY NEW SINKHOLES 

Ground-water pumping for crop freeze protection 

tends to induce sinkholes during the month of 

January in Hillsborough County. 

(Wilson and Shock 1996) 

FREEZING AND PUMPING 

During a 6-day period of record-breaking cold weather, 

ground water was pumped at night when temperatures 

fell below 39° F. 

The new sinkholes were attributed to the movement 

of sandy overburden material through a breached clay 

confining unit into cavities in the limestone below. 

Sinkhole fonmation ceased or 

slowed significantly when water 

levels recovered. 

Sinkholes induced by 

crop freeze protection 

Number of 

reported 

new 40 

sinkholes 

Heavy pumping during 

the dry months of March 

through May also induced 

new sinkholes 

Jan Feb Mar Apr May June Ju~ Aug Sept Oct Nov Dec 

1964 to 1992 

J 

E 

~ 
Sinkholes, West-Central Florida 

"Construction practices often 'set the 

stage' for sinkhole occurrence." 

- ].G. Newton, 1986 

135 

"A giant sink hole opened up on Thursday, September 19 [ 1975} at a drilling site 

near Tampa, Florida and swallowed up a well-drilling rig, a water truck, and a 

trailer loaded with pipe all valued at $100,000. The well being drilled was down 

200ft when the ground began to give way to what turned out to be a limestone 

cavern. Within 10 minutes all the equipment was buried way out of sight in a crater 

measuring 300ft deep, and 300ft wide. Fortunately, the drilling crew had time to 

scramble to safety and no one was hurt." 

-from National Water Well Association newsletter 

One factor confounding the relation between pumping wells and the 

distribution of induced sinkholes is the nonuniform hydraulic connection between the well and various buried cavities. The development of secondary porosity is not uniform. Dissolution cavities 

often form along structural weaknesses in the limestone, such as 

bedding planes, joints, or fractures-places where water can more 

easily infiltrate the rock. The distribution of cavities can be controlled by the presence of these features and thus may be preferentially oriented. It is not uncommon for a pumping well to have more 

impact on cavities that are well-connected hydraulically- although 

farther away from the pumping well-than on nearby cavities that 

are less well-connected hydraulically. Proximity to pumping wells is 

not always a reliable indicator for predicting induced sinkholes. 

When structures such as buildings and roadways are constructed, 

care is usually taken to divert surface-water drainage away from the 

foundations to avoid compromising their structural integrity. Associated activities may include grading slopes and removal or addition 

of vegetative cover, installing foundation piles and drainage systems, 

and ditching for storm drainages and conduits for service utilities. 

The altered landscapes typically result in local changes to established 

pathways of surface-water runoff, infiltration, and ground-water 

recharge. Pavements, roofs, and storm-drainage systems can dramatically increase the rate of ground-water recharge to a local area, 

thus increasing flow velocity in the bedrock and potentially inducing 

sinkholes. A common cause of induced sinkholes in urban areas is 

broken water or sewer pipes. Pipelines strung through karst terrane 

are subject to uneven settling as soils compact or are piped into dissolution cavities. The result can be cracked water pipes or the separation of sewer line sections, further aggravating erosion and 

perpetuating the process. 

Loading by heavy equipment during construction or, later, by the 

weight of the structures themselves may induce sinkholes. A number 

of engineering methods are commonly used to prevent this type of 

sinkhole damage (Sowers, 1984), including drilling and driving pilings into competent limestone for support, injecting cement into 

subsurface cavities, and construction of reinforced and spread foundations that can span cavities and support the weight of the construction. Compaction by hammering, vibratory rollers, and heavy 

block drops may be used to induce collapse so that areas of weakness can be reinforced prior to construction. 
136 

Excessive spray-effluent irrigation 

Inducing sinkholes by surface loading 

In April1988 several cover-collapse sinkholes developed in an 

area where effluent from a wastewater treatment plant is 

sprayed for irrigation in northwestern Pinellas County. The 

likely cause was an increased load on the sediments at land surface due to waste-disposal activities, including periodic 

land spreading of dried sludge 

as well as spray irrigation. The 

118-acre facility is located 

within a karst upland characterized by internal drainage 

and variable confinement between the surficial aquifer system and the Upper Floridan 

aquifer. 

Collapsing Cavities 

Spray-effluent volume applied for 1988 was equivalent to 290 

inches per year (Trommer, 1992). Ponding of effluent occurred as the surficial sediments became saturated. The increased weight or load of the saturated sediments probably 

contributed to the ponding by 

causing some subsidence. At 

the beginning of the rainy season, several cover-collapse 

sinkholes developed suddenly, 

draining the effluent ponds into the aquifer system. 

Sin kholes developed suddenly where water ponded 

due to excessive spray-effluent irrigation. 

Oohn T rommer) 

LINKING SURFACE AND GROUND WATER 

Within several days of sinkhole formation, discharge at 

Health Springs, 2,500 feet downgradient in the ground-water 

flow path, increased from 2 cubic feet per second to 16 cubic 

feet per second (Trommer, 1992). Water-quality sampling of 

the spring during the higher flow detected constituents indicative of the spray effluent. Within 2 weeks, discharge at 

Health Springs had dropped to the normal rate of2 cubic feet 

per second. The existence of a preferential ground-water flow 

path linking the upland spray field with the spring was confirmed by timing the movement of artificially dyed ground 

water between a well in the spray field and the spring (Tihansky and Trommer, 1994). The ground-watervelocity 

based on the arrival time of the dye was about 160 feet per 

day, or about 250 times greater than the estimates of theregional ground-water velocity (0.65 feet per day) in this area. 

The dye-tracer test demonstrates how sinkholes and enhanced secondary porosity can provide a pathway directly 

linking surface-water runoff and the aquifer system. Sinkholes beneath holding ponds and rivers can convey surface 

waters directly to the Upper Floridan aquifer, and the introduction of contaminated surface waters through sinkholes 

can rapidly degrade ground-water resources. 
Sinkholes, West-Central Florida 

Sinkhole collapse beneath a gypsum stack 

Inducing sinkholes by surface loading and pumping 

The sands and clays of the overburden sediments support a 

large phosphate mining and processing industry in westcentral Florida. A gaping sinkhole formed abruptly on June 

27, 1994, within a 400-acre, 220-foot high gypsum stack at 

a phosphate mine. The gypsum stack is a flat-topped pile of 

accumulated phosphogypsum-a byproduct of phosphateore chemical processing. The phosphogypsum precipitates 

when acidic mineralized water (about pH 1.5) used in processing the ore is circulated and evaporated from the top of 

the continually growing stack of waste gypsum. The waste 

slurry of slightly radioactive phosphogypsum results from 

the manufacture of phosphoric acid, a key ingredient in 

several forms offertilizer. 

The sinkhole likely formed from the collapse of a preexisting dissolution cavity that had developed in limestone 

deposits beneath the stack. Its development may have been 

accelerated by the aggressive chemical properties of the 

acidic waste slurry. Infiltration of the applied waste slurry 

into the underlying earth materials was unimpeded because 

there was no natural or engineered physical barrier immediately beneath the stack. Enlargement of cavities by dissolution and erosion combined with the increasing weight of 

the stack would have facilitated the sinkhole collapse. This 

effect may have been exacerbated by the reduction of fluidpressure support for the overburden weight due to localized 

ground-water-level declines; the phosphate industry withdraws ground water from the Upper Floridan aquifer to 

supply water to the ore-refining plant. 

Before the collapse, acidic 

water was ponded on top 

of the stack to evaporate, 

leaving gypsum as a precipitate. 

Gypsum stack 

Acidic water percolated into 

the stack and ground-water 

system, thus accelerating 

development of the sinkhole. 

Rubble from 

the fai led stack 

Horizonal distance not to scale 

The nearly vertical shaft tapered to a diameter of 

about I 06 feet at a depth 

of 60 feet and extended 

more than 400 feet below 

the top of the stack. 

Ground-water samples collected from the Upper 

Floridan aquifer confinmed that the aquifer had 

been locally contaminated with stack wastes. Officials began pumping nearby wells to capture the 

contaminated ground water and prevent its 

movement off-site. 

PREVENTING SINKHOLE COLLAPSE 

There are approximately 20 gypsum stacks located within 

the sinkhole-prone region of west-central Florida and, with 

the exception of new construction, all of these stacks are 

unlined. Because of potential environmental impacts from 

the phosphate industry, the State of Florida created the 

Phosphogypsum Management Rule to manage all aspects 

of phosphate chemical facilities. All new gypsum stacks are 

lined at their bases to impede the infiltration of process 

water and have specially designed water-circulation systems 

to prevent the escape of waste slurry. Ground-water-quality 

and water-level monitoring are also required. Efforts are 

being made to close all unlined stacks and 

reduce impacts on the underlying groundwater system. All new gypsum stacks must 

undergo an assessment of the susceptibility 

to subsidence activity and ground-water contamination. Geophysical surveys are used to 

locate potential zones of weakness so that 

any cavities or preexisting breaches can be 

plugged or avoided. 

137 
138 Collapsing Cavities 

A swarm of sinkholes suddenly appeared on a forest floor 

Development of a new irrigation well triggered hundreds of 

sinkholes in a 6-hour period 

Hundreds of sinkholes ranging in diameter from 1 foot to 

more than 150 feet formed within a 6-hour period on February 25, 1998, during the development of a newly drilled 

irrigation well (a procedure that involves flushing the well in 

order to obtain maximum production efficiency). Unconsolidated sand overburden collapsed into numerous cavities 

within an approximately 20-acre area as pumping and surging operations took place in the well. 

The affected land is located near the coast in an upland region that straddles parts of Pasco and Hernando counties. A 

20-foot-thick sediment cover composed primarily of sand 

with little clay is underlain by cavernous limestone bedrock. 

The well was drilled through 140 feet oflimestone, and a 

cavity was reported in the interval from 148 to 160 feet 

depth, where drilling was terminated. Very shortly after development began, two small sinkholes formed near the drill 

rig. As well development continued, additional new sinkholes of varying sizes began to appear throughout the area. 

Trees were uprooted and toppled as sediment collapse and 

slumping took place, and concentric extensional cracks and 

crevices formed throughout the landscape. The unconsolidated sandy material slumped and caved along the margins 

of the larger sinkholes as they continued to expand. The.first 

two sinkholes to form eventually expanded to become the 

largest of the hundreds that formed during the 6-hour development period. They swallowed numerous 60-foot-tall 

pine trees and more than 20 acres of forest, and left the well 

standing on a small bridge ofland. 

TEST BORINGS AND HYDROGEOLOGIC DATA 

INDICATE SUSCEPTIBILITY TO SINKHOLES 

The affected land contains several ponds formed by sinkholes long ago (paleosinkholes). Because west-central Florida is susceptible to sinkhole development, stability was tested 

Sinkholes induced during the 

development of an irrigation 

well affected a 20-acre area 

and ranged in size from less 

than 1 foot to more than 

150 feet in diameter. 

along the margins of these ponds to determine if the site had 

higher-than-normal risks of sinkhole occurrence. Many test 

borings were made to measure the structural integrity of the 

bedrock, revealing a highly variable limestone surface. Two 

of the borings, approximately 100 feet apart, were made 

within a few hundred feet of the well site. One boring indicated that there was firm limestone at depth, whereas the 

other never encountered a firm foundation. 

Irregularity in the limestone surface is typical of much of 

west-central Florida. Cavities, sudden bit drops, and lost circulation are frequently reported during drilling in this area. 

These drilling characteristics indicate the presence of significant cavernous porosity in the underlying limestone and, 

while commonly noted in drilling logs, only occasionally 

cause trouble during well construction. 

Sinkhole susceptibility in this area is high 

• The area is located within a mantled karst terrane where the 

limestone surface at depth is cavernous and highly irregular; 

the presence of nearby caves and springs suggests that major 

limestone dissolution has occurred. 

• Water-level gradients are downward. 

• Very little clay separates loose sand from limestone below. 

• Previous sinkhole occurrence is well documented; the presence 

of paleosinkholes is evident on topographic maps of the region . 
Sinkholes, West-Central Florida 

Cover-collapse sinkhole 

Winter Park, 1981 

A newly formed sinkhole 20 

miles north of Tampa is being examined by a team of 

scientists. 

(Tom Scott) 

139 

SINKHOLE IMPACTS CAN BE MINIMIZED 

Sinkholes have very localized structural impacts, but they may 

have far-reaching effects on ground-water resources. Sinkholes can 

also impact surficial hydrologic systems-lakes, streams, and wetlands-by changing water chemistry and rates of recharge or runoff. Because the Earth's surface is constantly changing, sinkholes 

and other subsidence features will continue to occur in response to 

both natural and human-induced changes. We have seen how specific conditions can affect the type and frequency of sinkholes, 

including a general lowering of ground-water levels, reduced runoff, increased recharge, or significant surface loading. Recognition of these conditions is the first step in minimizing the impact 

of sinkholes. 

In areas underlain by cavernous limestone with thin to moderate 

thickness of overburden, increased sinkhole development and 

property loss are strongly correlated to human activity and cultural 

development. There are several reasons for this correlation. First, 

rapid growth and development makes it more likely that new sinkholes will be reported, and the construction of roads and industrial 

or residential buildings increases exposure to the risk of property 

damage. Second, land-use changes in rapidly developing areas are 

often loosely controlled and include altered drainage, new impoundments for surface water, and new construction in sinkholeprone areas. Finally, the changing land use is often associated with 

population increases and increasing demands for water supplies, 

which may lead to increases in ground-water pumpage and the 

lowering oflocal and regional ground-water levels. 

Although we cannot adequately predict sinkhole development, we 

may be able to prevent or minimize the effects of sinkholes or reduce their rate of occurrence. Well-documented episodes of accelerated sinkhole activity are directly related to ground-water 

pumping events that lower ground-water levels. In many instances, 

the changes in ground-water levels are only a few tens of feet. It is 
140 Collapsing Cavities 

likely that many induced sinkholes can be prevented by controlling 

fluctuations in ground-water levels. 

The overall regional decline in water levels in the Upper Floridan 

aquifer has been a long-standing concern of water-resource managers. Local declines around municipal well fields, often much greater 

than the regional declines, have led to dewatering of lakes and wetlands, upconing of poorer-quality water, saltwater intrusion, and 

accelerated sinkhole development. The Southwest Florida Water 

Management District has been working with other water-resources 

agencies to establish critical levels for ground water within the westcentral Florida area. The establishment of minimum ground-water 

levels will help minimize sinkhole impacts by ameliorating some of 

the conditions that cause them. 

Land-use planners, resource managers, and actuaries have been able 

to estimate the probability of sinkhole occurrence and associated 

risks. The Florida Department of Insurance designed insurance 

premiums for four sinkhole probability zones (Wilson and Shock, 

1996) on the basis of insurance claims for sinkhole damages and 

hydrogeologic conditions. West -central Florida was delineated as an 

area having the highest frequency of sinkhole activity. The use of 

scientific information to assess risks and establish insurance rates 

demonstrates the benefits of understanding the hydrogeologic 

framework and potential effects of water-resource development. 

This scientific understanding is key to assigning meaningful risks to 

both property and the environment, and essential for formulating 

effective land- and water-resources management strategies. 
THE RoLE oF SciENCE 

Land Subsidence in the United States 

Scientists use equations that 

represent physical and chemical processes to analyze subsidence. 

Ground-water flow 

Oxidation of organic matter 

Devin Galloway, S.E. lngebritsen, and 

Francis S. Riley 

U.S. Geological Survey, 

Menlo Park, California 

Marti E. lkehara 

National Geodetic Survey, 

Sacramento, California 

Michael C. Carpenter 

U.S. Geological Survey, 

Tucson, Arizona 

T he Panel on Land Subsidence of the U.S. National Research 

Council (NRC) ( 1991) recognized three information needs: 

"First, basic earth-science data and information on the magnitude and distribution of subsidence [. .. 1 to recognize and to assess 

future problems. These data [. .. 1 help not only to address local subsidence problems but to identify national problems. [ ... 1 Second, research 

on subsidence processes and engineering methods for dealing with 

subsidence [. .. 1 for cost -effective damage prevention or control. [ ... 1 

And third, although many types of mitigation methods are in use in 

the United States, studies of their cost-effectiveness would facilitate 

choices by decision makers." (emphases added) 

The third need can only be met after we learn how to better measure the total impact of subsidence problems and the effectiveness 

of our attempted solutions. It is clear that in order to assess the total 

impact we would need to inventory the total costs to society of 

overdrafting susceptible aquifer systems. Presently this is impractical because there are only sparse estimates of subsidence costs, and 

most of these are directly related to damages to tangible property. 

Additional consideration could be given to many of the indirect 

costs of excessive ground-water withdrawal and subsidence. In particular, it is our impression that the impact of subsidence on our 

surface-water resources and drainage-riparian and wetland habitat, drainage infrastructure, and flood risk-is large. Though much 

knowledge could be gained from risk-benefit analyses that include 

the indirect costs of subsidence, in this concluding chapter we focus 

on the role of science as identified by the NRC panel-recognition 

and assessment of subsidence, research on subsidence processes, 

and mitigation methods. 

RECOGNITION 

The occurrence ofland subsidence is seldom as obvious as it is in 

the case of catastrophic sinkholes such as those in Winter Park, 

Florida, or at the Retsof Salt Mine in Genesee Valley, New York. Dis-
142 

An abandoned water-supply 

well protrudes above ground 

in Las Vegas, 1997. 

(Pictured is Terry Katzer) 

Subsidence in the United States 

covery of such catastrophic subsidence is difficult only when the 

localized collapse occurs in a remote area. Where ground-water 

mining or drainage of organic soils are involved, the subsidence is 

typically gradual and widespread, and its discovery becomes an 

exercise in detection. Gazing out over the San Joaquin Valley, California, one would be hard-pressed to recognize that more than 30 

feet of subsidence has occurred in some locations. In the absence 

of obvious clues such as protruding wells, failed well casings, broken pipelines, and drainage reversals, repeat measurements of 

land-surface elevation are needed to reveal the subsidence. 

The problem of detection in regional land subsidence is compounded by the large areal scale of the elevation changes and the 

requirement for vertically stable reference marks-bench markslocated outside the area affected by subsidence. Where such stable 

bench marks exist and repeat surveys are made, subsidence is fairly 

easily measured using professional surveying instruments and 

methods. In fact, this is one of the common ways in which subsidence is first detected. Often, public agencies or private contractors 

discover that key local bench marks have moved only after repeat 

surveys that span several years or longer. Prior to the discovery, 

when the cumulative subsidence magnitude is small, the apparent 

errors in the surveys may be adjusted throughout the network under the assumption that the discrepancies reflect random errors of 

the particular survey. The subsidence may then go undetected until 

later routine surveys, or until suspicions arise and steps are taken 

to confirm the current elevations of the affected bench marks. 

Subsidence is sometimes obvious 

Protruding well casings are common in agricultural areas and 

some urban areas where ground water has been extracted from 

alluvial aquifer systems. The land surface and aquifer system are 

displaced downward relative to the well casing, which is generally 

anchored at a depth where there is less compaction. The stressed 

well casings are subject to failure through collapse and dislocation. 

Submersible pumps, pump columns, and the well itself may be 

damaged or require rehabilitation. Deep wells are most vulnerable 

and are also the most expensive to repair and replace. Typical repair costs amount to $5,000-$25,000 or more, and replacement 

costs are in the range of $40,000-$250,000! Where the frequency 

of well-casing failures is high, land subsidence is often suspected 

and is often the cause. 

The formation of earth fissures in alluvial aquifer systems is another indication that compaction and land subsidence may be occurring. Other possible indicators ofland subsidence include 

changes in flood-inundation frequency and distribution; stagnation or reversals of streams, aqueducts, storm drainages, or sewer 

lines; failure, overtopping or reduction in freeboard along reaches 

of levees, canals, and flood -conveyance structures; and, more generally, cracks and/or changes in the gradient oflinear engineered 

structures such as pipelines and roadways. 
The Role of Science 

Drought conditions in the Sacramento Valley during 1976-77 

reduced the amount of surface 

water available for irrigation 

and, for the first time, more 

ground water than surface water was used to irrigate crops. 

During the summer of 1977 

many irrigation wells that penetrated the valley-fill deposits 

were damaged. Most of the 

damaged wells occurred in the 

southwestern part of the valley. 

The damage seems to have 

been caused by compaction of 

the aquifer system which resulted in the vertical compression and rupture of well casings. 

0 IOMiles 

,-------, 

0 I 0 I(Jiometers 

This bench mark was established in 1995 to monitor 

potential land subsidence in 

the Antelope Valley, Mojave 

Desert, California. 

.. • 

• • • • J" • • 

• ~ 

• • (' "' "' <::.. 

• ~ ?~ • • ? e-7 ~'o 

• <" ~ ...... _, <" ~ 

• A I'" o & (') 

•• • 0 

···'. "' ~ . f • •• 

. • ... •••• ., .... r+ . .,. • • L 

. . ' • • 7'l • -· "' . '. '· -::> • -<. 

• • • • crO •• •• (1) 

. '···. Intact well 

"' ... ,... Damaged well ' 

ASSESSMENT 

143 

Some correlation was observed 

between subsidence and the 

compression of nearby well casings . 

A Feet 

A' 

4- ---- Subsidence 

-

2 

-

0 

0 

,h I, 

4 

Casing 

compres sion 

I -

8 12 

Miles 

(Borchers and others, 1998) 

Differential surveys measure relative changes in the position of the 

land surface. The observable position is typically a geodetic mark 

that has been established to some depth (usually greater than 10 feet 

when in soil), so that any movement can be attributed to deepseated ground movement and not to surficial effects such as frost 

heave. Sometimes geodetic marks, especially those used to measure 

vertical movement (bench marks), are established in massive artificial foundations, such as bridge abutments, that are well-coupled to 

the earth. Any vertical or horizontal movement of a geodetic mark is 

measured in relation to other observation points. When the bench 

mark can be assumed to be stable or its movement is otherwise 

known and measurable, it can be used as a control point, and the 

absolute position of the observation point can be determined. By 

this method, land subsidence has been measured using repeat surveys of bench marks referenced to some known, or presumed stable, 

reference frame. Access to a stable reference frame is essential for the 

measurements needed to map land subsidence. In many areas where 

subsidence has been recognized, and other areas where subsidence 

has not yet been well documented, accurate assessment has been 

hindered or delayed by the lack of a sufficiently stable vertical reference frame (control). 

Known positions are linked into a network 

"Sufficiently stable" is a somewhat relative term that has meaning in 

the context of a particular time-frame of interest and magnitude of 

differential movement. Because of continuous and episodic crustal 
144 

For more information on geodetic control, 

visit the National Geodetic Survey web site at 

http:/ /www.ngs.noaa.gov/faq.shtml 

Subsidence in the United States 

motions caused mostly by postglacial rebound, tectonism, volcanism, and anthropogenic alteration of the Earth's surface, it is occasionally necessary to remeasure geodetic control on a national scale. 

Networks of geodetic control consist of known positions that are 

determined relative to a horizontal or vertical datum or both. 

Two reference networks are used for horizontal and vertical geodetic control for the United States, the North American Datum of 

1983 (NAD83) and the North American Vertical Datum of 1988 

(NAVD88). NAD83 replaces the older North American Datum of 

1927 (NAD27) and is the current geodetic reference system for 

horizontal control in the United States, Canada, Mexico, and Central America. It is the legally recognized horizontal control datum 

for the Federal government of the United States and for 44 of the 50 

individual States. NAVD88 replaces the National Geodetic Vertical 

Datum of 1929 (NGVD 1929), which was based on local mean sea 

levels determined at 26 tidal gauges. The principal sea-level reference for NAVD88 is the primary tidal gauge at Father Point/ 

Rimouski, Quebec, Canada. The vertical datum is based on the 

Earth's geoid-a measurable and calculable surface that is equivalent to mean sea level. 

In partnership with other public and private parties, the National 

Geodetic Survey (NGS) has implemented High Accuracy Reference 

Networks (HARNs) in every State. HARN observation campaigns 

(originating in Tennessee in 1986 and ending in Indiana in 1997) 

resulted in the establishment of some 16,000 survey stations. The 

updated networks were needed not only to replace thousands of 

USGS survey party spirit lev- historic bench marks and horizontal-control marks lost to develop- eling near Colusa, Sacramento Valley, in 1904 and ment, vandalism, and natural causes, but also to provide geodetic 

their field notes. r~=-~::-:=~------ monuments easily accessible by roadways. These updated reference 

- ., .. ,, " , . .,, networks will facilitate the early and accurate detection and mea- - A ".:J..J t6. ....t ~· ~~-"-: "'p .oIl:•. - "'' -- ···--... surement of land subsidence. ~~~·~ .. ':···-7:~r±----=~--~ 

Qohn Ryan, donated 

courtesy ofThomas E. Ryan) 

Spirit leveling was once a common method 

of determining elevation 

Before the advent of the satellite-based Global Positioning System (GPS) in the 1980s, the most common 

means of conducting land surveys involved either 

the theodolite or, since the 1950s, the geodimeter (an 

electronic distance measuring device, or EDM). If only 

vertical position were sought, the spirit level has been 

the instrument of choice. The technique of differential 

leveling allows the surveyor to carry an elevation from a 

known reference point to other points by use of a precisely 

leveled telescope and graduated vertical rods. Despite its 

simplicity, this method can be very accurate. When surveying to meet the standards set for even the lower orders of 

accuracy in geodetic leveling, 0.05-foot changes in elevation 

can be routinely measured over distances of miles. At large 

scales, leveling and EDM measurement errors increase. When 
The Role of Science 

A full constellation of the Global 

Positioning System (GPS) includes 24 satellites in orbit 

12,500 miles above the Earth . 

The satellites are positioned so 

that we can receive signals 

from six of them at any 

one time from any point 

on the Earth. 

A GPS antenna mounted on a 

tripod at a known distance 

above a geodetic mark near 

Monterey, California, receives 

signals from GPS satellites. The 

operator is entering station information into a receiver that 

stores the signals for later processing. 

Horizontal control station 

• Vertical control station 

• GPS-network bench mark 

- GPS network vector 

Subsidence (feet) 

1930- 1992 

0 I to3 

03to5 

• Greater than 5 0 

Qay Prendergast. 1992) 

145 

the scale of the survey is small (on the order of 5 miles or less) and 

the desired spatial density is high, spirit leveling is still commonly 

used because it is accurate and relatively inexpensive. Large regional 

networks warrant use of the more efficient Global Positioning System (GPS) surveying for differential surveys. 

GPS-Giobal Positioning System uses Earth-orbiting satell ites 

A revolution in surveying and measurement of crustal motion occurred in the early 1980s when tests of the satellite-based NAVSTAR 

GPS showed that it was possible to obtain 1 part in 1 million precision between points spaced from 5 to more than 25 miles apart. GPS 

uses Earth-orbiting satellites to trilaterate positions based on the 

time required for radio signals transmitted from satellites to reach a 

receiving antenna. An accurate three-dimensional position can be 

determined from trilateration of the range distances between the 

receiver and at least four satellites. Since July 17, 1995, NAVSTAR has 

been operational with a full constellation of 24 satellites, and in 

North America provides essentially continuous coverage with at least 

6 satellites in view. Guidelines have been formulated for establishing 

GPS-derived ellipsoid heights with accuracy standards at either the 

2-cm (.0656 ft) or the S-cm (.164ft) level (Zilkoski and others, 1997). 

In land-subsidence and other crustal-motion surveys, the relative 

positions of two points can be determined when two GPS receivers, 

one at each observation point, receive signals simultaneously from 

the same set of 4 or more satellites. When the same points are reoccupied following some time interval, any relative motion between 

the points that occurred during the time interval can be measured. 

Geodetic networks of points can be surveyed in this fashion. Such a 

network, one of the first of its kind designed specifically to monitor 

land subsidence, was established in the Antelope Valley, Mojave 

/ 

\/ \_ 

Antelope Valley~) ) / 

20 Miles 

20 Kilometers 

) 

\ 

I ~- ........ 

This geodetic network was used 

to measure historical subsidence 

in Antelope Valley, Mojave 

Desert, California. 

Geodetic surveying of 85 stations in Antelope Valley using 

GPS requ ired about 150 persondays during 35 days of observation in 1992. Results from the 

GPS surveys and conventional 

leveling surveys spanning more 

than 60 years showed a maximum subsidence of about 6.6 

feet; more than 200 square 

miles had subsided more than 2 

feet since about 1930 (lkehara 

and Phillips, 1994). 
146 

Pulley cable 

Fulcrum arm 

Dial gauge 

Reference surface 

Counter weights 

Extensometer 

pipe 

Pedestal 

Part of a two-stage 

counter-weighted pipe extensometer that measures 

compaction in a shallow 

aquifer near Lancaster, 

This two-stage, counterweighted pipe extensometer 

measures compaction simultaneously in shallow and deep 

aquifers in Antelope Valley, 

Mojave Desert, California. 

As the aquifer system compresses, the land surface subsides along with the extensometer table. The 

extensometer pipe anchored 

deeper in the aquifer system 

appears to rise relative to the 

table. This relative movement 

represents the amount of vertical displacement occurring in 

the aquifer system between 

the shallow-seated piers supporting the table and the bottom of the extensometer pipe. 

Subsidence in the United States 

Desert, California in 1992 (Ikehara and Phillips, 1994). It was designed to determine the subsidence of previously leveled bench 

marks and enable precise measurement of points separated by tens 

of miles for future subsidence monitoring. Other large GPS-based 

geodetic networks for subsidence monitoring have been established 

in Albuquerque, New Mexico; the Avra Valley, Arizona; Las Vegas, 

Nevada; the Lower Coachella Valley, California; the Sacramento-San 

Joaquin Delta, California; and the Tucson basin, Arizona. GPS surveying is also a versatile exploratory tool that can be used in a rapid 

mode to quickly but coarsely define subsidence regions, in order to 

site more precise, site-specific and time-continuous measurement 

devices such as extensometers. 

Extensometers measure subsidence and horizontal displacement 

Borehole extensometers generate a continuous record of change in 

vertical distance between the land surface and a reference point or 

"subsurface bench mark" at the bottom of a deep borehole (Riley, 

1986). In areas undergoing aquifer-system compaction, the extensometer is the most effective means of determining precise, continuous deformation at a point. If the subsurface bench mark is 

established below the base of the compacting aquifer system, the 

extensometer can be used as the stable reference or starting point for 

local geodetic surveys. Designs that incorporate multiple-stage extensometers in a single instrument are being used to measure aquifer-system compaction simultaneously in different depth intervals. 

SHALLOW EXTENSOMETER DEEP EXTENSOMETER 

Analog chart 

recorder 

instrument 

table 

"'----U.. Dial gauge -Shallow-seated 

Reference surface pier 

Concrete slab 

.,. . 

, . • ' I I 

•' 

. . . · · .. : . · . ShanoV.: oquif~r . ·. · · · · . . · : 

· .. Exte~someter P,ipe: · · · 

; .. . :. ' : -. ~ .. ' ; -. .·· '[)eepaquifer. · ... ·.:,:; ·:: ·' i:. 

; ' . 

. . .. . '· .. ._ - '. ; - Not to scale 

. · .. · 
The Role of Science 

Measurements of water level 

and compaction from borehole 

extensometers form the basis of 

stress-strain analysis. These data 

are from the Hueco Basin, El 

Paso, Texas . 

This quartz-tube horizontal extensometer is being installed 

near Apache Junction, Arizona. 

The quartz tube is placed inside 

the pipe housing and attached 

to a post on one side of a fissure. A displacement sensor, 

such as a dial gage, is attached 

to the post on the opposite side 

of the fissure and pushes against 

the quartz tube. Fissure opening 

and closing is observed in the 

dial-gage reading. An electronic 

sensor can be substituted for the 

dial gage for continuous measurement of fissure movement. 

( hal'ge m appl1ed 

st '"ss measured as 

c.,angc m depth to 

w te-r level 

:teet' 

Change in applied 

stress measured as 

change in 

water level 

(feet) 

-1--_;_'IJ""'r Pipe housing 

Quartz tube 

-:::iilrr"-'--i"if- Post 

2 

0 

2 

0 

2 Compaction 

(thousandths of feet) 

4 

Vertical strain x I I I ,000,000 

0 4 8 12 

rends related to permanent 

V -"'";:;;; 4' . .t-- ---fil'--:t.!':ir.6c;_- compaction of the aquifer system 

. 

9/2/93 

Trends related to recoverable expansion 

and compression of the aquifer system 

0 I 2 3 4 

Compaction (thousandths of feet) 

._ Relative expansion Relative compression _. (Heywood, 1995) 

147 

As a stand-alone instrument, the borehole extenso meter may be 

regarded simply as a sentinel against the undetected onset of unacceptable rates of aquifer-system compaction. However, when used 

in conjunction with good well logs and water-level data from an 

adjacent observation well, the deformation history generated by an 

extensometer can provide the basis for stress-strain analysis (Riley, 

1969) and inverse modeling that defines the average compressibility 

and vertical hydraulic conductivity of the aquitards (Helm, 1975). 

This capability derives from the fact that the compaction measured 

by the extensometer is directly related to the volume of water produced by the aquitards. Major improvements in stability and sensitivity allow recently constructed extensometers to record the minute 

elastic compression and expansion that inevitably accompany even 

very small fluctuations in ground-water levels in unconsolidated 

alluvial aquifer systems, as well as the relatively large deformations 

typical of the irreversible compaction of aquitards. Reliable estimates of aquitard properties are necessary for constraining predictive modeling, whether the objective is the prevention or mitigation 

ofland subsidence or simply the optimal use of the storage capacity 

of the aquifer system. 

Several kinds of horizontal extensometers measure differential horizontal ground motion at earth fissures caused by changes in groundwater levels (Carpenter, 1993). Buried horizontal extensometers 

constructed of quartz tubes or invar wires are useful when precise, 

continuous measurements are required on a scale of 10 to 100 feet. 

Tape extensometers measure changes across intermonument distances up to 100 feet with a repeatability of 0.01 inches. The tape 
148 

The extensometer is attached at both ends 

to a monument (yellow cylinder), which is a 

I 0-foot rod driven into the ground and encased with a concrete-filled pipe for stability. 

Tape extensometers measure 

horizontal ground motion over 

distances of up to 100 feet. 

Dial gage for measurement along tape 

Subsidence in the United States 

Dial gage for tape tension 

extensometer is used in conjunction with geodetic monuments specially equipped with ball-bearing instrument mounts, which can 

serve as both horizontal and vertical control points. Arrays or lines 

of monuments can be extended for arbitrary distances, usually in 

the range of 200 to 600 feet. 

Radar interferometry is a new tool for measuring subsidence 

Interferometric Synthetic Aperture Radar (InSAR) is a powerful 

new tool that uses radar signals to measure deformation of the 

Earth's crust at an unprecedented level of spatial detail and high 

degree of measurement resolution. Geophysical applications of radar interferometry take advantage of the phase component of reflected radar signals to measure apparent changes in the range 

distance of the land surface (Gabriel and others, 1989; Massonnet 

Radar* is an active sensor, transmitting a signal of electromagnetic energy. Satellite-borne radar using one 

antenna transmits a pulsed train of 

microwaves. 

Simplified geometry for a European Remote Sensing (ERS) Satellite Synthetic Aperture Radar (SAR) imaging can "synthesize" an effectively 

larger antenna (about 3 miles long) 

with a spatial resolution on the order 

of 16 feet by pulsing the microwaves 

every 16 feet of satellite travel. The waves reflect off the ground 

surface, and echoes are received by 

the moving antenna, producing a 

recorded image of the scanned 

ground that is continuous along the 

track of the satellite and about 60 

miles wide. 

The restricted size of the satellite 

antenna limits the spatial resolution 

to 3 to 6 miles on the ground. 

• RADAR: RAdio Detection And Ranging 

Side-looking, L/"J / 

slanted antenna ~ To L ~ 32.81 feet (10m) 

Range distance 

to center of 

swath ~ 5 28 miles 

1 D ~ 3.28 feet (1 m) 

Radar ''line-of-sight'' 

Wavelengths ~ 2.2 inches 

F~c:- 0 0 C/ 0 9 Frequency ~ 5.3 billion 

cycles per second 

Not to scale 

The 3- to 4-mile-wide footprints overlapped at 16-foot intervals along the 

ground track are processed through a 

technique similar to medical x-ray imaging. Numerous 16-foot echoes are 

averaged to improve signal coherence, and the actual spatial resolution 

is on the order of 260 feet or better. 

lnterferograms are made by 

differencing successive SAR images 

taken from the same orbital position 

but at different times. Under favorable radiometric conditions 1 / 2-inch 

to 1/1 0-inch resolution is possible in 

the line-of-sight (range) of the radar. 
The Role of Science 

Differential interferogram 

made from lnSAR images 

acquired June 1993 and 

March 1997 over the Tucson and Green Valley areas 

of Arizona. The interferegram (center) is shown 

overlain on the radar amplitude image. 

One color cycle represents 

about 1.1 inch of subsidence. 

More than two cycles can be 

seen on this Tucson image. 

0 

0 4 Kilometers 

4 Miles 

149 

and Feigl, 1998). Ordinary radar on a typical Earth-orbiting satellite 

has a very poor ground resolution of about 3-4 miles because of the 

restricted size of the antenna on the satellite. Synthetic Aperture 

Radar (SAR) takes advantage of the motion of the spacecraft along 

its orbital track to mathematically reconstruct (synthesize) an operationally larger antenna and yield high-spatial-resolution imaging 

capability on the order of tens of feet. The size of a picture element 

(pixel) on a typical SAR image made from satellite-borne radar may 

be as small as 1,000 square feet or as large as 100,000 square feet, 

depending how the image is processed. 

For landscapes with more or less stable radar reflectors (such as 

buildings or other engineered structures, or undisturbed rocks and 

ground surfaces) over a period of time, it is possible to make highprecision measurements of the change in the position of the reflectors by subtracting or "interfering" two radar scans made of the 

same area at different times. This is the principle behind InSAR. 

Under ideal conditions, it is possible to resolve changes in elevation 

on the order of 0.4 inches ( 10 mm) or less at the scale of 1 pixel. 

Interferograms, formed from patterns of interference between the 

phase components of two radar scans made from nearly the same 

antenna position (viewing angle) but at different times, have demonstrated dramatic potential for high-density spatial mapping of 

ground-surface deformations associated with tectonic (Massonnet 

and others, 1993; Zebker and others, 1994) and volcanic strains 

(Massonnet and others, 1995; Rosen and others, 1996; Wicks and 

(Data acquisition and interferometric processing by the NPA Group, (rockham 

Park. Eden bridge. Kent TNS 6SR. UK Image copyright NPA 1998. Image data 

copyright European Space Agency 1993, 

1997. For information contact 

ren@npagroup.co.uk) 
150 Subsidence in the United States 

Extraction of oil, brine, and 

ground water from the Lost 35-DAY INTERFEROGRAM 8-MONTH INTERFEROGRAM 

Hills and Be/ridge oil reservoirs in the San Joaquin Valley, California, caused compaction, forming surface 

subsidence bowls. Near Lost 

Hills more than 1.5 inches 

of subsidence occurred in 

35 days and nearly 3 inches 

occurred in 8 months. 

0 0.8 1.6 

Subsidence (inches) 

A Lost Hills oil field A' 

Approximate 

rate of change 

0 _..... '1 ' •• ~,o 

.~~fornia ~ 

0 4 8 

Subsidence (inches) 

in land-surface O.Ol _ altrtude 

(inches 

per day) 0.04 -

red primarily in agricultural 

fields where the ground surface 

had been significantly modified 

by plowing or crop growth. 

Aqueduct _ 

35-day -

8-month -

4 6 

Distance (miles) 

The Lost H ills o il field is 

near Interstate 5 and 

the California Aqueduct. 

(Fielding and others. 1998) 

others, 1998). InSAR has also recently been used to map localized 

crustal deformation and land subsidence associated with geothermal fields in Imperial Valley, California (Massonnet and others, 

1997), Long Valley, California (W. Thatcher, USGS, written communication, 1997), and Iceland (Vadon and Sigmundsson, 1997), and 

with oil and gas fields in the Central Valley, California (Fielding and 

others, 1998). InSAR has also been used to map regional-scale land 

subsidence caused by aquifer-system compaction in the Antelope 

Valley, California (Galloway and others, 1998), Las Vegas Valley, NeDifferent methods of measuring land subsidence 

METHOD Component Resolution 1 

displacement (milimeters) 

Spirit level vertical 0 .1-1 

Geodimeter horizontal 

Borehole vertical 0 .01-0.1 

extenso meter 

Horizontal 

extensometer: 

Tape horizontal 0.3 

lnvar wire horizontal 0.0001 

Quartz tube horizontal 0.00001 

GPS vertical 20 

horizontal 5 

lnSAR range 10 

1 Measurement resolution attainable under optimum 

conditions. Values are given in metric units to conform with standard geodetic guidelines. (One inch 

is equal to 25.4 millimeters and 1 foot is equal to 

304.8 millimeters.) 

2Number of measurements generally necessary to 

define the distribution and magnitude of land subsidence at the scale of the survey. 

3 A pixel on an lnSAR displacement map is typically 

40 to 80 meters square on the ground. 

Spatial density2 Spatial scale 

(samples/survey) (elements) 

10- 100 line-network 

10-100 line-network 

1-3 point 

1-10 line-array 

line 

line 

10-100 network 

100,000- map pixe/ 3 

10,000,000 
The Role of Science 

An acoustic profile taken 

across a sinkhole lake in westcentral Florida shows how 

high-resolution seismic-reflection techniques can image 

geologic characteristics associated with subsidence. 

(T1hansky and others. 1996) 

151 

vada (Amelung and others, 1999), and Santa Clara Valley, California 

(Ikehara and others, 1998). 

RESEARCH ON SUBSIDENCE PROCESSES 

The areal and vertical distribution of subsidence-prone materials, 

their current state of stress, and their stress history govern the potential for subsidence. These factors vary in importance and can be 

determined with varying degrees of difficulty for the three major 

types of subsidence considered in this Circular. 

In the case of organic-soil subsidence (oxidation), the subsidenceprone material is generally surficial, and both thickness and areal 

extent are often readily mapped. The state of stress and the stress 

history are largely irrelevant, as the subsidence rate is mainly determined by the degree of drainage. In aquifer-system compaction, the 

subsidence-prone (fine-grained) material is buried and must be 

mapped indirectly by drilling, sampling, assembling drilling logs of 

the subsurface lithology, and by various borehole and surface geophysical techniques. These methods produce spatially discrete information-often one-dimensional or, in the case of surface geophysics, 

quasi-two dimensional with integrated depth information. The interpretation is often ambiguous and extrapolation of the spatially 

limited data to other areas of interest is laden with uncertainty, making the mapping imperfect. Mapping of subsidence-prone materials 

is perhaps most difficult for those materials subject to catastrophic 

collapse, because the failures are so localized and frequently evolve 

over short time scales. Acoustic profiling has been used successfully 

to map possible locations of buried cavities in west-central Florida. 

For both aquifer-system compaction and cavity collapse, the current 

stress and stress history are critically important. 

Water 

Lake bottom 

Recent lake Intact sedimentary structure 

sediments with little disruption 

Units have failed or have 

been disrupted, leaving little 

identifiable geologic structure. 

Dipping refiectors along margins of buried cavities where 

geologic units are deformed 

and are sagging into underlying 

void space 
152 

Preconsolidation stress is usually deduced from paired timeseries of ground-water levels 

and subsidence. Estimates of 

preconsolidation stress such as 

these from Antelope Valley, 

Mojave Desert, California, are 

used to evaluate the historical 

compaction of aquifer systems. 

Subsidence in the United States 

In typical alluvial ground-water basins, an accurate initial estimate 

of preconsolidation stress (critical head) is particularly important 

for successful evaluation of the historical compaction of aquifer 

systems (Hanson, 1989; Hanson and Benedict, 1994). Prior to the 

development of ground-water resources in a basin, the alluvial sediments are typically overconsolidated-the initial preconsolidation 

stress of the aquifer system is larger than the intergranular or effective stresses. Land subsidence becomes obvious only after substantial water-level drawdowns have caused increased intergranular 

stresses and initiated inelastic compaction. Holzer (1981) identified 

a variety of natural mechanisms that can cause such an 

overconsolidated condition in alluvial basins, including removal of 

overburden by erosion, prehistoric ground-water-level declines, 

desiccation, and diagenesis. Few investigations have examined the 

elastic responses of the aquifer system to changes in effective stress 

under natural conditions, before large-scale ground-water withdrawal has begun to cause irreversible subsidence. As a result, information on critical aquifer hydraulic head, representing the native 

preconsolidation stress of the system, is usually deduced from 

paired time-series of ground-water levels and land subsidence 

(Holzer, 1981; Anderson, 1988, 1989) measured at wells and nearby 

bench marks, or inferred from ground-water-flow models (Hanson 

and others, 1990; Hanson and Benedict, 1994). 

Similar uncertainties exist for systems that have undergone some 

period of lowered ground-water levels and land subsidence followed 

by ground-water-level recovery and slowing or cessation of subsidence. The problem of determining the new preconsolidation stress 

thresholds in these aquifer systems is equally as difficult as determining the native preconsolidation stresses in undeveloped aquifer 

systems. The difficulty is compounded when the developed aquifer 

systems contain thick aquitards affected by hydrodynamic lag. 

Preconsolidation stress 

was exceeded some time 

during this period 

Estimated range of 

predevelopment (native) 

preconsolidation stress 

1920 1940 

8 

1960 

Bench marks 

0 Established 1929 

0 Established 19 30 

0 Established 1961 

0 

0 

1980 

Change in 

land-surface 

altitude 

(feet below 

original surveyed 

elevation) 
The Role of Science 

This three-layer digital model of Antelope 

Valley, Mojave Desert, California is a mathematical representation of the physical processes of ground-water flow and aquifersystem compaction. Separate model layers 

represent different depth horizons within 

the aquifer system. Flow and compaction 

properties are specified for each of the 

2,083 active cells in the model. 

The computer model solves for hydraulic 

head for each cell and computes groundwater fluxes between cells and within individual cells between the elastic and inelastic 

storage components. These values are then 

used to calculate the amount of compaction, if any, for each cell. The total amount 

of compaction in all three layers is the computed land subsidence at that location. 

Differences between measured and simulated compaction and water levels are minimized through a history-matching process. 

Model parameters are iteratively adjusted 

to find the best match between simulated 

and measured values. Once the set of possible model parameters is constrained by 

the history match, the model may be used 

cautiously to predict future land subsidence. 

In this example, 18 sites were used to 

match water levels and 10 sites were used 

to match compaction; one of each is 

shown. 

Compaction 

(inches) 

153 

0 

Antelope Valley-........_ 

30 t 

Model layer 2 

(intermediate) 

40 . Model layer 3 

Tl. (deep) 

I~J. LTt 

0 

2 

~ 2300 "S- • ,/ Measured 

Simulated .,. • 

~~2200 

.--.---r---,--,---.---,----,r--> 2 I 00 

Water-level 

altitude 

(feet above 

sea level) 

1915 1935 1955 1975 1995 1915 1935 1955 1975 1995 

Simulation models are useful analytical tools 

Since the advent of high-speed digital computers, scientists have 

had the ability to numerically simulate the flow of ground water 

and associated aquifer-system compaction in multiple dimensions. 

In actual practice, ground-water flow may be simulated in one, two, 

or three dimensions, but compaction is typically simulated as a 

one-dimensional process (Helm, 1978). Though poroelastic theory 

developed by Biot (1941) provides a means for coupling groundwater flow and skeletal deformation in three dimensions, scientists 

commonly invoke the one-dimensional theory of hydrodynamic 

consolidation developed by Terzaghi (1925), which is described in 

some detail in the introduction to the section on "Mining Ground 

Water:' Multidimensional flow of ground water is described by a 

variant of the well-known diffusion equation that also describes 

conduction of heat and electricity. 

Simulation models can aid visualization of complex three-dimensional processes. They are important analytical tools, and can also 

be used to help devise data-acquisition and water-management 

strategies. Though simulation models are powerful tools, it is im-
154 

Simulated and lnSAR-detected land subsidence in the 

Antelope Valley are compared for the time period October 1993-December 1995. 

CALIFORNIA 

Subsidence in the United States 

portant to recognize their limitations. The common assumption of 

one-dimensional consolidation is motivated by an obvious truism: 

most aquifer-system compaction takes place in the vertical dimension. Nevertheless, the widespread occurrence of earth fissures indicates that horizontal deformation can be locally significant. A more 

general and more important limitation of simulation models is that 

their solutions are nonunique. The relevant hydrogeologic parameters (permeabilities, compressibilities, and boundary conditions) 

are never exactly known, which would be required for a unique solution. Nevertheless, simulation models may be used-with caution-in a predictive mode, and there are formal procedures for 

dealing with parameter uncertainties. 

lnSAR images offer new insights 

In the Antelope Valley, Mojave Desert, California, a radar interferogram for the period October 20, 1993 to December 22, 1995 revealed up to 2 inches of subsidence in areas previously affected by as 

much as 6 feet of subsidence since 1930 (Galloway and others, 

1998). The regions of maximum subsidence detected during the 26-

month period correlated well with declining ground-water levels. In 

another part of Antelope Valley formerly affected by ground-water 

depletion and subsidence, but where ground-water levels recovered 

throughout the 1990s, about 1 inch of subsidence was detected on 

the interferogram. This suggests residual (time-delayed) compaction due to the presence of thick aquitards. Computer simulations of 

aquifer-system compaction compared favorably with the subsidence 

detected by the interferogram for the same period. The computer 

simulation was weakly constrained due to the scarcity of conventional field measurements; these results highlight the potential use 

of spatially detailed InSAR subsidence measurements to provide 

better constraints for computer simulations of land subsidence. 

I 0 K1lometers 0 0.4 0.8 I .2 1.6 2 

Subsidence (inches) 
The Role of Science 

One color cycle represents 

about 4 inches of subsidence. 

0 5 M1les 

0 5 K•lometers 

This large subsidence bowl is 

bounded on the southeast by the 

Eglington fault. 

~.,.,.,..,~--~J 

155 

This lnSAR-based surface-deformation map 

shows subsidence in the Las Vegas Valley 

between April1992 and December 1997. It 

was obtained by stacking or summing the 

three time-sequential interferograms appearing on the cover of this Circular. The 

subsidence is caused by aquifer-system 

compaction and controlled in part by the 

surface faults, which have also been the focal point of earth-fissure formation 

(Amelung and others, 1999). 

New InSAR-based surface-deformation maps for Las Vegas Valley 

demonstrate the intimate connection between faults and subsidence. An interferogram for the Las Vegas Valley between April21, 

1992, and December 5, 1997, delineates two main features-a subsidence bowl in the northwest and an elongated subsiding zone in 

the central part of the valley. The northwest subsidence bowl is 

nearly circular along its western extent and includes the area of 

maximum subsidence of nearly 7.5 inches. Its southeastern boundary is aligned along the Eglington scarp, one of several Quaternary 

faults cutting the valley-floor alluvium. Little subsidence is detected 

immediately southeast of the fault. Similarly, the central subsidence 

zone follows the general trends of several mapped faults. The map 

suggests that the spatial distribution of land subsidence in Las Vegas 

Valley is controlled by Quaternary faults to a much greater degree 

than previously suspected. The faults may separate compressible 

from less-compressible deposits, or they may act as barriers to 

ground-water flow, impeding the horizontal propagation of fluidpressure changes and creating ground-water-level differences across 

the faults. 

The potential for renewed subsidence in Santa Clara Valley, California, is a concern for the Santa Clara Valley Water District. One of the 

District's objectives in managing water resources is to limit groundwater extractions that would cause inelastic (irreversible) compaction of the valley's aquifer system. Seasonal and longer-term 

elevation changes were measured from successive satellite radar 
156 

lnSAR imagery reveals seasonal and 

long-term land-surface-elevation 

changes influenced by ground-water levels and fault alignment in the 

Santa Clara Valley. 

San Francisco Bay 

Change in land-surface 

aH:itude (inches) 

+

"'" ~~: 

--0 

Subsidence 

--0.6 

Subsidence in the United States 

1985 1990 1995 

D~~ltO 100 --~~ ---------+~~~~~~--~ Relative 

6.3 compaction 

(feet be''JW 

land surface) 

(feet) 

200 --------~~~rr~w--r---------1 

0 

r------1 

due to recovering 

ground-water 

levels. 

5 Miles 

Seasonal, recoverable subsidence 

was measured between January 

and August 1997. Subsidence 

patterns during this period are 

similar to historical subsidence 

patterns ( 1934-1967). 

0 5 Kilometers 

Change in land-surface 

altitude (inches) 

Uplift 

~ :. s, Aod~~ fooh 

Subsidence -

- -1.2 (lkehara and others, 1998) 

passes during 1992 to 1997. The longer-term ( ~5 year) measurement indicates no change for most of the southwestern Santa Clara 

Valley and land-surface uplift of up to 1.4 inches in the northern 

and eastern parts of the valley. This uplift is correlated to the recovery of ground-water levels that has been occurring for several years 

as a result of reduced pumpage and increased recharge. In contrast, 

the seasonal ( 6-8 month) interferograms reveal a large region in 

San Jose undergoing seasonal elastic deformation related to groundwater-level fluctuations. The eastern extent of this deformation appears to be truncated by a Quaternary fault, the Silver Creek Fault, 

several miles west and roughly parallel to the tectonically active 

Hayward Fault. The InSAR maps are generally consistent with compaction measured in borehole extensometers. 

MITIGATION MEASURES 

When development of natural resources causes subsidence, governments sometimes exercise their power either to prohibit the re-
The Role of Science 

General approaches to mitigation of 

subsidence will rarely apply to all 

types of subsidence. 

157 

source development or to control it in ways that minimize damage. 

This may be done through regulation. With adequate monitoring 

programs and institutional mechanisms in place, optimal benefits 

may be achieved for both subsidence mitigation and resr)urce development. The Panel on Land Subsidence (NRC, 1991) found that 

more research is needed in this area of optimal resource allocation 

and adaptive approaches to land- and water-use management. 

In order to wisely and conjunctively manage our land and water 

resources we first need to define the relevant interacting processes. 

In the case of land subsidence and ground-water resources, this 

means understanding the hydrogeologic framework of the resource 

as well as the demands or stresses that we place on it. It a.lso means 

identifying a desired state of the land and water resources-a set of 

goals and objectives that describe some desirable outcomes. These 

goals and objectives may require guidelines for decision making 

(policy) to modify usage of the resources in order to attain the desired state. The selection and management of these policies can be 

based on measures of the condition of the hydrogeologi,: system. 

Land-subsidence and water-management problems are linked 

In a typical basin, ground water is in part a renewable re<-ource; a 

certain amount may be extracted without seriously deplo.ting the 

amount of water stored. This is the concept of the "safe yield" of a 

basin. In subsidence-prone alluvial aquifer systems, unless we wish 

to mine a significant volume of water in storage in the fine-grained 

sediments, the volume of water withdrawn cannot great1

y exceed the 

natural and artificial replenishment. It may be necessary to maintain 

ground-water levels above critical thresholds in subsidence-prone 

areas within the basin in order to avoid incurring new O:"' additional 

subsidence. Another important consideration is climatic variability, 

which affects the amount of water available for natural and artificial 

replenishment. This restricted concept of"safe yield" addresses only 

the volume of extracted water with respect to a dynamic equilibrium between the water recharging and discharging a bzsin. Beyond 

this, to conserve an aquifer system from a water-quality perspective, 

it may be necessary to maintain certain minimal flow-through rates. 

Because aquifer systems have the capacity to store water, the amount 

of natural outflow from a basin may not be equal to the amount 

replenished in the same year. Thus the "optimal yield" of a groundwater basin is not necessarily a constant value. It may vary from 

year-to-year depending upon the state of the aquifer system and the 

availability of alternative local and imported supplies. T>e concept 

of optimal yield incorporates the dynamic nature of the groundwater basin and the adaptability of the management system (Bear, 

1979). However, over the long term, the "annual safe yield" of a basin 

would be roughly equivalent to its average replenishment. 

Adaptability has emerged as a conscious element of insftutional 

design in basin-management programs (Blomquist, 1992). Managing basins according to optimal yield assumptions has allowed water 
158 Subsidence in the United States 

users to respond to changed conditions of water supply, including 

severe drought and relative abundance. Ground-water management plans typically address both demand and supply by adjusting 

the demands placed on the water-supply system through conservation and water-rationing programs, by adjusting the supply 

through conjunctive use of ground water and surface water, and by 

augmenting the supply through aquifer storage and recovery programs. Adaptable management alternatives contribute to tre stability and sustainability of land and water resources in many htsins. 

In basins susceptible to detrimental effects related to the loTvering 

of ground-water levels, such as the three types of subsidence presented in this Circular, land and water resources are linked. For 

alluvial ground-water basins subject to aquifer-system comoaction, 

threshold values of aquifer-system stress define the boundary between nonpermanent (recoverable) and permanent (nonre--:overable) compaction and loss of land-surface elevation. For re,Q:ions 

affected by the dissolution and collapse of soluble rocks, the 

threshold stress values are more ambiguous but nevertheless real 

and somewhat manageable. For oxidation of organic soils, the 

threshold for detrimental effects is very nearly defined by the position of the water table. In each case, management of the lan<i-subsidence problem is inextricably linked to other facets of 

water-resource management. 

Socioeconomic risks versus benefits 

Ground-water basins have value not only as perennial sources of 

water supply, but also as reservoirs for cyclical recharge anc discharge. While augmenting base water-supply needs met from a 

variety of water sources, ground-water basins may provide water at 

peak-demand periods to modulate the variability inherent in surface-water supplies. The conjunctive surface- and ground-,,Tatermanagement programs in some southern California basins make 

more efficient use of basin storage capacity than the fixed-yield 

management programs of other, nonconjunctively manage -I basins. Storing water underground in wet years for use in dry years, 

and encouraging water users to take more surface and imported 

water when it is plentiful and to pump more ground water when it 

is not, capitalizes on one of the strengths of the ground-water resource. Restricting pumping to the same amount each year regardless of basin conditions does not. In some cases, the most valuable 

use of ground-water basins is to lessen the immediate shod< of 

short-term variability of water supply (Blomquist, 1992). In subsidence-prone basins, the need to maintain minimum wate~ levels 

for subsidence control may place a significant constraint or conjunctive-use schemes. 
GLOSSARY 

Land Subsidence in the United States These detlnitions are based on the American Geological Institute's Glossary of 

Geology (4th edition) and Glossary of 

Hydrology, and USGS Water Supply Paper 

2025, "Glossary of selected terms useful in 

studies of the mechanics of aquifer systems 

and land subsidence due to fluid withdrawal" (Poland, and others, 1971). 

Aquifer A saturated, permeable, geologic unit that can transmit significant 

quantities of ground water under ordinary hydraulic gradients and 

is permeable enough to yield economic quantities of wzter to wells. 

Aquifer, Artesian See Aquifer, Confined, and Artesian. 

Aquifer, Confined An artesian aquifer that is confined between two aquitards. 

Aquifer, Unconfined A water-table aquifer in which the water table forms the upper 

boundary. 

Aquifer System A heterogeneous body of interbedded permeable and p'">orly permeable geologic units that function as a water-yielding hydraulic 

unit at a regional scale. The aquifer system may comprife one or 

more aquifers within which aquitards are interspersed. Confining 

units may separate the aquifers and impede the vertical exchange of 

ground water between aquifers within the aquifer system. 

Aquitard A saturated, but poorly permeable, geologic unit that impedes 

ground-water movement and does not yield water freely to wells, but 

which may transmit appreciable water to and from adjar.ent aquifers 

and, where sufficiently thick, may constitute an importznt groundwater storage unit. Areally extensive aquitards may function regionally as confining units within aquifer systems. See also Confining Unit. 

Artesian An adjective referring to confined aquifers. Sometimes the term artesian is used to denote a portion of a confined aquifer where the altitudes of the potentiometric surface are above land surface (flowing 

wells and artesian wells are synonymous in this usage). But more 

generally the term indicates that the altitudes of the potentiometric 

surface are above the altitude of the base of the cont1nirg unit ( artesian wells and flowing wells are not synonymous in this case). See 

Aquifer, Confined. 

Blue hole A subsurface void, usually a solution sinkhole, developed in carbonate rocks that are open to the Earth's surface and contains tidally influenced waters of fresh, marine, or mixed chemistry. 

Cave A natural underground open space or series of open sp::tces and passages large enough for a person to enter, generally with a connection 

to the surface; often formed by solution oflimestone. 
160 Subsidence in the Uni'"~d States 

Cavern A cave, with the implication of a large size. 

Cenote Steep-walled natural well that extends below the water table; generally caused by collapse of a cave roof; term reserved for featues 

found in the Yucatan Peninsula of Mexico. 

Confining Unit A saturated, relatively low-permeability geologic unit that is areally 

extensive and serves to confine an adjacent artesian aquifer or aquifers. Leaky confining units may transmit appreciable water to and 

from adjacent aquifers. See also Aquitard. 

Compaction In this Circular, compaction is used in its geologic sense and refers 

to the inelastic compression of the aquifer system. Compaction of 

the aquifer system reflects the rearrangement of the mineral grain 

pore structure and largely nonrecoverable reduction of the porosity 

under stresses greater than the preconsolidation stress. Con1paction, 

as used here, is synonymous with the term "virgin consolidation" 

used by soils engineers. The term refers to both the process and the 

measured change in thickness. As a practical matter, a very small 

amount ( 1 to 5 percent) of the compaction is recoverable as a slight 

elastic rebound of the compacted material if stresses are reduced. 

Compaction, Residual Compaction that would ultimately occur if a given increase in applied stress were maintained until steady-state pore pressuF~s were 

achieved. Residual compaction may also be defined as the difference 

between ( 1) the amount of compaction that will occur ultin1ately for 

a given increase in applied stress, and (2) that which has occurred at 

a specified time. 

Compression In this Circular, compression refers to the decrease in thickness of 

sediments, as a result of increase in vertical compressive strrss. 

Compression may be elastic (fully recoverable) or inelastic (nonrecoverable). 

Consolidation In soil mechanics, consolidation is the adjustment of a saturated soil 

in response to increased load, involving the squeezing of water from 

the pores and a decrease in void ratio or porosity of the soil. In this 

Circular, the geologic term "compaction" is used in preferen r.e to 

consolidation. 

Datum See Geodetic Datum. 

Ellipsoid, Earth A mathematically determined three-dimensional surface obtained 

by rotating an ellipse about its semi-minor axis. In the case of the 

Earth, the ellipsoid is the modeled shape of its surface, which is relatively flattened in the polar axis. 

Ellipsoid, Height The distance of a point above the ellipsoid measured perpendicular 

to the surface of the ellipsoid. 

Exfoliation The process by which concentric scales, plates, or shells of rock, 

from less than a centimeter to several meters in thickness, are 

stripped from the bare surface of a large rock mass. See spall. 

Geodetic Datum A set of constants specifying the coordinate system used fo'" geodetic control, for example, for calculating the coordinates of points 

on the Earth. 
Glossary 161 

Geoid, Earth The sea-level equipotential surface or figure of the Earth. If the Earth 

were completely covered by a shallow sea, the surface of this sea 

would conform to the geoid shaped by the hydrodynamir: equilibrium of the water subject to gravitational and rotational forces. 

Mountains and valleys are departures from this reference geoid. 

Head, Hydraulic A measure of the potential for fluid flow. The height of tl'~ free surface of a body of water above a given subsurface point. 

Hydraulic Conductivity A measure of the medium's capacity to transmit a particular fluid. 

The volume of water at the existing kinematic viscosity that will 

move in a porous medium in unit time under a unit hydraulic gradient through a unit area. In contrast to permeability, it is a function of 

the properties of the liquid as well as the porous medium. 

Hydro compaction The process of volume decrease and density increase that occurs 

when certain moisture-deficient deposits compact as thr~r are wetted 

for the first time since burial. The vertical downward me vement of 

the land surface that results from this process has also been termed 

"shallow subsidence" and "near-surface subsidence:' 

Karst A type of topography that is formed on limestone, dolon1ite, gypsum 

and other rocks, primarily by dissolution, and that is characterized 

by sinkholes, caves, and subterranean drainage. 

Karstification Action by water, mainly chemical but also mechanical, that produces 

features of a karst topography. 

Karst, Mantled A terrane of karst features, usually subdued, and covered by soil or a 

thin alluvium. 

Load We refer to Load as synonymous with Stress. 

Overdraft Any withdrawal of ground water in excess of the Safe Yield. 

Paleokarst A karstified area that has been buried by later deposition of sediments. 

Permeability The capacity of a porous rock, sediment, or soil for transmitting a 

fluid. Unlike hydraulic conductivity, it is a function only of the 

medium. 

pH A measure of the acid/base property of a material sampl~. The negative logarithm of the hydrogen ion concentration; pH 7 is neutral 

with respect to distilled, deionized water; pH less than 7 is more 

acidic; pH greater than 7 is more basic. 

Piezometric Surface See Potentiometric Surface. 

Plutonic A loosely defined term with a number of current usages. We use it to 

describe igneous rock bodies that crystallized at great depth or, more 

generally, any intrusive igneous rock. 

Porosity The percentage of the soil or rock volume that is occupied by pore 

space, void of material. The porosity is defined by the ratio of void 

space to the total volume of a specimen. 

Potentiometric Surface An imaginary surface representing the total head of ground water 

and defined by the level to which the water will rise in a tightly cased 

well. See Head, Hydraulic. 
162 

Recharge 

Subsidence in the United States 

The process involved in addition of water to the saturated zone, 

naturally by precipitation or runoff, or artificially by spreacing or 

injection. 

Sinkhole A depression in a karst area. At land surface its shape is genorally 

circular and its size measured in meters to tens of meters; underground it is commonly funnel-shaped and associated with subterranean drainage. 

Safe Yield See Yield, Safe. 

Specific Storage The volume of water that an aquifer system releases or take:: into 

storage per unit volume per unit change in head. The specific storage is equivalent to the Storage Coefficient divided by the thickness 

of the aquifer system. 

Spall A chip or fragment removed from a rock surface by weathering; especially by the process of exfoliation. See exfoliation. 

Spring Any natural discharge of water from rock or soil onto the lend surface or into a surface-water body. 

Storage The capacity of an aquifer, aquitard, or aquifer system to re1

':'ase or 

accept water into ground-water storage, per unit change in hydraulic 

head. See Storage Coefficient and Specific Storage. 

Storage Coefficient The volume of water that an aquifer system releases or take." into 

storage per unit surface area per unit change in head. 

Strain Relative change in the volume, area or length of a body as a result of 

stress. The change is expressed in terms of the amount of displacement measured in the body divided by its original volume, area, or 

length, and referred to as either a volume strain, areal strain, or onedimensional strain, respectively. The unit measure of strain is dimensionless, as its value represents the fractional change from the 

former size. 

Stress In a solid body, the force (per unit area) acting on any surface within 

it; also refers to the applied force (per unit area) that createf the internal force. Stress is variously expressed in units of pressure, such 

as pounds per square inch, kilograms per square meter, or Pascals. 

Stress, Applied The downward stress imposed on a specified horizontal plene 

within an aquifer system. At any given level in the aquifer system, 

the applied stress is the force or weight (per unit area) of seiiments 

and moisture above the water table, plus the submerged weight (per 

unit area), accounting for buoyancy of the saturated sediments overlying the specified plane at that level, plus or minus the net seepage 

stress generated by flow (upward or downward component) 

through the specified plane in the aquifer system. 

Stress, Effective Stress (pressure) that is borne by and transmitted through the 

grain-to-grain contacts of a deposit, and thus affects its porosity 

and other physical properties. In one-dimensional comprefsion, effective stress is the average grain-to-grain load per unit area in a 

plane normal to the applied stress. At any given depth, the effective 
Glossary 

Stress, Geostatic (Litho static) 

Stress, Preconsolidation 

Stress, Seepage 

Subsidence 

Subsidence, Near-Surface 

Subsidence, Shallow 

Transmissivity 

Vug 

Water Table 

Yield, Operational 

Yield, Optimal 

163 

stress is the weight (per unit area) of sediments and mo~sture above 

the water table, plus the submerged weight (per unit area) of sediments between the water table and the specified depth, plus or minus the seepage stress (hydrodynamic drag) produced l~y downward 

or upward components, respectively, of water movement through 

the saturated sediments above the specified depth. Effective stress 

may also be defined as the difference between the geostatic stress 

and fluid pressure at a given depth in a saturated deposit, and represents that portion of the applied stress which becomes effective as 

intergranular stress. 

The total weight (per unit area) of sediments and water above some 

plane of reference. Geostatic stress normal to any horizontal plane of 

reference in a saturated deposit may also be defined as the sum of 

the effective stress and the fluid pressure at that depth. 

The maximum antecedent effective stress to which a deposit has 

been subjected and which it can withstand without undorgoing additional permanent deformation. Stress changes in the range less than 

the preconsolidation stress produce elastic deformations of small 

magnitude. In fine-grained materials, stress increases beyond the preconsolidation stress produce much larger deformations that are principally inelastic (nonrecoverable). Synonymous with "v:rgin stress." 

Force (per unit area) transferred from the water to the nedium by 

viscous friction when water flows through a porous medium. The 

force transferred to the medium is equal to the loss of hydraulic head 

and is termed the seepage force exerted in the direction of flow. 

Sinking or settlement of the land surface, due to any of several processes. As commonly used, the term relates to the vertical downward 

movement of natural surfaces although small-scale horizontal components may be present. The term does not include landslides, 

which have large-scale horizontal displacements, or settlements of 

artificial fills. 

See Hydrocompaction. 

See Hydrocompaction. 

The rate at which water at the prevailing kinematic viscosity is transmitted through a unit width of aquifer under a unit hydraulic gradient. See also Hydraulic Conductivity. 

A small cavity or chamber in rock that may be lined with crystals. 

The surface of a body of unconfined ground water at which the pressure is equal to atmospheric pressure. 

See Yield, Optimal. 

An optimal amount of ground water, by virtue of its use, that should 

be withdrawn from an aquifer system or ground-water basin each 

year. It is a dynamic quantity that must be determined f-:-om a set of 

alternative ground-water management decisions subjec+ to goals, 

objectives, and constraints of the management plan. 
164 Subsidence in the United States 

Yield, Perennial The amount of usable water from an aquifer that can be eco!lomically consumed each year for an indefinite period of time. It is a 

specified amount that is commonly specified equal to the m c:-an annual recharge to the aquifer system, which thereby limits the 

amount of ground water that can be pumped for beneficial use. 

Yield, Safe The amount of ground water that can be safely withdrawn from a 

ground-water basin annually, without producing an undesirable result. Undesirable results include but are not limited to depletion of 

ground-water storage, the intrusion of water of undesirable quality, 

the contraventions of existing water rights, the deterioration of the 

economic advantages of pumping (such as excessively lowered water levels and the attendant increased pumping lifts and associated 

energy costs), excessive depletion of streamflow by induced infiltration, and land subsidence. 
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Paine, J.G., and Morton, R.A., 1986, Historical shoreline changes in Trinity, Galveston, West, and East Bays, Texas Gulf 

Coast: University of Texas Bureau of Economic Geology Circular 86-3,58 p. 

Pratt, W.E., and Johnson, D.W., 1926, Local subsidence of the Goose Creek oil field: Journal of Geology, v. 34, p. 577-590. 

Titus, J.G., and Narayanan, V.K., 1995, The probability of sea level rise: U.S. Environmental Protection AgenC)', EPA 230-R95-008. 

White, W.A., Tremblay, T.A., Wermund, E.G., Jr., and Handley, L.R., 1993, Trends and status of wetland and aouatic habitats in the Galveston Bay system, Texas: Galveston Bay National Estuary Program Publication GBNEP-31, 225 p. 

LAS VEGAS, NEVADA 

Acevedo, William, Gaydos, Leonard, Tilley, Janet, Mladinich, Carol, Buchanan, Janis, Blauer, Steve, Kruger, KeHey, and 

Schubert, Jamie, 1997, Urban land use change in the Las Vegas Valley: U.S. Geological Survey, accessed July 27, 1999, http:/ 

I geochange.er. usgs.gov/ sw/ changes/ anthropogenic/population/las_ vegas. 

Bell, J.W., 1981a, Subsidence in Las Vegas Valley: Nevada Bureau of Mines and Geology Bulletin 95,83 p., 1 plate, scale 

1:62,500. 

Bell, J. W., 1981 b, Results of leveling across fault scarps in Las Vegas Valley, Nevada, April1978-June 1981: Nevada Bureau 

of Mines and Geology Open-File Report 81-5,7 p. 

Bell, J.W., and Helm, D.C., 1998, Ground cracks on Quaternary faults in Nevada-Hydraulic and tectonic, in Borchers, 

J.W., ed., Land subsidence case studies and current research: Proceedings of the Dr. Joseph F. Poland symposium on land 

subsidence, Association of Engineering Geologists Special Publication No.8, p. 165-173. 

Bell, J.W., and Price, J.G., 1991, Subsidence in Las Vegas Valley, 1980-91-Final project report: Nevada Bureau of Mines 

and Geology, Open-File Report 93-4, 10 sect., 9 plates, scale 1:62,500. 
168 Subsidence in the United States 

Bernholtz,A., Brothers, K., and Katzer, T., 1994,Artificial ground-water recharge in Las Vegas Valley, Clark County, Nevada- Storing today, treating tomorrow?: International Symposium on Artificial Recharge of Ground Water, 2nd, Orlando, 

Fla., July 1994, [Proceedings], p. 548-557. 

Bur bey, T.J., 1995, Pumpage and water-level change in the principal aquifer of Las Vegas Valley, 1980-90-Nevada Division 

of Water Resources Information Report 34, 224 p. 

Carpenter, Everett, 1915, Ground water in southeastern Nevada: U.S. Geological Survey Water-Supply Paper 365, 8~ p. 

Coache, Robert, 1996, Las Vegas Valley water usage report, Clark County, Nevada, 1996: Nevada Division ofWater Resources Report, [50+] p. 

Dettinger, M.D., 1989, Reconnaissance estimates of natural recharge to desert basins in Nevada, U.S.A., by using cHoride 

balance calculations: Journal of Hydrology, v. 106, p. 55-78. 

Domenico, P.A., Stephenson, D.A., and Maxey, G.B., 1964, Ground water in Las Vegas Valley: Nevada Department of Conservation and Natural Resources Division of Water Resources Bulletin 29,53 p. 

Hafen, L.R., and Hafen, A.W, 1954, Old Spanish Trail, Santa Fe to Los Angeles-With extracts from contemporary records 

and including diaries of Antonio Armijo and Orville Pratt: University of Nebraska Press, 375 p. 

Harrill, James R., 1976, Pumping and ground-water storage depletion in Las Vegas Valley, Nevada, 1955-74: Nevad::-1. Department of Conservation and Natural Resources, Division of Water Resources Bulletin No. 44, 70 p. 

Holzer, T.L., 1979, Leveling data-Eglington fault scarp, Las Vegas Valley, Nevada: U.S. Geological Survey Open-File Report 

79-950,7 p. 

- 1984, Ground failure induced by ground-water withdrawal from unconsolidated sediment, in Holzer, T.L., ed., Maninduced land subsidence: Geological Society of America Reviews in Engineering Geology, v. 6, 221 p. 

Houghton, J. G., Sakamoto, C. M., and Gifford, R.O., 1975, Nevada's weather and climate: Nevada Bureau of Mines and 

Geology Special Publication 2, 78 p. 

Jones, F. L., and Cahlan, J. F. 1975, Water, a history of Las Vegas-Volume I: Las Vegas Valley Water District, 171 p. 

Livingston, Penn, 1941, Underground leakage from artesian wells in the Las Vegas area, Nevada: U.S. Geological Survey 

Water-Supply Paper 849-D, p. 147-173. 

Malmberg, G. T., 1965, Available water supply of the Las Vegas ground-water basin, Nevada: U.S. Geological Surve~r WaterSupply Paper 1780, 116 p., 13 plates. 

Maxey, G.B., and Jameson, C. H., 1948, Geology and water resources of Las Vegas, Pahrump, and Indian Springs Valleys, 

Clark and Nye Counties, Nevada: Nevada State Engineer Water Resources Bulletin 5, 121 p. 

Mendenhall, WC., 1909, Some desert watering places in southeastern California and southwestern Nevada: U.S. Geological 

Survey Water-Supply Paper 224, 98 p. 

Mifflin, M.D., and Wheat, M.M., 1979, Pluvial lakes and estimated pluvial climates of Nevada: Nevada Bureau of 11ines and 

Geology Bulletin 94, 57 p., 1 plate. 

Mindling, A.L., 1971, A summary of data relating to land subsidence in Las Vegas Valley: Center for Water Resources Research, Desert Research Institute, University of Nevada, Reno; 55 p. 

Morgan, D.S., and Dettinger, M.D., 1996, Ground-water conditions in Las Vegas Valley, Clark County, Nevada, par'" 2, Hydrogeology and simulation of ground-water flow: U.S. Geological Survey Water-Supply Paper 2320-B, 124 p., 2 plrtes. 

Nevada Department of Conservation and Natural Resources, Division of Water Resources and Water Planning, 1992, Hydrographic basin summaries, 1990-1992, [variously paged]. 

Plume, R. W, 1989, Ground-water conditions in Las Vegas Valley, Clark County, Nevada, part I, Hydrogeologic framework: 

U.S. Geological Survey Water-Supply Paper 2320-A, 15 p. 

Quade, J., Mifflin, M.D., Pratt, WL., McCoy, W., and Burckle, L., 1995, Fossil spring deposits in the southern Great Basin and 

their implications for change in water-table levels near Yucca Mountain, Nevada, during Quaternary time: Geolog::cal Society of America Bulletin, v. 107, p. 213-230. 

Riley, F.S., 1969,Analysis of borehole extensometer data from central California: International Association ofScie1tific 

Hydrology Publication 89, p. 423-431. 

U.S. Department of Commerce, 1997, Las Vegas metro area leads nation in population growth, census bureau reports: Bureau of the Census, accessed July 27, 1999, at URL http:/ /www.census.gov/ftp/pub/Press-Release/cb97-212.html 
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SOUTH -CENTRAL ARIZONA 

Anderson, S.R., 1988, Potential for aquifer compaction, land subsidence, and earth fissures in the Tucson Basi.n, Pima 

County, Arizona: U.S. Geological Survey Hydrologic Investigations Atlas 713,3 sheets, scale 1:250,000. 

Anderson, S.R., 1989, Potential for aquifer compaction, land subsidence, and earth fissures in Avra Valley, Pina and Pinal 

Counties, Arizona: U.S. Geological Survey Hydrologic Investigations Atlas 718,3 sheets, scale 1:250,000. 

Anderson, T.W., Freethey, G.W., and Tucci, P., 1992, Geohydrology and water resources of alluvial basins in south-central 

Arizona and parts of adjacent states: U.S. Geological Survey Professional Paper 1406-B, 67 p., 3 plates, scale 1:1,000,000. 

Anning,D.W., and Duet, N.R., 1994, Summary of ground-water conditions in Arizona, 1987-90: U.S. Geological Survey 

Open-File Report 94-476,2 sheets. 

Arizona Department of Water Resources, 1999,Arizona water information, statewide overview, supply and d<:>mand (1994): 

accessed July 27, 1999 at URL http://www.adwr.state.az.us/AZWaterinfo/statewide/supplyde.html. 

Carpenter, M.C., 1993, Earth-fissure movements associated with fluctuations in ground-water levels near the Picacho 

Mountains, south -central Arizona, 1980-84: U.S. Geological Survey Professional Paper 497-H, 49 p. 

Carpenter, M.C., and Bradley, M.D., 1986, Legal perspectives on subsidence caused by ground-water withdra··val in Texas, 

California, and Arizona, U.S.A.: International Symposium on Land Subsidence, 3rd, Venice, 1984, [Proceedings, Johnson, 

A.I., Carbognin Laura, and Ubertini, L., eds.], International Association of Scientific Hydrology Publication 151, p. 817-828. 

City of Tucson Water Department, 1995, Annual static water level basic data report, Tucson Basin and Avra Valley, Pima 

County, Arizona, 1995: City of Tucson Water Planning and Engineering Division, 140 p. 

Eaton, G.P., Peterson, D.L., and Schumann, H. H., 1972, Geophysical, geohydrological, and geochemical recornaissance of 

the Luke salt body, Central Arizona: U.S. Geological Survey Professional Paper 753, 28 p. 

Feth, J.H., 1951, Structural reconnaissance of the Red Rock quadrangle, Arizona: U.S. Geological Survey Open-File Report, 30 p. 

Holzer, T.L., 1980, Reconnaissance maps of earth fissures and land subsidence, Bowie and Willcox areas, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map MF-1156, 2 sheets, scale 1:24,000. 

Holzer, T.L., 1984, Ground failure induced by ground-water withdrawal from unconsolidated sediment, in Holzer, T.L., ed., 

Man-induced land subsidence: Geological Society of America Reviews in Engineering Geology, v. 6, p. 67-1 0~. 

Holzer, T.L., Davis, S.N., and Lofgren, B.E., 1979, Faulting caused by groundwater extraction in south-centralArizona: Journal of Geophysical Research, v. 84, p. 603-612. 

Johnson, N.M., 1980, The relation between ephemeral stream regime and earth fissuring in south-central Arizona: Tucson, 

Ariz., University of Arizona, M.S. thesis, 158 p. 

Laney,R.L., Raymond, R.H., and Winikka, C.C., 1978, Maps showing water-level declines, land subsidence, and earth fissures in south -central Arizona: U.S. Geological Survey Water-Resources Investigations Report 78-83, 2 sheets, scale 

1:125,000. 

Leonard, R.J., 1929,An earth fissure in southern Arizona: Journal of Geology, v. 37, p. 765-774. 

Peterson, D.E., 1962, Earth fissuring in the Picacho area, Pinal County, Arizona: Tucson, Ariz., University of Arizona, M.S. 

thesis, 35 p. 

Robinson, G.M., and Peterson, D.E., 1962, Notes on earth fissures in southern Arizona: U.S. Geological Survey Circular 466,7 p. 

Schumann, H.H., 1995, Land subsidence and earth fissure hazards near Luke Air Force Base, Arizona: in Prince, K.R., Galloway, D.L., and Leake, S.A., eds., U.S. Geological Survey subsidence interest group conference, Edwards Air Fo"'ce Base, Antelope Valley, California, November 18-19, 1992-abstracts and summary: U.S. Geological Open-File Report 94-532, p. 18-21. 

Schumann, H.H., and Cripe, L.S., 1986, Land subsidence and earth fissures caused by groundwater depletion in southern 

Arizona, U.S.A.: International Symposium on Land Subsidence, 3rd, Venice, 1984, [Proceedings, Johnson, A.!., Carbognin 

Laura, and Ubertini, L., eds.], International Association of Scientific Hydrology Publication 151, p. 841-851. 

Schumann, H.H., and Genauldi, R.B., 1986, Land subsidence, earth fissures, and water-level change in southern Arizona: 

Arizona Bureau of Geology and Mineral Technology Geological Survey Branch Map 23, 1 sheet, scale 1:1,00(',000. 

Schumann, H.H., and Poland, J.F., [1969-1970], Land subsidence, earth fissures, and groundwater withdraw;;! in southcentral Arizona, U.S.A: International Association of Scientific Hydrology Publication 88, p. 295-302. 
170 Subsidence in the United States 

Strange, W.E., 1983, Subsidence monitoring for the State of Arizona: National Oceanic and Atmospheric Administration 

National Geodetic Information Center, Rockville, Md., 80 p. 

PART II-Drainage of Organic Soils 

INTRODUCTION 

Darby, H. C., 1956, The Draining of the Fens (2nd ed.): Oxford, Cambridge University Press, 314 p. 

Lucas, R.E., 1982, Organic soils (Histosols)-Formation, distribution, physical and chemical properties and management 

for crop production: Michigan State University Farm Science Research Report 435, 77 p. 

Nieuwenhuis, H.S., and Schokking, F., 1997, Land subsidence in drained peat areas of the Province of Friesland, Tre Netherlands: Quarterly Journal of Engineering Geology, v. 30, p. 3 7-48. 

Schothorst, C.J., 1977, Subsidence oflow moor peat soils in the western Netherlands: Geoderma, v. 17, p. 265-291. 

Stephens, J.C.,Allen, L.H., Jr., and Chen, Ellen, 1984, Organic soil subsidence in Holzer, T.L., ed., Man-induced land subsidence: Geological Society of America Reviews in Engineering Geology, v. 6, p. 107-122. 

Waksman, S.A., and Purvis, E.R., 1932, The influence of moisture upon the rapidity of decomposition oflowmoor peat: Soil 

Science, v. 34, p. 323-336. 

Waksman, S.A., and Stevens, K.R., 1929, Contribution to the chemical composition of peat, part 5. The role of microorganisms in peat formation and decomposition: Soil Science, v. 28, p. 315-340. 

Wosten, J.H.M., Ismail, A.B., and van Wijk, A.L.M., 1997, Peat subsidence and its practical implications: A case study in 

Malaysia: Geoderma, v. 78, p. 25-36. 

SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA 

Atwater, B.F., 1980, Attempts to correlate Late Quaternary climatic records between the San Francisco Bay, the SacramentoSan Joaquin Delta, and the Mokelumne River, California: Dover, Del., University of Delaware, Ph.D. dissertation, 215 p. 

California Department of Water Resources, 1993, Sacramento-San Joaquin Delta atlas: Sacramento, State of California Department ofWater Resources, 121 p. 

California Department of Water Resources, 1995, Delta levees: Sacramento, State of California Department of Water Resources, 19 p. 

Delta Protection Commission, 1995, Land use and resource management plan for the primary zone of the Delta: VTalnut 

Grove, Delta Protection Commission, 60 p. 

Deverel, S.J., and Rojstaczer, S.A., 1996, Subsidence of agricultural lands in the Sacramento-San Joaquin Delta, California: 

Role of aqueous and gaseous carbon fluxes: Water Resources Research, v. 32, p. 2,359-2,367. 

Dillon, Richard, 1982, Delta Country: Novato, Calif., Presidio Press, 134 p. 

Rojstaczer, S.A., and Deverel, S.J., 1993, Time dependence of atmospheric carbon inputs from drainage of organic soils: 

Geophysical Research Letters, v. 20, p. 1,383-1,386. 

Rojstaczer, S.A., Hamon, R.E., Deverel, S.J., and Massey, C.A., 1991, Evaluation of selected data to assess the causes of subsidence in the Sacramento-San Joaquin Delta, California: U.S. Geological Survey Open-File Report 91-193, 16 p. 

Tans, P.P., Fung, I.Y., and Takahashi, Y., 1990, Observational constraints on the global atmospheric C02 budget: Sci"'nce, v. 

247,p.1,431-1,438. 

Thompson, John, 1957, The settlement geography of the Sacramento-San Joaquin Delta, California: Palo Alto, Calif., 

Stanford University, Ph.D. dissertation, 551 p. 

Weir, W.W., 1950, Subsidence of peat lands of the Sacramento-San Joaquin Delta, California: Hilgardia, v. 20, p. 37-55. 

FLORIDA EVERGLADES 

Allison, R.V., 1956, The influence of drainage and cultivation on subsidence of organic soils under conditions of Everglades 

reclamation: Soil and Crop Science Society of Florida Proceedings, v. 16, p. 21-31. 

Bodle, M.J., Ferriter,A.P., and Thayer, D.D., 1994, The biology, distribution, and ecological consequences ofMelaleKa 

quinquenervia in the Everglades, in Davis, S.M., and Ogden, J.C., The Everglades-The ecosystem and its restorati(m: 

Delray Beach, Fla.,St. Lucie Press, p. 341-355. 
References 

Craft, C.B., and Richardson, C.J., 1993a, Peat accretion and N, P, and organic C accumulation in nutrient-enriched and 

unenriched Everglades peatlands: Ecological Applications, v. 3, p. 446-458. 

171 

Craft, C.B., and Richardson, C.J., 1993b, Peat accretion and phosphorus accumulation along a eutrophication gradient in 

the northern Everglades: Biogeochemistry, v. 22, p. 133-156. 

Davis, J.R., Jr., 1946, The peat deposits of Florida: Florida Geological Survey Bulletin 30,247 p. 

Davis, S.M., and Ogden, J.C., eds., 1994, The Everglades-The ecosystem and its restoration: Delray Beach, F:a., St. Lucie 

Press, 826 p. 

Deren, C.W., Snyder, G.H., Miller, J.D., and Porter, P.S., 1991, Screening for heritability of flood-tolerance in flle Florida 

(CP) sugarcane breeding population: Euphytica, v. 56, p. 155-160. 

Douglas, M.S., 1947, The Everglades- River of grass: St. Simons Island,Fla., Mockingbird Press, 308 p. 

Gascho, G.J., and Shih, S.F., 1979, Varietal response of sugarcane to water table depth, part 1. Lysimeter performance and 

plant response: Soil and Crop Society of Florida Proceedings, v. 38, p. 23-27. 

Glaz, Barry, 1995, Research seeking agricultural and ecological benefits in the Everglades: Journal of Soil and Water Conservation, v. 50, p. 609-612. 

Johnson, Lamar, 1974, Beyond the fourth generation: Gainesville, Fla., The University Presses of Florida, 230 p. 

Jones, L.A., Allison, R. V., and others, 1948, Soils, geology, and water control in the Everglades region: University of Florida 

Agricultural Experiment Station Bulletin 442, 168 p., 4 maps. 

Kang, M.S., Snyder, G.H., and Miller, J.D., 1986, Evaluation of Saccharum and related germplasm for tolerance to high 

water table on organic soil: Journal of the American Society of Sugar Cane Technologists, v. 6, p. 59-63. 

Light, S.S., and Dineen, J.W., 1994, Water control in the Everglades: A historical perspective, in Davis, S.M., and Ogden, 

J.C., The Everglades-The ecosystem and its restoration: Delray Beach, Fla., St. Lucie Press, p. 47-84. 

Lucas, R.E., 1982, Organic soils (Histosols)-Formation, distribution, physical and chemical properties and management 

for crop production: Michigan State University Farm Science Research Report 435, 77 p. 

Matson, G.C., and Sanford, Samuel, 1913, Geology and ground water of Florida: U.S. Geological Survey Wate~-Supply 

Paper 319,445 p. 

Mcivor, C. C., Ley, J.A., and Bjork, R.D., 1994, Changes in freshwater inflow from the Everglades to Florida Bay including 

effects on biota and biotic processes: A review, in Davis, S.M., and Ogden, J.C., The Everglades-The ecosystem and its 

estoration: Delray Beach, Fla., St. Lucie Press, p. 117-146. 

Ogden, J.C., 1994,A comparison of wading bird nesting colony dynamics (1931-1946 and 1974-1989) as an indication of 

ecosystem conditions in the southern Everglades, in Davis, S.M., and Ogden, J.C., The Everglades-The ecosystem and its 

restoration: Delray Beach, Fla.,St. Lucie Press, p. 533-570. 

Porter, G.S., Snyder, G.H., and Deren, C.W., 1991, Flood-tolerant crops for low input sustainable agriculture i1 the Everglades agricultural area: Journal of Sustainable Agriculture, v. 2, p. 77-101. 

Ray, J.D., Miller, J.D., and Sinclair, T.R., 1996, Survey of arenchyma in sugarcane roots (abs.): Fifth Symposiun of the International Society of Root Research, July 14-18, 1996, Clemson, S.C., p. 118. 

Shih, S.F., Glaz, Barry, and Barnes, R.E., Jr., 1997, Subsidence lines revisited in the Everglades agricultural area, 1997: University of Florida Agricultural Experiment Station Bulletin 902, 38 p. 

Shih, S.F., Stewart, E. H., Allen, L.H., Jr., and Hilliard, J.E., 1979, Variability of depth to bedrock in Everglades organic soil: 

Soil and Crop Society of Florida Proceedings, v. 38, p. 66-71. 

Smith, G., 1990, The Everglades agricultural area revisited: Citrus and Vegetable Magazine, v. 53, no. 9, p. 40-42. 

Smith, T.S., and Bass, O.L., Jr., 1994, Landscape, white-tailed deer, and the distribution of Florida panthers in the Everglades, in Davis, S.M., and Ogden, J.C., The Everglades-The ecosystem and its restoration: Delray Beach, Flc-., St. Lucie 

Press, p. 693-708. 

Snyder, G.H., and Davidson, J.M., 1994, Everglades agriculture: Past, present, and future, in Davis, S.M., and Ogden, J.C., 

The Everglades-The ecosystem and its restoration: Delray Beach, Fla., St. Lucie Press, p. 85-115. 

Stephens, J.C.,Allen, L.H., Jr., and Chen, Ellen, 1984, Organic soil subsidence, in Holzer, T.L., ed., Man-induced land subsidence: Geological Society of America Reviews in Engineering Geology, v. 6, p. 107-122. 
172 Subsidence in the United States 

Stephens, J.C., and Johnson, Lamar, 1951, Subsidence of organic soils in the upper Everglades region of Florida: So;l Science 

Society of Florida Proceedings, v. 11, p. 191-23 7. 

PART III-Collapsing Cavities 

INTRODUCTION 

Ege, J.R. 1984, Mechanisms of surface subsidence resulting from solution extraction of salt, in Holzer, T.L. ed., Mar-induced 

land subsidence: Geological Society of America Reviews in Engineering Geology, v. 6, p. 203-221. 

Martinez, J.D., Johnson, K.S., and Neal, J.T., 1998, Sinkholes in evaporite rocks: American Scientist, v. 86, p. 38-51. 

White, W.B., Culver, D.C., Herman, J.S., Kane, T.C., and Mylroie, J.E., 1995, Karst lands: American Scientist, v. 83, p. 450-459. 

THE RETSOF SALT MINE COLLAPSE, NEW YORK 

Alpha Geoscience, 1996, Geologic and hydrogeologic investigation of the Genesee River Valley, prepared for AKZO Nobel 

Salt Inc., Clarks Summit, Pa.,: Albany, N. Y.,Alpha Geoscience Project no. 95132,31 p., 10 app., 4 plates. 

Dunn Corporation, 1992, Hydrogeologic report for the AKZO ash processing plant: Report to Akzo Nobel Salt, Inc., Clarks 

Summit, Pa., 35 p. 

Moran, R.P., Scovazzo, V.A., and Streib, D.L., 1995, Impact analysis-Retsof Mine, Akzo Nobel Salt, Inc.: Report 24-55, prepared for the New York State Department of Conservation, J. T. Boyd Co., Inc., 54 p. 

Nittany Geoscience, 1995, Groundwater recharge calculations for the Retsof Mine: May 12, 1995, Letter Report, 6 F· 

NYSDEC (New York State Department of Environmental Conservation), 1997, Collapse and flooding of Akzo Nol,ol's 

Retsof salt mine, Livingston Co., N.Y.: Feb. 1997 Draft report of the Department Task Force, Feb. 1997,114 p. 

Riley, F.S., 1969,Analysis of borehole extensometer data from central California, International Association Of Scie:-1tific 

Hydrology Publication 89, p. 423-431. 

Shannon and Wilson, Inc., 1997, Task 3, final report-Retsof Mine collapse, Technical Assistance Grant Committe,~, Retsof, 

New York: Seattle, Wash., Shannon and Wilson Inc., 15 p. 

Van Sambeek, L.L., 1994, Predicted ground settlement over the Akzo Nobel Retsof Mine, prepared for Akzo Nobel Salt Inc., 

Clarks Summit, Pa., Project RSI-0525: Rapid City, S.Dak., RE/SPEC Inc., [27+] p. 

Van Sambeek, L.L., 1996, Dissolution-induced mine subsidence at the Retsof Salt Mine: Meeting Paper, Solution Mining 

Research Institute, October 20-23,1996, Cleveland, Ohio, p. 289-309. 

Young, R.A., 1975, The effects of a Late Wisconsin glacial readvance on the postglacial geology of the Genesee Valley, 

Livingston County, N.Y. [abs.]: Geological Society of America Abstracts with Programs, v. 7, p.135-136. 

SINKHOLES, WEST-CENTRAL FLORIDA 

Atkinson, T., 1977, Diffuse flow and conduit flow in limestone terrain in the Men dip Hills, Somerset (Great Britain): Journal 

of Hydrology, v. 35, p. 93-110. 

Bengtsson, T.O., 1987, The hydrologic effects from intense ground-water pumpage in east-central Hillsborough County, 

Florida, in Beck, B. F. and Wilson, W.L. eds, Karst hydrogeology: engineering and environmental applications: proceedings 

of a conference sponsored by the Florida Sinkhole Research Institute, February 9-11, 1987, College of Engineerinf, University of Central Florida, Orlando: Boston, Mass., A.A. Balkema, p. 109-114. 

Brooks, H.K., 1981, Guide to the physiographic divisions of Florida: Gainesville, Florida Cooperative Extension Service, 

Institute of Food and Agricultural Sciences, University of Florida, 11 p., 1 plate. 

Brucker, R.W., Hess, J.W., and White, W.B., 1972, Role of vertical shafts in the movement of ground water in carbonate aquifers: Ground Water, v. 10, p. 5-13. 

Culshaw, M.G., and Waltham, A. C., 1987, Natural and artificial cavities as ground engineering hazards: Quarterly Journal of 

Engineering Geology, v. 20. p.139-150. 

Ford, D., and Williams, P., 1989, Karst Geomorphology and Hydrology: Boston, Mass., Unwin Hyman, 601 p. 

Lattman, L.H., and Parizek, R.R., 1964, Relationship between fracture traces and the occurrence of ground water in carbonate rocks: Journal of Hydrology, v. 2, p. 73-91. 
References 173 

Lewelling, B. R., Tihansky,A. B., and Kindinger, J. L., 1998,Assessment of the hydraulic connection between ground water 

and the Peace River, west-central Florida: U.S. Geological Survey Water-Resources Investigation Report 97-4211,96 p. 

Littlefield, J.R., Culbreth, M.A., Upchurch, S.B., and Stewart, M.T., 1984, Relationship of modern sinkhole deY~lopment to 

large-scale photolinear features: Multidisciplinary Conference on Sinkholes, 1st, Orlando, Fla., October 15-17, [Proceedings, Beck, B.F., ed., Sinkholes-Their geology, engineering and environmental impact: Boston, Mass., A.A. Balkema], 

p. 189-195. 

Metcalfe, S.J., and Hall, L.E., 1984, Sinkhole collapse due to groundwater pumpage for freeze protection irrigation near 

Dover, Florida, January, 1977: Multidisciplinary Conference on Sinkholes, 1st, Orlando, Florida, October 15-17, [Proceedings, 

Beck, B.F., ed., Sinkholes-Their geology, engineering and environmental impact: Boston, Mass.,A.A. Balkema], p. 29-33. 

Mularoni, R. A., 1993, Potentiometric surface of the Upper Floridan Aquifer, west -central Florida, September 1992: U.S. 

Geological Survey Open-File Report 93-49, 1 plate. 

Newton, J.G., 1986, Development of sinkholes resulting from man's activities in the eastern United States: U.S. Geological 

Survey Circular 968, 54 p. 

Quinlan, J,F., Davies, G.J., and Worthington, S.R.,1993, Review of groundwater quality monitoring network design: Journal 

of Hydraulic Engineering, v. 119, p.1,436-1,441. [Discussion, with reply, p. 1,141-1,142] 

Ryder, P.D., 1985, Hydrology of the Floridan aquifer system in west-central Florida: U.S. Geological Survey Professional 

Paper 1403-F, 63 p., 1 plate. 

Sinclair, W.C., 1982, Sinkhole development resulting from ground-water development in the Tampa area, Florida: U.S. Geological Survey Water-Resources Investigations Report 81-50, 19 p. 

Sinclair, W.C., and Stewart, J.W, 1985, Sinkhole type, development, and distribution in Florida: U.S. Geological Survey Map 

Series 110, 1 plate. 

Southeastern Geological Society, 1986, Hydrogeological Units of Florida: Florida Geological Survey Special PubF:ation 28,9 p. 

Sowers, G.F., 1984, Correction and protection in limestone terrane: Multidisciplinary Conference on Sinkholes, 1st, Orlando, Florida, October 15-17, [Proceedings, Beck, B.F., ed., Sinkholes-Their geology, engineering and environmental 

impact: Boston, Mass., A.A. Balkema], p. 373-378. 

Stewart, M., and Parker, J., 1992, Localization and seasonal variation of recharge in a covered karst aquifer sys+em, Florida, 

USA: International Contributions to Hydrogeology, v. 13, Springer-Verlag, p. 443-460. 

Tihansky, A.B., and Trommer, J. T., 1994, Rapid ground -water movement and transport of nitrate within a karst aquifer 

system along the coast of west-central Florida [abs.]: Transactions American Geophysical Union, v. 75,April19, 1994-

Supplement, p. 156. 

Trommer, J. T., 1992, Effects of effluent spray irrigation and sludge disposal on ground water in a karst region. northwest 

Pinellas County, Florida: U.S. Geological Survey Water-Resources Investigations Report 91-4181, 32 p. 

Watts, WA., 1980, The Late Quaternary vegetation history of the southeastern United States: Annual Review of Ecology 

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Watts, WA., and Stuive~, M., 1980, Late Wisconsin climate of northern Florida and the origin of species-rich deciduous 

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Watts, W.A., and Hansen, B.C.S., 1988, Environments of Florida in the Late Wisconsinan and Holocene, in Purdy, B.A., ed., 

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176 

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