Table Of ContentH Y D R O L O G Y
Daniel B. Stephens
Cover illustration by:
Andrea J. Kron
cARTography by Andrea Kron
Los Alamos, New Mexico
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PREFACE
This book on vadose zone hydrology is unique in both its title and scope. Its intent
is to present elements of physical processes that are most often encountered by
hydrogeologists and groundwater engineers in their projects undertaking character
ization and monitoring of the vadose zone.
The contents of thé book generally parallel a short course of the same title I have
taught since 1990. This short course, in turn, is an abbreviation of a formal, graduate-
level course in vadose zone hydrology developed during my academic career in the
Geoscience Department at New Mexico Tech in Socorro from 1979 to 1989. Credit
for developing that course, the first with such a title in a hydrogeological curriculum
to the best of my knowledge, goes to Professor Lynn Gelhar, who provided encour
agement and seed money for teaching and research materials that fostered my career
and that of many of my graduate students as well.
To understand more about my point of view in writing this book, it is perhaps best
for me to highlight my training and the professors who most influenced me. With a
bachelor’s degree in geology from Penn State and after a couple of years in the field,
I entered a master’s program at Stanford intent on obtaining formal training as a
hydrogeologist. There my interest in unsaturated flow was stimulated by Professor
Irwin Remson of the Applied Earth Science Department during his inspirational
lectures on capillary barriers created by gravel layers and tales about Ike Winograd’s
work at the Nevada Test Site. After several years of consulting as a geologist, most
of which was devoted to nuclear power plant and oil shale sites, I began a doctoral
program at the University of Arizona the same year Professor Shlomo Neuman, a
geologist and engineer, joined the hydrology faculty. Meetings with Professor Neuman
lead to discovery of mutual interest in field permeability tests in boreholes and how
capillary effects could influence the results of tests above the water table. To prepare
for research on the subject, I took graduate courses in traditional soil physics from
Professors Dan Evans and Art Warrick, which gave me a different perspective on
applications of the science of unsaturated flow to agricultural problems. During my
four years at Arizona, I became friends with Dr. L. Grey Wilson, a soil physicist, with
whom I shared many interests for applying soil physics to problems of a hydro-
geological nature, such as waste disposal and groundwater recharge. To these gentle
men, I remain forever in debt.
The nature of this book is a unique blend of soil physics and hydrogeology,
written from the perspective of a hydrogeologist. The book is intended to be used
primarily as a tool for those who already have received training in geology, hydrol
ogy, or engineering and who are dealing with problems involving the vadose zone.
The audience in mind includes primarily consultants and regulators without formal
training in soil physics; however, the book could be a reference or supplementary text
for undergraduate courses in hydrology and soil science, as well as civil, geological,
and environmental engineering. The book does not delve into derivations or complex
mathematics, but it does display equations for the primary purpose of recognizing the
data needs for predictive models in design or regulatory decision making. The level
of presentation is significantly less intense and less broad in coverage than most
introductory soil physics texts. On the other hand, the text is much more detailed and
extensive in coverage of this topic than any current hydrogeology texts. The book
includes an introduction to physical processes, including basic theory of flow, along
with discussions and examples of some of the processes at the field scale that are
essential for hydrogeologists to recognize. Considerable attention is devoted to
recharge, inasmuch as this process often is the most crucial and difficult to evaluate
in vadose zone problems of concern to hydrogeologists. The book includes a chapter
devoted to a review of field and laboratory methods to characterize the hydraulic
properties in the vadose zone, with case studies. The final two chapters deal with the
timely subject of vadose zone monitoring. In this area more than any other, the field
is rapidly evolving toward new and more sophisticated methods to detect contami
nants above the water table. Case studies are presented covering seepage detection,
landfill monitoring, and soil gas investigations.
In addition to thanks to my mentors and graduate students who kept me so
enthusiastic about vadose zone hydrology, I want to acknowledge several people who
have contributed directly to this book by permitting the use of portions of their prior
publications for use here. Jeff Havlena was the senior author for much of the
discussion on the borehole permeameter. Doug Reaber and Todd Stein contributed
the portion of the text on the landfill case study, and Jeff Forbes was primarily
responsible for a significant part of the case study on soil gas. I am indebted to
reviewers of early drafts of and portions of this manuscript including Dr. Jim Yeh,
Dr. Jan Hendrickx, Dr. Michael Sully, and Jeff Havlena. I am especially grateful to
Dr. Mark Ankeny for his critical comments and persistence in reviewing the entire
manuscript; however, I take full responsibility for any errors or omissions. Addition
ally, I am happy to thank Violet Tveit, Pamela Mathis, and Deborah Salvato for their
exceptional organizational skills in assisting in preparation of the manuscript, and
Linda Hirtz and Jhanine Huntsman for preparing many of the tedious illustrations.
And finally, I want to thank my family, Deborah, Jake, and Jordan, for their love and
patience during this ordeal.
The Author
Daniel B. Stephens, Ph.D., is Principal Hydrologist and President of Daniel B.
Stephens & Associates, Inc., Albuquerque, New Mexico. Formerly chairman of the
Geoscience Department at New Mexico Institute of Mining and Technology (NMIMT)
in Socorro, New Mexico, he began private consulting in 1976 and founded Daniel B.
Stephens & Associates, Inc. (DBS&A) in 1984. Dr. Stephens is an adjunct professor
of geology at the University of New Mexico in Albuquerque and an adjunct professor
of hydrology at NMIMT.
Dr. Stephens received his bachelor’s degree in geological science from Penn
State University, his master’s degree in hydrology from Stanford University, and his
doctorate in hydrology from the University of Arizona. Dr. Stephens is a certified
professional hydrogeologist and a registered geologist in California and Arizona. He
is a member of the American Society for Testing and Materials, the American
Geophysical Union, the Geological Society of America, and the American Associa
tion of Ground-Water Scientists & Engineers. He has served on ASTM committees
that establish guidelines and set standards for determining hydrologic properties and
monitoring the vadose zone.
Dr. Stephens is internationally recognized as an authority on vadose zone hydrol
ogy. For several years. Dr. Stephens taught the vadose zone hydrology course at New
Mexico Institute of Mining and Technology. This pioneering course, at the time one
of the few vadose zone hydrology courses available at any university, covered the
theory and application of vadose zone characterization and monitoring. Dr. Stephens
has been invited to speak in-vadose zone issues to national symposia sponsored by
diverse groups such as the American Geophysical Union, the Soil Science Society of
America, the American Association of Ground Water Scientists and Engineers, the
New Mexico Geological Society, the New Mexico Environment Department, and the
U.S. Nuclear Regulatory Commission.
Dr. Stephens has been a technical director of hundreds of environmental and
hydrogeological consulting projects, dozens of which include vadose zone issues
such as landfill siting, seepage analysis, and monitoring systems. He has also been
a pioneer in developing methods to characterize the hydrologic properties of soil. He
developed the first field method that includes capillary effects to determine the
saturated hydraulic conductivity of soil from a borehole permeameter. Through
extensively instrumented field sites. Dr. Stephens and his colleagues have discovered
new physical processes which induce significant horizontal flow components to soil
water movement.
Dr. Stephens has published over 28 articles in peer-reviewed professional jour
nals and given over 47 presentations and articles in symposia proceedings. In
addition to his expertise in vadose zone hydrology. Dr. Stephens specializes in
recharge in semiarid environments, application of numerical models, and aquifer
monitoring and contamination problems.
This book is dedicated to my loving parents,
Dallas W. and Jean E. Stephens
Table of Contents
Chapter 1
Basic Concepts and Theory......................................................................................1
I. Energy Status of Porewater............................................................................3
II. Water Content.................................................................................................8
III. Soil-Water Retention Curves..........................................................................9
IV. Darcy’s Equation and Unsaturated Flow Parameters...................................16
A. Hydraulic Gradient..................................................................................17
B. Unsaturated Hydraulic Conductivity and Relative Permeability...........18
C. Hysteresis in Hydraulic Conductivity.....................................................22
D. Anisotropy..............................................................................................23
E. Soil-Water Diffusivity.............................................................................24
V. Flow Equations for Variably Saturated Porous Media ................................27
VI. Temperature Effects.....................................................................................29
VII. Gas-Phase Flow............................................................................................31
VIII. Chemical Transport Processes............................. 32
A. Liquid-Phase Transport.................................. 33
1. Hydrodynamic Dispersion............................................. 33
2. Chemical Interactions........................................................................35
3. Colloidal Transport............................................................................39
B. Gaseous-Phase Transport........................................................................41
1. Gas Diffusion....................................................................................41
2. Gas Partitioning..................................................................................42
Chapter 2
Soil-Water Budget..................................................................................................45
I. Infiltration.....................................................................................................46
II. Evaporation and Transpiration......................................................................46
III. Water Storage and Deep Percolation..........................................................52
A. Storage....................................................................................................53
B. Deep Percolation and Recharge..............................................................53
1. Physical Methods...............................................................................53
a. Soil Lysimeters.............................................................................53
b. Water Balance..................................................... 54
c. Plane of Zero Flux............................. 55
d. Darcy Flux in the Vadose Zone....................................................55
e. Soil Temperature...........................................................................56
f. Electromagnetic Methods..............................................................57
g. Darcy Flux in Aquifers.................................................................57
h. Water-Level Fluctuations..............................................................58
i. Stream Gaging...............................................................................59
2. Numerical Models of Soil-Water Flow.............................................61
3. Groundwater Flow Models................................................................62
4. Chemical Methods in the Vadose Zone............................................63
a. Tritium..........................................................................................64
b. Chlorine-36...................................................................................64
c. Chloride Mass Balance.................................................................65
d. Stable Isotopes..............................................................................65
5. Chemical Tracers in Aquifers..........................................................67
a. Tritium.................... 68
b. Tritium/Helium-3.........................................................................70
c. Krypton-85...................................................................................70
d. Carbon-14.....................................................................................70
e. Chlorine-36...................................................................................71
f. Chloroflourocarbons.....................................................................71
Chapter 3
Physical Processes Relevant to Deep Soil-Water Movement................................73
I. Infiltration.....................................................................................................73
A. Factors Affecting Infiltration.................................................................74
B. Influence of Soil Air..............................................................................78
C. Effect of Infiltration on Soil-Water Status........................ 79
D. The Displacement Process.....................................................................81
E. Heterogeneity and Multidimensional Flow............................................83
F. Preferential Flow.....................................................................................89
II. Drainage and Redistribution........................................................................94
A. General Processes...................................................................................94
B. Propagation of Pulses of Infiltration.......................................................95
C. Field Capacity and Water Storage..........................................................98
D. Effects of Vegetation on Soil-Water Movement..................................101
E. Pneumatic and Thermal Effects............................................................103
Chapter 4
Recharge................................................................................................................107
I. Diffuse Natural Recharge............................................................................109
II. Local Recharge............................................................................................113
A. Channel Infiltration...............................................................................115
B. Depression-Focused Recharge..............................................................122
C. Culturally Modified Recharge..............................................................125
III. Case Study Considering Combined Recharge Mechanisms......................127
Chapter 5
Characterizing Hydraulic Properties.....................................................................135
I. Planning Site Characterization....................................................................135
II. Saturated Hydraulic Conductivity...............................................................138
A. Laboratory Methods..............................................................................138
B. Field Methods........................................................................................141
1. Air-Entry Permeameter....................................................................142
2. Borehole Permeameters...................................................................144
3. Disc Permeameter/Tension Infiltrometer.........................................149
4. Sealed Double-Ring Jnfiltrometer...................................................151
5. Air and Gas Permeameters..............................................................153
a. Vertical Air and gas permeameters.............................................154
b. Borehole-Type Air and Gas Permeameters................................155
c. Natural Air Pressure Method.......................................................156
6. Other In Situ Test Methods.............................................................156
III. Field Permeameter Case Studies...............................................................157
A. Fluvial Sand...........................................................................................157
1. Borehole Permeameter Tests...........................................................157
2. Comparison to Different Permeameters..........................................159
B. Loam.....................................................................................................161
C. Clay Site................................................................................................164
1. Slim-Diameter Borehole Permeameter Tests..................................165
2. Large-Diameter Borehole Permeameter Tests................................165
3. Comparison of Results of Different Permeameters........................166
4. Wetting Front Behavior...................................................................167
IV. Unsaturated Hydraulic Conductivity..........................................................171
A. Laboratory Methods..............................................................................171
1. Steady-State Methods......................................................................171
2. Transient Methods............................................................................173
a. Instantaneous Profile Method.................. 173
b. Bruce-Klute Method....................................................................174
c. Pressure-Plate Method.................................................................175
d. One-Step Outflow Method..........................................................176
e. Centrifuge Method......................................................................177
B. Field Methods........................................................................................177
1. Instantaneous Profile Method..........................................................178
2. Constant-Flux Methods....................................................................179
a. Ccrust Method.............................................................................180
b. Sprinkler Method........................................................................180
3. Flow Net Method.............................................................................181
C. Estimating Unsaturated Hydraulic Conductivity..................................183
1. Emprical Approach..........................................................................183
2. Calculation From Water Retention Data.........................................185
V. Specific Moisture Capacity and Moisture Retention.................................187
A. Laboratory Methods..............................................................................187
1. Water Column Method....................................................................187
2. Pressure Cell Method.......................................................................189
B. Field Methods........................................................................................191
VI. Unsaturated Hydraulic Conductivity Case Study.......................................191
A. Approach...............................................................................................192
1. Fluvial Sand.....................................................................................192
2. Dune Sand........................................................................................193
B. Results and Discussion.........................................................................193
1. Fluvial Sand.....................................................................................193
2. Dune Sand........................................................................................199
