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AN ASSESSMENT 01= THE ECOLOGICAL IMPACTS OF
GROUND WATER OVERDRAFT ON
WETLANDS AND RIPARIAN AREAS IN THE UNITED STATES
by
Cheryl Grantham
Idaho Water Resources Research institute
University of Idaho
Moscow, Idaho 83844-3011
May, 1996
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Table of Contents
Page
Acknowledgments and Disclaimers .......................Hi
Abstract. 1 v
Section 1 - Introduction 1
Section 2 - Methods .............. 3
Section 3 - Historical Background...................... .5
Wetlands Protection. , 5
Ground Water Use. ,6
Ground Water Overdraft........... ..... 8
Section 4 - Overview of the Ecology of Wetlands and Riparian Areas . . 13
Ecological Importance of Wetlands and Riparian Areas. ... .13
Types of Ecological Impacts Resulting From Overdraft .... 15
Sections- Hydrogeologic Mechanisms Capable of Causing
Ecological Impacts from Ground Water Overdraft . . . . . ... 19
Ground Water Level Fluctuation. 20
Reduction of Ground Water Discharge. 22
Induced Recharge .23
Land Subsidence. . ...-.-. .24
Changes in River Channel Morphology, and Stability. ...... .25
Salt Water Intrusion. 27
Changes in Ground Water Geochemistry..-.._ 28
Section 6 - Locations of Ecological Impacts of Ground Water ,
Overdraft on Wetlands and Riparian Areas............... 31
International .32
England .. . r. .32
Northwest European Lowlands, the Netherlands .. , . 32
.. Spain ... .......,.'... 34
United States..... "..;..., ........ 37
Arizona. 37
California.............. .48
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Florida -.... .. 56
Idaho .. .. 62
Indiana . .63
Nevada ... 64
Oklahoma ^ 69
South Carolina 70
Texas ... .71
Wisconsin . .... 73
Section 7 - Evaluation of the Adequacy of Existing Information on
Ecological Impacts of Ground Water Overdraft 75
Technical Perspective 76
Regulatory Policy Perspective 78
Legal Perspective 79
Section 8 - Recommendations 81
Section 9 - Conclusions .85
Section 10 - Bibliography ........87
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Acknowledgments and Disclaimers
Funding for this project was provided by the Office of Policy, Planning, and
Evaluation of the United States Environmental Protection /Agency
(Cooperative Agreement CX 822008-01). The author gratefully
acknowledges the assistance .of EPA Project Offiqer Rodges Ankrah in the
conceptualization and initiation of this project. Completion of the project
was accomplished with assistance from John Simons, EPA Project Officer
with the Office of Water. Administrative assistance was provided
throughout the project by the staff of the Idaho Water Resources Research
Institute of the University of Idaho.
The author appreciates the comments and suggestions made by Paul Jehn
(Ground Water Protection Council), Sydney Bacchus (Institute of Ecology,
University of Georgia), Harriet Hill (U.S. EPA, Region IX), and John Simons
(U.S. EPA). Assistance from many of the authors whose work is cited in this
report also is appreciated greatly.
Contents of this publication do not necessarily reflect the views and
policies of the Idaho Water Resources Research Institute, nor does mention
of trade names or commercial products constitute their endorsement by the
Idaho Water Resources Research Institute.
The views expressed by the author are her own and do not necessarily
reflect those of the United States Environmental Protection Agency.
Mention of trade names, products, or services does riot convey, and should
not be interpreted as conveying, official EPA approval, endorsement, or
recommendation.
- Printed on Recycled Paper -
in
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AN ASSESSMENT OF THE ECOLOGICAL IMPACTS OF GROUND WATER
OVERDRAFT ON WETLANDS AND RIPARIAN AREAS
IN THE UNITED STATES -
by
Cheryl Grantham
Abstract
One potential, consequence of ground water overdraft that frequently is
overlooked in the allocation of ground water resources is the impact of
pumping on surface ecosystems dependent on ground water. Many
researchers have noted the almost anomalous dearth of information on the
linkage between hydrogeological factors and ecosystem impacts. As ground
water use continues to increase and the health of the remaining wetlands
and riparian habitat comes under increasing scrutiny, there is a growing need
for an adequate technical understanding of this emerging issue to form the
basis of consistent public policy. The-purpose of this report is to collect and
evaluate information on the location, nature, and extent of ecological effects
that have been shown to have occurred in wetlands and riparian areas as a
result of ground water pumping.
As transition environments between aquatic and terrestrial ecosystems,
wetlands and riparian areas are among the most spatially and temporally
complex natural systems on earth. These habitats provide food web support
for all trophic levels as well as sites for breeding, nesting, rearing, resting,
refuge; feeding, and overwintering. It is estimated that over 70% of the
original area of riparian ecosystems in the United States has been cleared
and much of the rest has been affected by a myriad of land and water uses
during this century. .
Ground water overdraft is capable of causing ecological impacts in wetlands
and riparian areas in a variety of ways. Because the stage and duration of ,
the natural hydroperiod are among the most crucial aspects affecting the
location, composition, and overall health of these ecosystems, ground water
drawdown that results in significant hydroperiod perturbation can have a
major impact. Examples include ground water level fluctuation in excess of
species' limits of tolerance, reduction of ground water discharge on either a
seasonal or long-term basis, and induced recharge of underlying saturated
zones derived from drainage, of surficial aquifers on which ecosystems are
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dependent. Overdraft can result in geomorphological changes that impact
wetland hydrology such as land subsidence and alteration of river channel
morphology and stability. Lastly, changes in the natural geochemical
environment in excess of species' tolerance may result from overdraft.
Examples include salt water intrusion as well as other modifications of the
chemical characteristics of the ground water and the associated solid media.
Literature on ecological impacts from ground water overdraft was found for
several countries as well as ten states. The nature and extent of these
impacts are described as well as pertinent geological and hydrogeological
characteristics of the sites. Some impacts are localized while others have
occurred on a regional basis throughout an entire drainage basin or aquifer
system.
The extent to which these impacts have been studied varies widely. In a few
cases, impacts are reasonably well documented. For the most part the
potential for ecological impacts from ground water overdraft is largely
overlooked in the context of water allocation as well as ecological
assessment. The locations of impacts described in this report are not
hydrologically or ecologically unique. Therefore it is probable that the
existing body of literature on this subject significantly underestimates the
extent of ecological impacts that result from ground water overdraft.
To more adequately and accurately address this issue, it is recommended
that studies to identify and quantify ecological consequences from overdraft
be undertaken more frequently in a broad array of academic and resource
management settings* including regional water use planning programs,
wetland surveys, and regional hydrological studies. A major objective
should be an enhanced predictive capability for early identification of "high
risk" hydrologic settings and "high risk" wetland and riparian communities
and species. Efforts to assess and mitigate impacts from ground water
overdraft must be approached with a full understanding of the complexity of
hydrologic systems and wetland ecosystems. Many human activities and
natural processes can result in ecosystem changes and identification of
causal relationships requires detailed, site-specific, and usually long-term
studies.
With care to avoid erroneous oversimplification, this information should be
used to improve planning and policy regarding ground water use and wetland
protection. Permitting processes pursuant to applicable federal, state, and
local statutes should include an assessment of the potential for impact from
subsurface drainage for projects that may affect wetlands and riparian
areas. A major objective of the activities should be an ecosystem approach
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to overall water management in which there is scientifically valid feedback
between the occurrence of ground water overdraft, water stress tolerance
limits of affected species, and allowed pumping pates. -
VII
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Section 1
Introduction
Utilization of ground water resources typically results in increased capture
of local recharge and/or withdrawal of ground water from storage.
Withdrawal-of ground water from storage is commonly called ground water
mining or overdraft (Bredehoeft et al.,J982). It is manifested by a decline
in the water table, a reduction in artesian pressure or an irreversible
compaction of certain fine grained sediments (U.S. Geological Survey, 1984).
There is controversy regarding ,the definition of overdrawing and whether it
should necessarily be avoided (Smith, 1989). One perspective is that
^overdrafting or ground water mining is "no.more unsafe than the mining of
any other mineral resource, provided it's recognized and planned" (U.S.
Geological Survey, 1984). However, it is frequently the case that the extent
of drawdown is'not planned and the associated consequences are not
accurately anticipated. The impacts of some pumping may not be noticeable
until they are irreversible or the consequences may be borne by parties other
than those receiving the benefits of the water which was pumped (House
Committee on Natural Resources, 1994). At present many of our legal
institutions do not adequately address these situations pertaining to ground
water pumping (Bredehpeft et al., 1982).
One potential consequence of ground water overdraft which is frequently
overlooked in the allocation of ground water resources is the impact of
pumping on surface ecosystems which are dependent on ground water. For
example, many wetlands are located in ground water discharge zones and
many riparian communities are dependent on shallow alluvial aquifers. In
arid environments in particular, ground water may be the only perennial
water source available to some wetland and riparian communities. In
locations throughout the country where water level decline has occurred in
the course. of ground^ water development, the potential for large-scale
impacts in wetland habitats is great.
In response to a growing body of information on water level decline, the
National Water Commission referred to ground water mining as^one of the
"three principal problems of ground water law, management and
administration" (Fort et al., 1993). Predicting and preventing adverse
impacts from ground water im^ is a complicated task. Habitat impacts
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are varied and the remedies are often poorly defined and diverse. A broad
group of participants is involved in developing public policy including parties
with interests in water allocation as well as habitat protection.
Many researchers have noted the almost anomalous dearth of information on
the linkage between hydrogeological factors and wetland community
functions (for example Fort et al., 1993; Busch et al., 1992; Llamas, 1989).
As ground water use continues to increase and the health of the remaining -
wetlands and riparian habitat come under increasing scrutiny, there is a
growing need for an adequate technical understanding of this emerging issue
to form the basis of consistent public policy.
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Section 2
Methods
The purpose of this report is to collect and evaluate information on the
location, nature and extent of ecological impacts which have been shown to
have resulted from ground water; overdraft. It is hoped that this document
will assist those who seek to predict or remedy these impacts in their
jurisdictions.
There is considerable controversy ovqr the legal definition of wetlands when
used in the context of. implementing various statutes. Wetlands as described
in this report are broadly and scientifically defined and no attempt has been
made to conform to an individual statutory definition. As used herein, a
wetland is an area "where saturation with water (either permanent or
intermittent) is the dominant factor determining the nature of soil
development and the types of plant and animal communities living in the
soil, on its surface, and in the overlying water" (U.S. Geological Survey,
1984). As sudh, wetlands are transitional between terrestrial and aquatic
systems where the water table is usually at or near the surface or the land
is at least intermittently covered by shallow water. Considerable difficulty
in determining the location and extent of some wetlands results from
situations in which human activities have either permanently or temporarily
created an artificial hydrological regime (Tiner, 1990).
Riparian ecosystems are those flobdplain, bottomland and streambank
communities which occur along watercourses, both perennial and
intermittent. Riparian areas generally occur entirely within the 100-year
flopdplain of streams and rivers and are characterized by vegetation types,
which are adapted to and tolerant of relatively high soil moisture conditions
(Swift, 1984).
i ' ,-...-
For this report, existing literature was reviewed to collect information on
sites at which ecological impacts have occurred as a result of ground water
pumping. Locations included in this study are limited to those where
pumping of. water supplies is,known to be the, primary causal factor of
ecological impacts. Suspected impacts or sites at risk generally were
excluded as the determination of causation is a complicated task. As a
result of this limitation and the fact that many potential sites have not been
investigated by , ecological researchers,-"'ti's number of sites listed in this
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report undoubtedly underestimates the extent of ecological impacts
resulting from ground water pumping.
Sites described in this report are limited further by excluding ecological
impacts from intentional water table suppression such as reclamation
drainage to make land suitable for agriculture or development. Over the last
century many millions of acres have been deliberately drained for cultivation
and development. These activities are not the focus of this report. In
adddition, wetland and riparian impacts from other sources of hydrological
modification such as surface water impoundment and channel modification
are excluded where a differentiation can be made. For this study, discussion
is confined to impacts incidental to ground water resource development.
This report is organized to give the reader an historical, ecological and
hydrogeological context with regard to this issue. This information is found
in Sections 3 and 4. For those interested in a predictive capability,
hydrogeological mechanisms capable of causing ecological impacts in
wetlands and riparian areas are discussed in Section 5. Section 6 contains a
summary of documented sites at which impacts have occurred. An evaluation
of the adequacy of the existing information as a basis for development of
public policy is found in Section 7. Lastly, recommendations for future work
are made in Section 8.
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Section 3
Historical Background on Wetlands Protection,
Ground Water Use, and Ground Water Overdraft
Wetlands Protection ,
For much of our nation's history, wetlands of all types have been regarded as
areas to be converted into some other "more productive use" (Wentz, 1988).
Drainage of wetlands has been seen as a progressive, public-spirited
enhancement of the natural environment designed to alleviate flood danger
and reclaim land for agriculture. Likewise, wetland loss has been viewed as
a relatively minor consequence of receiving the benefits of large surface
water storage projects (Dugan, 1990).
In arid areas, the close association of phreatophytic plant species wjth the
availability of shallow ground water has been understood since the early
part of this century (Meinzer, 1927). Because most phreatophytic species
are of low economic value, the water they transpire has been defined as
"consumptive waste" for most of this century (Robinson, 1958). The
scientific literature on wetlands prior to the early 1970s is filled with
water conservation studies describing the advantages of removal of wetland
and riparian vegetation for the purpose of "salvaging evapotranspiration" for
human needs (Winter, 1988). Replacement of native vegetation with
agricultural crops or grasslands lor grazing was widely undertaken,
particularly in the southwest (for exiample 'Culler et al., 1970; Heindl, 1961;
van der Leeden, 1991). ,
It is estimated that more than 70% of the original area of riparian
ecosystems in the United States has been cleared and less than 5% of the
original riparian vegetation remains in the southwest (Johnson and Hajgnt,
,1984). Wetland loss in the midwestern farm belt states of Illinois,, Indiana,
Iowa, Michigan, Minnesota, Ohio, and Wisconsin accounts for approximately
one-third of all wetland loss in the history of the nation. The highest
percentage loss, 91%, has taken place in-California and the highest loss of
acreage (9,286,713 acres) has occurred in Florida (a 46% loss). All states
except for Alaska, Hawaii, and New Hampshire have lost more than 20% of
their original wetland acreage. The most significant historical loss of
wetlands has resulted from agricultural practices (87%) (National Research
Council, 1991).
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Wetlands and riparian areas were not widely regarded as ecological systems
with essential functions for supporting indigenous flora and fauna until the
1960s (Wentz, 1988). Increasingly, scientific literature has documented the
importance of wetlands and riparian areas as integral parts of the
surrounding watersheds and stream corridors. Their importance for primary
production and nutrient cycling for associated terrestrial and aquatic
ecosystems has been demonstrated repeatedly as well as their role in
providing spawning, nesting, rearing and refuge habitat for many species
(Crance and Ischinger, 1989). Management agencies now also consider the
value of riparian areas in providing a vegetated buffer against erosion and
flooding as well as irreplaceable recreational and aesthetic values (Harrison
and Kellogg, 1989). ,
Multiple objective water resource planning and management decisions today
have replaced the more single-minded objectives of water conservation and
land reclamation which predominated earlier in the century (Wentz, 1988).
Recent statutes including reauthorized versions of the Water Resources
Development Act, the Clean Water Act, and the 1990 Farm Act which
established the Agricultural Wetland Reserve Program have provided impetus
to achieve a "no net loss" of remaining wetland acreage (National Research
Council, 1991). Regulatory efforts have improved in evaluating cumulative
impacts on wetland ecosystems. Although wetland restoration is a
technically and politically elusive goal, for the most part, efforts have
improved in recent years (National Research Council, 1991). However,
because wetland preservation and restoration frequently are impediments to
development, ongoing controversy surrounds the desirability of protection in
some circumstances (Lehr, 1991). For example, recently proposed
Congressional bills seek to weaken the wetlands protection provisions
currently included in the Clean Water Act. At present, the future direction
of wetlands protection is difficult to predict.
Ground Water Use
The United States is fortunate to have a vast- ground water resource. By
volume it is estimated that over 90% of the fresh water in the United States
is in the form of ground water, a volume equivalent to about 35 years of
surface runoff nationwide. Of this volume, about half is considered to be
extractable if no consideration is given to changes in stream flow, effects
on the environment, and the cost of extraction (U.S. Water Resources Council,
1978).
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The United States is also a major user of ground water. In an international
survey of countries for which data were available, the United States ranked
second in terms of volume of ground water used (Llamas et al., 1992). The
pumpage of fresh ground water in 1980 was estimated to be about 88 billion
gallons per day. This amounted to about 10% of the total natural flow
through all of the nation's ground water systems (U.S. Geological Survey,
1984). Ground water supplies drinking water for about half of the country's
population and about 35% of the water used for irrigated agriculture (Smith,
1989). It is also a major source of the water used in industrial processes
and power generation.
Ground water withdrawals have increased steadily and significantly during
most of the twentieth century. As overall water utilization has increased,
ground water_ has supplied an increasingly larger portion Tof total water
needs. Between 1950 and 19?5 surface water withdrawals increased
annually at a rate of 2% compared to a 4% annual increase in ground water
pumped (U.S. Water Resources Council, 1978). Total ground water
withdrawals more than doubled between 1950 and 1980 (Solley and Pierce,
1988). Among the factors responsible for the sustained increase in ground
water use are significant expansion of irrigation, water supply requirements
of growing urban areas, particularly those in arid areas of the west and
southwest, water demands associated with energy production, objections to
the construction of surface reservoirs, and the fact that ground water
usually requires less treatment than surface, water as a potable water
supply (U.S. Geological Survey, 1984).
Agriculture is the largest consumer of water, acccounting for 83% of the
total water consumed in 1975 (U.S. Water Resources Council, 1978). Perhaps
the key driving force for the development of ground water resources during
this century has been the development of irrigation technology which has
permitted a dramatic increase in irrigated acreage nationwide (Smith,
1989). Centrifugal pumps jn shallow dug wells were commonly used on
small tracts in the 1920s. The more sophisticated turbine pumps, deep well
drilling technology and rural electrification which followed by the middle of
the century brought many more acres into irrigated agriculture. Development
of wheel line and center pivot application systems reduced labor
requirements, further encouraging expansion of irrigated acreage (Collins
and Cline, 1991). Consequently, ground water useage has increased 5 to 10
fold from 1950 to_ the present in most heavily irrigated areas of the nation
(Smith, 1989). A major exception is the recent reduction in pumpage in some
areas of extreme water level decline such as parts of the High Plains states
which depend on the depleted Ogallala aquifer. Rising energy costs of
greater lifts have provided an economic incentive for water preservation, a
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return to dryland farming and withdrawal of land from agriculture (Kromm
and White, 1986). -
Many states are looking to ground water to meet most of their future growth
in water use. In a recent survey, the most frequent ground water,availability
issues identified by states were water level decline in response to intensive
pumping (35 states) and legal disputes arising from increasing competition
for available ground water supplies (26 states) (U.S. Geological Survey,
1984).
Ground Water Overdraft
Declines in water tables and potentiometric surfaces have occurred in all
states to some extent. Areas with declines in excess of 40 feet in at least
one aquifer are depicted in Figure 1 (U.S. Geological Survey, 1984).
In terms of volume, the areas of greatest overdraft tend to be relatively
localized regions where water availability falls short of water demand. It is
estimated that 61% of the overdraft in the western states occurs in Arizona,
California, Texas, and Nebraska (U.S. Water Resources Council, 1978). For
example, about two-thirds of the ground water withdrawn in Arizona in 1985
was pumped from storage (U.S. Geological Survey, 1990). Similarly about
77% of the ground water pumped in central and coastal Texas was derived
from storage. Other areas in which ground water is withdrawn significantly
in excess of recharge are parts of Oklahoma, Kansas, New Mexico, Nevada,
South Dakota, and eastern Colorado (U.S. Water Resources Council, 1978; U.S.
Geological Survey, 1985).
Most regional generalizations about the extent of ground water overdraft and
the potential for ecological impacts are inappropriate for several reasons.
First it is difficult to summarize the overall extent of water level decline in
an area because many areas are underlain by more than one aquifer and
declines have not occurred, or have not occurred to the same extent, in all
aquifers present. In most areas, the most significant declines have occurred
in a semiconfined water bearing zone and not in the overlying unconfined
aquifers (U.S. Geological Survey, 1984). Under some circumstances, these
instances of overdraft are less likely to result in ecological impacts as the
extent of interconnection with surface water bodies may be more limited
than with some shallow water table aquifers.
Secondly, extensive water level decline is not necessary for ecological
impacts in wetlands and riparian preas. Any perturbation of the natural
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hvdroperiod to which a hydric ecosystem is adapted may be sufficient to
result in adverse impacts. Three important aspects of a wetland hydroperiod
are- 1) the depth or stage of the fluctuating ground and surface water, 2) the
duration of the fluctuating water levels, and 3) the periodicity or-
seasonality of the water level fluctuations (Bacchus, in press).
Consequently, determination of the affects of ground water pumping must
consider ecosystem sensitivity to alteration of rates of change and duration
of the natural hydroperiod as well as the overall magnitude of drawdown.
Thirdly many of the instances of water level decline depicted in Figure 1
took place many years ago during the initial stages of water development.
Today water levels in those areas are stable and associated ecosystems have
adapted accordingly. Examples include parts of South Dakota and declines in
Iowa and the Chicago-Milwaukee area (U.S. Geological Survey, 1985).
i|
A fourth and extremely important caveat regarding identification of
ecological impacts in wetlands and riparian areas resulting from ground
water overdraft pertains primarily to the southwest. Interpretation of
water level decline data is complicated by- a widespread phenomenon called
channel incision or arroyo cutting which is believed to be independent of
ground water pumping. These dramatic changes in stream channel
morphology have had a major impact on wetland and riparian communities
throughout the southwest.
Prior to the mid-nineteenth century, many streams in the southwest were
associated with wide floodplain aquifers. These shallow water sources
supported extensive riparian habitats and numerous headwater wetlands.
Frequent floods resulted in adequate seed dispersal and constant
replenishment of fertile alluvium. Mature hardwoods forests as well as
extensive marsh vegetation were dependent on these shallow alluvial
aquifers (Hendrickson and Minckley, 1984).
Between 1865 and 1915, a regional decline in water tables which is thought
to be predominantly unrelated to ground water pumping occurred throughout
the southwest (Betancourt, 1990). A combination of human impacts
Including stream ditching and draining, timber harvest from riparian zones
and uplands, and excessive cattle grazing is believed to have interacted with
drought and floods to cause rapid and widespread channel incision and
headward erosion of watercourses throughout the region (Stromberg, 1994).
With smaller flows confined to vertical walled channels, alluvial water
tables declined many tens of feet in some locations. Many valleys which
were previously swampy or which had ground water within 10 feet of the
surface were drained (Bryan, 1928jr -Marshes and grasslands were widely
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eliminated and ultimately riparian forests were diminished as welJ
(Hendrickson and Minckley, 1984).
In many cases these ecological changes resulting from channel incision were
initiated or became evident during periods of development of alluvial ground
water resources for agricultural and Domestic water supplies.
Differentiation of causal factors is difficult in many regions.
In summary, it is clear that in most cases regional generalizations about the
impact of ground water overdraft are not advised. The hydrology, water and
land use history, and ecological characteristics of each location must be
evaluated individually, Many instances of ecological impact are highly
localized and result from very minor perturbations of natural ground water
levels, On the other hand, some occurrences of water level decline may have
little or,no impact on wetland and riparian ecosystems. Sections 4 and 5
provide more detail on the hydrological and ecological considerations which,
contribute to an increased potential for wetland and riparian impacts.
Detailed descriptions of the locations and the ecological consequences of
ground water overdraft are found in Section 6.
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Section 4
Overview of the Ecology of Wetlands
and Riparian Areas
Ecological Importance of Wetlands and Riparian Areas
An understanding of the ecological processes and functions that control
wetland ecosystems is necessary to provide protection for these critical
areas, As transition environments between aquatic and terrestrial
ecosystems, wetlands are among the most spatially and temporally complex
natural systems on earth (Richter and Richter, 1992). And perhaps most
importantly, the ecological importance of an individual wetland is a function
of the presence of other wetlands (Swanson, 1988). Therefore, in many
regions in which wetlands are naturally limited or in which extensive loss
and alteration have taken a toll on predevelopment acreage, the remaining
wetland areas are absolutely critical for many plant and animal species.
Freshwater wetlands, although subject to controversy with regard to
definition, generally may be divided into three categories. Riverine wetlands
include those associated with both perennial and seasonal watercourses.
Lacustrine wetlands are associated with permanent and seasonal lakes and
ponds. Palustrine wetlands include emergent ecosystems such as marshes,
wet meadows, springs, potholes, and fens as well as forested wetland
ecosystems (Dugan, 1990). Examples range from small isolated depressional
wetlands such as glacial potholes or karst sinkholes to regional features
such as poorly drained low relief areas like the Florida Everglades (Brown
and Sullivan, 1988).
Recent research on wetland ecology has stressed the importance of
understanding the functional values of wetlands. Going beyond traditional
areas of interest such as species composition and community structure,
emphasis on wetlands functions enables researchers to evaluate impacts if
wetlands are eliminated or disturbed (National Research Council, 1991;
Brinson, 1993). From an ecological perspective, the most important function
is providing "food web support" for associated terrestrial and aquatic
ecosystems including primary production and nutrient cycling. Many
wetlands are among the most productive of natural ecosystems, exceeding
the best agricultural lands and rivaling the production of tropical rain
forests (National Research Council, 1991). Other important functions for
higher trophic levels include sites for breeding, nesting, rearing, resting,
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refuge, feeding, and overwintering. Healthy wetlands are important in
maintaining regional biodiversity. , -
Beyond a biological significance, wetlands are important for hydrologic
functions including flood conveyance, erosion protection, ground water
recharge, and potential water supplies. They also can contribute to improved
water quality by removing excess sediment, nutrients and other
contaminants from surface runoff and ground water recharge. Lastly,
wetlands are of significant economic value for timber harvest, development
sites and intrinsic aesthetic characteristics.
It is beyond the scope of this report to provide a thorough review of the
broad topic of the ecological importance of wetlands. Instead some of the
more crucial roles will be highlighted for the purpose of illustration.
One of the most important types of wetlands is forested areas in regions
which otherwise have few trees. Riparian areas are the only native forested
environments in the Great Plains (Segelquist et al., 1993) and most desert
areas (Walters et al., 1980). Overstory canopies provide perches, nest sites
and protection for birds in these locations where they would not otherwise
be available. Foliage, flowers, seeds, and fruits support insects, birds, and
mammals '(England et al., 1984). Therefore, loss of forested riparian areas is
significant for all trophic levels.
Waterfowl rely on wetlands for breeding grounds, winter feeding, and
feeding and resting sites in migration corridors. Decline of duck populations
has been linked directly to drainage and degradation of wetlands (Bellrose
and Trudeau, 1988).
In general, the ecology of mammals within wetland ecosystems is poorly
known relative to that in other environments. Mammals are less likely to be
obligatory inhabitants of wetlands than birds and other vertebrates.
However, it is clear that a diversity of mammals engages in opportunistic
exploitation of wetlands for diet, cover, and travel corridors (Fritzell,
1988).
Southwestern riparian systems support some of the richest biotas in North
America. At the same time, these areas are some of the world's most
endangered ecosystems because 70 to 95% have been lost (Johnson and
Haight, 1984). Plant species found in these systems are much less drought
resistant than surrounding desert flora and as such are vulnerable to
hydrologica! disturbance (Walters et al., 1980). -In turn, southwestern
riparian habitats support relatively large and diverse populations of
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mammals, other vertebrates, flowering plants and insects (Johnson and
Haight, 1984) as well as the highest density of noncolonial nesting birds in
the United States (Carothers et al., 1974). In particular, cotton wood/willow
habitat and mesquite bosques (forests) are extremely important for birds
and other animals (England et al., 1984). Mesquite bosques were formerly
the most abundant riparian type in the southwest and now are reduced to
relatively small isolated remnants, virtually none 9* which remain in
pristine condition (Stromberg, 1993b)~ ].
One of the most interesting facts which emerges from a review of the
literature on the ecological significance of wetlands and riparian areas is
their inordinate importance for rare and endangered species. Almost 35% of
all rare and endangered animal species either inhabit wetland areas or are
dependent on them, although wetlands constitute only about 5% of the
nation's lands (National Research Council, 1991). .Over 50% of the federally
designated animal species are wetland related and 28% of the listed plant
species are wetland dependent (Niering, 1988).
i . . " ( '
Reasons for this relationship between endangered species and wetlands
pertain to the nature of wetlands as well as the extensive loss of wetland
acreage. Many wetlands are small in size and lack surface water connection
to other bodies of flowing water. Thus small populations of endemic species
have a high degree of specialization and are vulnerable in the event of
wetland disturbance. Second, wetlands in arid areas, particularly in the
southwest, are refugia for Tertiary and Recent species which evolved during
a much wetter climate. These isolated wetland areas are often remnants of
pluvial lakes and support species which are unable to survive elsewhere in
current post-pluvial conditions (Hendrickson and Minckley, 1984). As a
result, many fish and mollusk species depend entirely on isolated wetlands
in the southwest. In addition to development-related habitat loss, this role
of wetlands as refugia accounts for the fact that about 60% of the federally
listed fish species are found in the desert southwest (Williams and Sada,
1985; Williams etal., 1989).
Types of Ecological Impacts Which May Result from Ground Water
Overdraft
Riparian wetlands are now considered to be the most modified land type in
the western United States and have undergone major changes in most of the
other regions of the country as well (Jackson and Patten, 1988). Historically
the most destructive alterations of wetlands have been associated with
changes in the hydrologic characteristics which support the wetland
15
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ecosystem (National Research Council, 1991). In many wetlands the
hydroperiod is the single most important factor which determines species
distribution and ecosystem health. Systems supported by ground water for
some or all of the year are therefore extremely susceptible to impact from
unnatural changes in water level.
A broad array of ecological impacts may result from ground water
withdrawals. Generalized examples are discussed here for the purpose of
illustration as it is beyond the scope of this report to review this subject in
species-specific detail. It is important to remember that ecological changes
occur along a continuum (Stromberg, 1994). Some absolute thresholds can be
recognized such as maximum .ground water depths beyond which a species
will not grow. But most changes are gradual such as the ongoing decline of a
species or a gradual reduction in disease resistance of individuals
experiencing water stress. In addition, there is often a time lag of as much
as a decade before the effects of water stress are noticeable. Furthermore,
many of the impacts discussed below can be caused by the other sources of
anthropogenic disturbance which are common in many wetlands today as well
as natural cycles of drought and disease. Consequently it is often far from
straightforward to identify impacts from ground water overdraft.
Ground water is a major determinant of riparian vegetation abundance,
community structure, species composition, and population health
(Stromberg, 1994). Riparian and wetland ecosystems undergo changes in
response to water stress in a hierarchical fashion. High levels of stress
cause ecosystem and community level changes and less severe stress can
evoke reponses in individuals of the least tolerant species (Stromberg,
1992). Among the most common impacts seen in wetland areas in response
to hydroperiod perturbation are changes in species composition, species
distribution and wetland extent (Stomberg, 1994; Harding, 1993). A typical
response may involve the loss of obligate wetland species followed by loss
of facultative riparian vegetation species. Hydroriparian and mesoriparian
species may be replaced by xeroriparian and upland species. Nuisance
species which are more tolerant of abnormal water fluctuations may become
established. On the other hand, in some ecosystems water stress results in
ongoing impoverishment of the whole community rather than promoting
succession to drought tolerant species (Walters et al., 1980).
As the competitive balance is disrupted in water stressed wetlands, native
species will frequently be replaced by opportunistic drought tolerant exotic
species which may be less desirable with respect to ecosystem function.
The extensive proliferation of saltcedar coupled with reduced survivorship
16
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of cottonwood and willow seedlings in water stressed riparian areas in the
southwest is an example (Stromberg, 1994).
Structural characteristics of most plant species are influenced by water
level fluctuations. Tree height and morphology can be altered drastically.
When ground water is deeper or not available, trees must invest more
resources into root production and therefore are shorter with less dense
canopies (Stromberg, 1994). For example, mesquite will occur as a tree if
shallow ground ^ water is available but populations will tend toward a shrub
morphology if ground water declines below about 5 to 15 m for significant
periods of time (Stromberg, 1993b). In fact, most major attributes of plants
can be influenced by water availability including total biomass, lifespan,
vegetation volume, leaf size, basal area, root mass and depth of penetration,
investment in reproductive structures, and susceptibiity to disease. Since
many of these parameters are also indicators of insect and avian abundance,
impacts from water stress are felt throughout the ecosystem (Stromberg,
1993b).
Other ecosystem responses to anthropogenic ground^ water level fluctuations
result from changes in soil parameters. Of primary importance in some
areas is an increased susceptibility to fire (Rochow, 1994; Harding, 1993)
Wetland soil which is high in organic matter is very combustible when
desiccated. Furthermore, with reduced soil moisture, mineralization of
nutrients from decaying organic matter is drastically reduced causing a
decline in soil fertility and productivity (Lieuranpe et al., 1994). In areas-
where water level decline has resulted in surface soil subsidence, tree roots
may become exposed. In the case of pondcypress roots which are adapted to
anaerobic conditions characteristic of organic soil and surface saturation,
exposure to air can result in death of the tree (Bacchus, 1995).
One of the key parameters which is indicative of the health of an ecosystem
is the age class diversity. Seed germination and seedling survival are often
indicators of the ability of populations to sustain themselves. In some
areas, seedlings will not germinate without alluvial ground water as rainfall
alone is insufficient (Segelquist et al., 1993). Frequently, seedlings of
riparian species are more sensitive to water level decline than mature
individuals with well developed root systems. Alternatively, some
streamside seedlings have been shown to utilize surface water while mature
individuals have evolved to use a presumably more seasonally dependable
ground water supply (Dawson and Ehleringer, 1991). Thus perturbations in
ground water levels can be differentially detrimental at various stages of
development for certain species. Knowledge of these variables is necessary
to interpret observed impacts.
1-7
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The seasonal cycle of anthropogenic ground water fluctuations-can have a
great influence on the extent of ecological impacts. Spring drawdown can be
detrimental to seed germination as well as populations of
macroinvertebrates and their associated predator species (Riley and
Bookhout, 1990). Summer drawdown can be harmful as alternative water
sources such as surface water or precipitation are least available to
compensate for the loss of ground water during the summer months
(McKnight, 1992; Lewis and Burgy, 1964).
Much of the preceding discussion has focused on impacts to plant species.
Impacts of water level decline are found at all trophic levels (Loftus et a!.,
1992). Often the elimination or degradation of one plant species will have
an impact on the entire floral and faunal assemblages.
18
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Section 5
Hydrogeologic Mechanisms Capable of
Causing Ecological impacts from
Ground Water .Overdraft
Introduction
Wetlands occur in geologic and hydrologic settings which enhance the
accumulation or retention of water. Water sources may include surface
water, ground water, precipitation or anthropogenic sources such as
irrigation or wastewater disposal. Two key aspects of a landscape need to
be considered to understand wetland hydrology: 1) the shape and hydraulic
characteristics of the land surface which affect movement of water across
it and 2) the geologic boundaries and hydraulic characteristics of the
subsurface which affect ground water flow systems (Winter, 1988). Ground
water dependent wetlands result from the interaction of these two sets of
surfaces. Examples include locations were the water table intersects the
land surface such as at breaks in surface slope, surface watercourses and
water table mounds as well as artesian discharge at springs and seeps.
The natural hydroperiod is among the most important factors which
determine the location, composition and health of wetlands and riparian
ecosystems. To preserve the natural hydroperiod, the sustainable yield for
allowable pumping must be based on the season, rate, and location of
pumping as well as the magnitude of the withdrawal (Dingman, 1994).
Ground water pumping which results in changes in excess of species' limits
of tolerance in any of these parameters will result in some degree of
ecological impact.
In this section the various hydrogeological changes which result from
pumping will be described in terms of the ways in which they may cause in
ecological impacts in wetlands and riparian areas. Hydroperiod
perturbations may result from changes in the seasonally, rate, and extent of
natural fluctuations in the following: 1) the water table and capillary rise in
the vadose zone, _2)'surface water flow, and 3) artesian discharge. In
addition, alteration of natural hydrologic conditions by ground water
pumping may cause or be accompanied by other changes in the physical
environment. Overdraft can result in geomorphological changes which
impact wetland hydrology such .as land subsidence and alteration of river
19
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channel morphology and stability. Lastly, changes in the natural geochemical
environment in excess of species' tolerance may occur including alteration
of equilibria between the ground water and the associated solid media as
well as salt water intrusion.
Ground Water Level Fluctuation
In riparian settings, alluvial soil moisture is determined by a complex
interaction of channel geometry, river stage and discharge, precipitation and
alluvial ground water dynamics (Segelquist et al., 1993). A growing body of
evidence suggests that riparian ground water is a primary source of water
for many riparian plant and tree species (Busch et al., 1992). Ground water
pumping may lower the water level beneath the depth of root penetration
either temporarily or permanently, thereby subjecting riparian vegetation to
lethal or sublethal water stress.
Riparian vegetation may be particularly vulnerable to impact from water
level decline for several reasons pertaining to the hydrolbgic variability
characteristic of riparian settings. In arid and mesic environments, alluvial
ground water may be the only water supply available during summer periods
of base flow. Consequently, even a small change in riparian water
availability can have a pronounced impact on riparian ecosystems
(Stromberg, 1993a). Similarly, periodic flood flows deposit seeds of some
riparian species such as Fremont cottonwood and Goodding willow in high
fioodplain settings distant from the active river channel. Seedling survival
may thus become entirely dependent on, shallow riparian ground water
(Stromberg, 1993c). In addition, the coarse -alluvium which is common in
many riparian environments has a low water retention capacity and reduced
capillarity, predisposing these environments to water stress (Mahoney and
Rood, 1992).
The rate, duration, seasonality and magnitude of hydroperiod perturbations
are all important in determining the effect of ground water withdrawal on
riparian and wetland ecosystems. Frequently, ground water pumping is
greatest during the dry summer months, particularly if ground water
withdrawal is needed for agricultural supplies. When pumping ceases or
declines after the growing season, water levels may recover. This seasonal
pattern can be detrimental to riparian ecosystems because recharge from
base flow and precipitation are at an annual low and plant water
requirements are highest during periods of greatest pumping.
20
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Many phreatophytjc species are capable of rapid root growth in response, to a
seasonally declining water table. Water level decline within these species-
specific limits may not be harmful and in fact, may promote extensive root
growth. For example, seedling survival of plains cottonwoods was stiown to
be highest at a drawdown rate of 0.4 cm/day and decreased with increasing
rates of water level decline (Segelquist et al., 1993). Likewise, the
maximum depth of root penetration varies by species and is a major factor in
determining the ability of a species to withstand a declining water table. A
related variable in determining riparian survivorship is the soil, texture.
Coarse alluvial soils enhance root penetration but are also readily drained
(Mahoney and Rood, 1992).
Time lags oh the order of years to decades may occur as impacted riparian
ecosystems adjust to reduced soil moisture conditions. This is because
mortality often occurs episodically and because individuals within a
population may vary in their tolerance to water stress (Stromberg, 1993a).
Thus the effects of water level decline may be unnoticed until considerable
impact has occurred.
Numerous examples of riparian and wetland impacts from ground water level
fluctuation are described in Section 6. One brief example will be mentioned
here as an illustration. Portions of the alluvial aquifer of the Carmel River
Valley (California) have experienced as much as 10 m of drawdown resulting
from pumping for municipal water supplies. When combined with a two year
drought which virtually eliminated annual recharge Trom river flow,
: phreatophyte mortality in the vicinity of the well fields was extensive.
Downstream of the well fields, aquifer drawdown was minimal and
vegetation experiencing the same weather-related conditions remained
'healthy (Kondolf and Curry, 1986).
Adverse impacts also may occur when ecosystems have successfully adapted
to anthropogenic ground water level decline and pumping is then stopped or
reduced. As the water level recovers, surface or subsurface inundation may
be injurious to , root tissue adapted to an aerobic, lower moisture regime.
Groeneveld (1989) identified root tissue damage resulting from rapid water
table fluctuations associated with changes in ground water pumping rates.
Likewise, when pumping ceases in dewatered mining and construction pits,
recovery of water levels, has the potential for similar adverse impacts on
any wetlands in the impacted area.
21
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Reduction of Ground Water Discharge
Ground water discharges to the land surface as a result of gravity, artesian
pressure, topography, and structural and stratigraphic geologic features.
Ground water discharge zones typically support important ecosystems such
as gaining reaches of rivers and-their associated riparian areas. Other more
localized discharge features include a wide variety of wetlands, marshes,
springs, and seeps. Ground water discharge may be the sole means of
sustenance for some of these settings, particularly in arid environments
where precipitation and surface discharge may be intermittent or
nonexistent during much of the growing season.
Anthropogenic withdrawal of ground water may reduce natural discharge to
these types of ecosystems in several ways. Excess pumping of ground water
in floodplain aquifers may reduce the baseflow in rivers to the detriment of
aquatic and riparian ecosystems. (Baseflow is that portion of the annual
discharge of a watercourse which is derived from ground water storage or
other delayed sources (Hall, 1968)). The hydraulic gradient may even be
reversed resulting in a previously gaining reach becoming a losing reach.
Such is the case in portions of the lower Carmel River in California. After
spring runoff discharge events, the river is sustained primarily by bank
storage from the alluvial aquifer. Timing of baseflow contributions is
critical to downstream migration of steelhead trout smolt and success of
willow seedlings. In recent years, localized pumping of the alluvial aquifer
for municipal water supplies has been shown to reverse the hydraulic
gradient in some reaches and deprive the river of a volume of baseflow
which was directly proportional to the rate of pumping (Kondolf et al., 1987).
Furthermore, in late summer in 1982 when pumping was coupled with
prolonged drought, the river dried up completely in the pumped reach and re-
emerged downstream.
Ground water pumping also may reduce or eliminate discharge at springs,
seeps, and wetlands. This has occurred on a widespread basis throughout the
southwest at localized headwater wetlands called cienegas (Stromberg,
1994) and at many former sites of artesian springs. Reduction of spring
flow is a serious concern at the major springs fed by the Edwards Aquifer in
Texas (Longley, 1992) and at springs in similar settings in the southeast
(House Committee on Uatural Resources, 1994). Likewise, seeps, springs and
marshes in the .Humboldt River Basin (Nevada) are threatened by ground
water pumping for dewatering gold mining pits (Bureau of Land Management,
1993a).
22
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As was mentioned in the previous section, the extent of ecological impacts
from a reduction of ground water discharge is determined by the rate,
duration, seasonally and magnitude of the reduced flow as well as various
ecological characteristics of the wetland and riparian communities. In many
cases, hydrophytic vegetation is rapidly replaced by species which are more
tolerant of water stress, resulting in a loss of food sources, shelter and
nesting sites for animals which rely on wetland areas.-
Induced Recharge
In many areas ground water in confined or semiconfined aquifers is under
sufficient artesian pressure to discharge vertically to ah overlying
unconfined aquifer or to discharge to the surface in the form of springs,
wetlands or a surface water body. If pumping of the confined aquifer
exceeds recharge, the hydraulic pressure in the confined system may be
reduced to the point that surface discharge ceases. Further pumping may
reverse the vertical gradient causing the unconfined aquifer to recharge the
underlying pumped aquifer. If recharge to the unconfined aquifer is
insufficient, the water table will decline. This sequence is called induced
recharge (Fetter, 1988). Clearly any riparian or wetland ecosystems which
are dependent on the artesian discharge or the unconfined shallow ground
water will be adversely impacted if induced recharge is significant.
Certain geologic settings predispose wetlands to being vulnerable to
hydroperiod alteration from induced recharge. In general, any discontinuity
in a confining layer will convey downward recharge in the event of excess
pumping of the confined aquifer. Examples include lenses of highly
permeable materials such_ as glacial or alluvial sand and gravel. Structural
discontinuities such as fractures, faults or karst features such as sinkholes
also serve as conduits for downward recharge. Geophysical methods such as
ground penetrating radar can be used to identify structural features which
present a risk to wetlands in the event of excess pumping (Bacchus, 1994;
Bacchus, 1995). ,
Documented examples of wetland impacts from induced recharge are not
abundant although the phenomenon is not uncommon. Pondcypress in
depressional wetlands" in the vicinity of municipal well fields in southwest
Florida have been adversely impacted by induced recharge derived from
surficial saturated zones. Other ecological impacts identified in wetlands in
this area include succession to upland plant species, soil subsidence and
increased susceptibility to fire (Rqchow and Rhinesmith, 1991).
23
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Las Vegas Valley (Nevada) provides another example. Development of ground
water resouces has resulted in over 300 ft of water level decline. Artesian
springs which were abundant in the nineteenth century have been eliminated
and the vertical hydraulic gradient has been reversed in some areas. As a
result, the shallow aquifer which previously had supported marsh vegetation
in portions of the valley now recharges the underlying semiconfined aquifer
(Katzer and Brothers, 1988).
Land Subsidence
Ground water decline can result in land subsidence which may adversely
affect riparian and wetland ecosystems in several ways. Subsidence can
occur either surficially or in the subsurface. Surface subsidence typically
occurs when moist highly organic soils are subjected to drying conditions
such as would result from water table decline or loss of artesian flow. With
exposure to the atmosphere, the organic matter is oxidized and surface
compaction may result. Wetland tree species such as pondcypress are
particularly vulnerable to damage from surface subsidence. Tree bases and
roots which are adapted to an anaerobic soil environment become exposed to
air and susceptible to fungal pathogens. The resulting decay may result in
the death of the tree (Bacchus, 1995). Loss of organic soils at rates of up to
15 cm within the first year of ground water withdrawal is not uncommon
from this type of subsidence in the Southeastern Coastal Plain (Bacchus,
1994).
Subsurface subsidence is by far more areally extensive. It occurs when
ground water is withdrawn from unconsolidated or poorly consolidated fine
grained sediments, typically clay. Collapse of the molecular structure of the
clay minerals results in permanent compaction of the dewatered materials.
Although irreversible once it has occurred, compaction will cease when
further ground water decline is halted (Freeze and Cherry, 1979). Another
type of subsurface subsidence occurs in the form of collapse from
dissolution of karst features (Bacchus, 1995).
The areal extent and elevation loss resulting from land subsidence can be
considerable. The maximum subsidence recorded in the United States is 29
ft measured on the west side of the San Joaquin Valley (California).
California also ranks first in terms of statewide total area affected by
subsidence induced by ground water withdrawal with over 6,200 rni2. Texas
and Arizona follow with 4,600 mi2 and 1,000 mi2 respectively (Poland,
1981). For perspective, it is interesting to note that the total volume of
aquifer storage lost as a result of aquifer compaction in California's Central
24
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Valley alone is: half the man-made surface storage capacity statewide
(Conniff, 1993). Management of this source of impact is difficult because
laws governing liability for subsidence are not settled in most jurisdictions
(Kopper and Finiayson, 1981).
Many areas of subsidence are in coastal regions where ground water is
derived from alluvial and shallow marine sediments (Johnson, 1981). As a
result, coastal wetland ecosystems are at increased risk due to salt water
intrusion and erosion and inundation from tides and storms (U.S. Water
Resources Council, 1978). For example, since 1943, several thousand acres
of bayfront property have been submerged into tidal reaches of Galveston
Bay, Texas (Neighbors, 1981) and similar impacts have been observed .around
southern San Francisco Bay, California (Fowler, 1981). Subsidence of about
450 mi2 in the Delta area formed at the confluence of the San Joaquin and
Sacramento Rivers (California) has resulted in submergence of islands to a
depth of 10 to 20 ft below sea level (Bertoldi, 1992). Specially adapted
intertidal communities such as salt marsh vegetation can be eliminated
when subsidence alters tidal elevations (National Research Council, 1991).
Subsidence also can alter or reverse surface water drainage patterns,
adversely affecting riparian and wetland ecosystems which are adapted to
the characteristics of a particular watercourse. Examples have been cited in
Galveston Bay (U.S. Water Resources Council, 1978) and the San Joaquin River
basin (Kopper and Finlayson, 1981). Subsidence can lead to reduction of the
gradient of a drainage basin. Riparian ecosystems are then exposed to
repeated flooding such as has occurred in the lower Santa Cruz River basin in
Arizona (Schumann et al., 1986). Where drainages cross the periphery of a
subsiding basin, the river channel gradient is increased resulting in
accelerated erosion. Removal of topsoil and deepening of the stream channel
may also adversely affect riparian ecosystems (Schumann et al., 1986).
Lastly, subsidence can result in surface features such as earth fissures and
sinkholes. These depressions can accelerate erosion and capture surface
flow and thereby alter the hydrologic characteristics to which riparian
ecosystems are adapted (Newton, 1981).
Changes in River Channel Morphology and Stability
River channel morphology and stability are determined by a complex and
dynamic equilibrium between many aspects of a drainage basin including the
soil and bedrock characteristics, the slope of the watercourse, and seasonal
discharge p£i'ems including -flooding cycles (Bloom, 1978). Riparian
25
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vegetation is adapted to these factors and changes outside species' specific
ranges of tolerance can adversely impact entire riparian ecosystems.
Ground water flow in shallow riparian aquifers plays an important role in
riverine hydrology and geomorphology. Excess pumping of alluvial aquifers
can deprive riparian vegetation of an adequate water supply, particularly in
the hot summer months during periods of low river flow. Loss of the
stabilizing effect of plant roots can greatly increase erosion of river banks
resulting in temporary or permanant loss of riparian habitat. Subsequently
.eroded sediment can be redeposited farther downstream with adverse effect
on plant species which may not be adapted to depositional environments. In
general, the relative importance of vegetation for bank stability is greater
for smaller streams but can be critical for all watercourses (Kondolf and
Curry, 1986).
The Lower Carmel River (California) is an example of the scenario described
above. Extensive pumping from the alluvial aquifer in recent decades has
resulted in massive death of riparian vegetation. Subsequently, several
relatively minor discharge events widened the river channel from 60 to over
400 ft in just 6 years (Groeneveld and Griepentrog, 1985). The overall
character of the river in the affected reach has changed from "a narrow,
stable meandering-channel to a wide shifting channel with braided reaches,
with obvious effects on the success with which the previous riparian
community can re-establish itself. Downstream reaches unaffected by
pumping have maintained healthy bank vegetation and have experienced no
major erosion (Kondolf and Curry, 1986).
Channel stability and morphology also can be affected when ground water
discharge to a watercourse is anthropogenically increased. In the case of
Las Vegas Wash (Nevada), ground water pumped from a deep aquifer is used
extensively to irrigate lawns and golf courses in the area. This has caused a
considerable increase in recharge to the shallow aquifer system. The
resulting increased discharge from the shallow subsurface to Las Vegas
Wash has been partially responsible for changing the wash from an
ephemeral to a perennial stream. As a consequence of this increased
discharge, erosion and headcutting have had a major impact on the riparian
vegetation. As the channel has been lowered by about 15 ft, water levels
have declined in the riparian communities. Plant communities have changed
from swamp and marsh vegetation to saltgrass and salt cedar. Headcutting
has resulted in the upgradient migration of the hydrophytic species (Burbey,
1993).
. 26
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Salt Water Intrusion
When ground water is pumped from fresh water aquifers that are in hydraulic
connection with saline water, the resulting gradient may induce a flow of
salt water toward the well. Overpumping of a well or well field in close
proximity to the salt water/fresh water interface can result in the interface
being drawn toward the well to the extent that salt water intrudes into the
fresh aquifer (Freeze and Cherry, 1979). This can occur under several
conditions. In coastal areas, sea water of greater density frequently
underlies surficial fresh water aquifers. In addition, an estimated two
thirds of the continental United States is underlain at some depth by saline
ground water, with the majority of .the shallowest located in the Central
Plains and Midwestern states (Atkinson et al., 1986). Excess pumping in
these areas can > draw underlying salt water upward toward or into a fresh
water zone, a phenomenon known as upconing (Fetter, 1988).
Salt water intrusion is a widespread problem in many parts of the country.
Atkinson et al. (1986) provide a comprehensive summary of the locations
where salt water intrusion, is threatening or contaminating fresh water
supplies. As an overview, salt water Intrusion has occurred in each of the
21 coastal states and has the potential to worsen as water demand increases
in coastal urban areas. Among the more critical problem areas are Long
Island (New York), the Biscayne Aquifer (Florida), and several basins in
California and Georgia. In total, only 8 states throughout the country have
not reported any instances of salt, water intrusion (Atkinson et al., 1986).,
Riparian areas and wetlands which are supported by fresh ground water
discharge can be impacted when overpumpihg results in intrusion of highly.
mineralized water. Riparian apd wetland communities are adapted to
specific ranges of salt tolerance. Increasingly saline conditions interfere
with water and nutrient uptake by plants (Bolen, 1964). As salinity
increases, plant growth usually is reduced and succession to more halophytic
species may occur. Such changes will typically be accompanied by impacts
to native animal species, particularly waterfowl, which depend on wetland
vegetation (Bolen, 1964). Evaporation and evapotranspiration accelerate salt
accumulation in the soil surface, exacerbating salinity impacts during the
growing season. In contrast, one potentially benejicial but relatively minor
effect of increased salinity has been noted in fine grained wetland soils.
Increased salinity enhances the internal cohesiveness of clay particles and
results in reduced erodibility of these soils (Jenkins and Moore, 1984).
Although salt water; intrusion is geographically widespread, and the
potential for ecological invents j& great, research on the nature and extent
27
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of such impacts is very limited. Attention is focused more frequently on the
impacts on drinking water supplies, water quality .for irrigated agricultural
supplies, and costly impacts borne by industrial users (Atkinson et a!.,
1986).
One comparatively well documented example of potential ecological impact
from salt water intrusion is occurring in portions of the Edwards Aquifer
(Texas). Overpumping for public water supply draws highly saline water
toward freshwater springs which threatens several endangered species of
amphibians- (Longley, 1992).
Changes in Ground Water Geochemistry
Studies on the interaction between ground water and surface water have
more commonly focused on the hydrologic balance in the system. As noted
above, alteration of the rate and direction of ground water flow can result in
ecological impacts to ecosystems. However, many geochemical aspects of
the surface water/ground water system and the associated solid media can
profoundly affect the biological communities which inhabit them. In general,
geochemical effects associated with changes in ground water discharge have
received relatively little attention compared to the ecological significance
of surface water chemical parameters (Hagerthey and Kerfoot, 1992).
Nutrient availability can be influenced significantly by ground water
discharge in a wetland. Reduction of ground water discharge can reduce the
rate of mineralization of detrital organic matter, resulting in decreased
delivery of essential nutrients such as carbon, nitrogen and phosphorus to
the root zone. Alternatively, reduced ground water discharge may result in
increased nutrient availability if ground water discharge is not available to
dilute nutrient concentrations and transport nutrients from the root zone
(Harding, 1993). Overall nutrient ratios as well as nutrient availability are
important in determining species composition and health in many wetlands.
Selective removal of the more soluble nutrients will alter optimal ratios to
which species are adapted.
Reduction of ground water discharge to wetlands can alter several other
important geochemical parameters of ecological significance including the
dissolved oxygen concentration, pH, redox potential, salinity and alkalinity
of the soil/water environment. Redox potential controls the solubility and
bioavailability of redox sensitive elements such as iron, manganese,
nitrogen, sulfur, and chromium (Stumm and Morgan, 1981). Reduction of
water discharge to a wetland also can affect physical features of the
28
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soil such as the temperature and the degree of aeration of the soil
(Grootjans and Ten Klooster, 1980). - Furthermore, without ground water
input to certain wetlands, surface outflow may be eliminated and
evapotranspiration may become tie major hydrologic output, thereby
promoting solute buildup in the soil (Davis, 1993).
Wetland ecosystems generally are adapted to a specific range with respect
to each of these parameters. Changes that exceed the tolerance of the
individual species will result in a loss of species and succession to more
tolerant species. For example, calcium-rich ground water discharge
traditionally has supported many rare and endangered plant species in a
nature reserve in the Netherlands. Increased ground water pumping for
drainage and drinking water supply over a period of 40 years has reduced the
ground water discharge. As a result, the extent of the calciphilous marsh
species has been reduced and succession to woody species such as Alniis has
been promoted (Wassen et al., 1989). Another example is provided by
changes in salinity in Las Vegas Wash (Nevada). Increased land application
of pumped ground water has resulted in greater flow through the salt-laden
shallqw alluvial deposits along the banks of the wash. The resulting
increase in salinity has adversely affected riparian vegetation in this area
(Burbey, 1993). . .
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Section 6
Locations of Ecological Impacts of
Ground Water Overdraft on
Wetlands and Riparian Areas
Introduction
The following section is a compilation of the literature which was found on
the location and extent of impacts on wetlands and riparian areas. The
majority of the information is from the United States although some
international examples have been included where documentation is
sufficient. Most of the locations are confined to an individual drainage basin
or aquifer system. However, several instances of more regional impacts
were identified.
This compilation is intended to be as-comprehensive as possible. Because
many sites are not well researched or well documented, it clearly is not
indicative of the magnitude of this issue in the United States. In some
cases, extensive hyd/ological modifications resulting from ground water
drawdown are described in the literature but no followup investigations
regarding the potential for impacts in associated wetland ecosystems have
been conducted.
Discussions of some locations of impacts have been developed into case
studies because detailed information was available. In some cases,
considerable information on geological settings and local hydrogeology has
been included. It is hoped that this information will be of use in a predictive
sense for other as yet unrecognized locations with similar characteristics.
3.1.
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INTERNATIONAL STUDIES
ENGLAND
Redgrave and Lopham Fens, East Anglia
Botanical and zoological data document ecological changes occurring over
the past 30 years in several valley wetlands in a national nature reserve in
East Anglia, England (Harding, 1993). Beginning in 1957, nearby pumping of
ground water for public water supplies was determined to be the primary
cause of impacts occuring at the species, community and ecosystem levels.
Because East Anglia has the highest concentration of such wetlands in
Britain and also has a substantial water supply deficit, there is the potential
for widespread and long term impacts (Harding, 1993).
Prior to the late 1950s, calcium-rich, nutrient-poor water discharged under
artesian pressure from a semiconfined aquifer and supported largely
herbaceous wetlands in the Redgrave and Lopham fens in East Anglia. With
the onset of pumping in 1957, artesian pressure was reduced to" the point
that surface discharge was eliminated and plant communities were
sustained exclusively by precipitation. This change in hydrology resulted in
a drastic alteration of the competitive balance of the dominant plant
species. By the 1970s, herbaceous species rapidly were being replaced by
scrub species such as Salix cinerea and Betula pubescens. The loss of ground
water discharge eliminated the specialized environmental conditions
required by semiaquatic and surface rooted species which can be sensitive to
reductions in water levels of as little as 2 to 3 cm.
Invertebrate species dependent on spring-fed and calcareous wetlands also
have declined. Seventy seven percent of the fen and bog species have been
lost. The spider Dolomedes plantarius currently faces extinction in this
area. An associated impact of the loss of ground water discharge is that the
wetlands are now susceptible to frequent damage from fires.
NORTHWESTERN EUROPEAN LOWLANDS
INCLUDING THE NETHERLANDS
The hydrology of the vast northwestern European lowlands has been altered
drastically by dish -age, primarily for land development and agriculture. Wet
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.meadows have become rare as they have been converted to highly productive
pastures. Remnants of native types of meadows are under protection as
nature reserves (Grootjans and Ten Klooster, 1980).
Vegetation changes associated with altered hydrology were investigated in
Germany as early as the 1930s. However, the objective of these studies was
to measure changes in agricultural productivity to estimate compensation
payments to farmers (Jalink, 1994). Beginning in the 1970s, considerable
research has been undertaken by Dutch scientists but studies limited to the
impacts of ground water pumping have not been performed because of the
widespread alteration, of the natural hydrology for drainage. Currently,
research is directed increasingly toward identifying ways in which changes
in surface water management and .land use can minimize the ecological
impacts of ground water pumping (Jalink, 1994). Regulations for well siting
and ground water allocation in the Netherlands currently do address the
importance of conservation of the remaining natural wetlands (Jansen and
Maas, 1993).
In very low relief landscapes such as are typical in the Netherlands, ground
water discharge zones called seepage faces are common (Jansen and Maas,
1993). Natural vegetation around these zones may be dependent entirely on
the soil chemistry and water availability resulting from the ground water
discharge. Excess pumping of ground water readily can threaten or eliminate
vegetation adapted, to these conditions as was shown by Grootjans and Ten
Klooster (1980) for three Dutch wetland reserves and by Jansen and Maas
(1993) for the Punthuizen wetland sanctuary.
Similarly, Wassen et al. (1989) have investigated the impacts of ground
water decline in the Naardermeer nature reserve in the Netherlands over the
past 40 years. The seepage areas have long supported T/7e/yper/s-reediands
and many rare and endangered plant species. During the last 40 years ground
water has been pumped for drainage and drinking water. This has reduced
ground water discharge in many seepage areas. Because the regional ground
water is calcium-rich, the distribution of endangered calciphilous plant
-species including Caricion davallianae has been restricted severely.
Diminished fresh seepage flow also has resulted in the acidification,
salination, and eutrophication of the studied marshes. Succession to woody
species such as filnus is widespread and has resulted in accelerated loss of
additional marsh species.
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SPAIN
Spain is the most arid country in Europe. Ground water demand for various
uses is large and has increased rapidly during the last two decades.
Similarly, the great ecological value of the wetlands in Spain did not become
appreciated widely until the 1970s. This growth of water demand already
has led to some serious conflicts between wetland conservation and ground
water development (Llamas, 1989). The problem is excerbated by a lack of
knowledge of the hydrology of many wetland ecosystems (Suso and Llamas,
1993).
Douro River Basin
Among the more systematically studied ecological impacts resulting from
ground water extraction are the ecosystem changes occurring in the Dquro
River Basin in central Spain (Bernaldez et aL, 1993). Because the basin is a
somewhat geographically isolated semiarid enclave, many species including
several endangered species of birds of prey, depend almost entirely on the
dispersed wetlands. Several different types of wetlands occur in the basin
including wet meadows, sedge meadows, marshes, phreatophytic woodlands,
ponds and sloughs. They include ground water discharge and recharge sites
and have differing degrees of interconnection with the regional and shallow
aquifers.
Over 40 years of declining water table levels are the result of ground water
pumping, primarily for crop irrigation. The average decline from 1970 to
1987 was 1 m/year. In the four subareas studied in detail by Bernaldez et
al., 39 to 82% of the wetland area has been lost (i.e. is now area which is no
longer classified as a wetland).
The following impacts were noted. 1) Non-phreatophytic annual species such
as Trisetaria panicea, Bromus tectorum and Vulpia spp. have increased,
indicating dryness and increasing nitrification due to the mineralization of
labile soil organic matter in the reduced moisture regime. 2) Perennial
plants such as Festuca rothmaleri, Phleum pratense and others have
disappeared. Mosaics of xerohalophytes have proliferated. 3) Desiccation of
slightly sandy soils has resulted in wind erosion of the A soil horizon and
enhanced surface salination which further impedes growth of vegetation.
Because many of the wetlands in this basin are interspersed in vast expanses
of irrigated cropland, they are reserves for a diverse flora and fauna which
34
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may,be dependent entirely on an individual isolated locality. Food webs of
plants, insects, aquatic invertebrates, reptiles, waterbirds and, mammals are
being impacted by the loss of relatively small wetland areas (Bernaldez et
a!., 1993).
Donana National Park
The Donana National Park (DNP) is recognized as one of the most important
natural environments of the European Community (Llamas, 1989; Suso and
Llamas, 1993). The area first received legal protection in 1969 and has been
designated'a Reserve of the Biosphere by the United Nations.
Donana National Park is situated in a tectonic basin filled with Plio-
Quaternary sediments in the Lower Guadalquivir Valley. Three ecosystems
can be distinguished in the DNP: stabilized eolian sands and moving coastal
dunes on the periphery and the central marshlands. An extensive partially
confined aquifer is located beneath thick clay deposits in the central area.
Ground water is recharged by rainfall on the sands and discharges at the
ecotone between the sands and marshes as well as contributing base flow to
surface streams. The permanent wetness of the ecotone renders it the most
productive and fertile zone of the DNP.
In 1979 the Spanish government approved and principally subsidized a
massive irrigation, project utilizing ground water from this aquifer (Llamas,
1988). Since then the drinking water demands in the area have increased as
well, primarily due to expansion of the tourist industry. The ground water
models used to project allowable pumping rates, were developed primarily
from the perspective of technological feasibility with little or no
consideration of ecological suitability (Suso and Llamas, 1993). More recent
modelling by leading Spanish researchers predicts the following ecological
impacts from the approved pumping scheme: 1) desiccation of the ecotone
wetlands in some locations leaving the abundant soil organic matter
vulnerable to fire, 2) considerable reduction of surface water influent to the
park as gaining reaches of streams become losing reaches, and 3) degraded
.ground water quality, primarily by nitrate (Suso and Llamas, 1993).
As a result of international public protest, the size of the originally
approved irrigation, project has been reduced. However, the researchers
cited above continue to predict that ecological impacts to the park's
wetlands are inevitable with the timeframe being dependent only on the
amount of natural precipitation available in the interim.
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Tabias de Daimiel National Park
One of the most extreme examples of ecological impacts from ground water
pumping is found in the Tabias de Daimiel National Park. The internationally
recognized park is located on the Central Plateau of Spain. Prior to ground
water development, the park consisted of an approximately 20 km2 marshy
area around the confluence of the Guardiana and Giguela Rivers. The area
was the natural discharge zone for the extensive underlying aquifer system
which is composed of calcareous and detrital material of continental origin
(Llamas, 1988). Prior to development, the swamps of the Tabias were
covered by about 1 m of water except during extremely dry periods in the
summer.
The major land use in the surrounding region was dryland farming until the
hydrogeology of the extensive La Mancha aquifer became understood more
widely about 20 years ago. From 1974 to 1987, ground water irrigated
acreage increased from 30,000 to 130,000 ha and annual ground water
pumping increased from 200 hm3 to 600 hm3 (Llamas, 1989). The average
annual recharge rate is only about 260 hm3 from a combination of sources
including ground water, surface water and rainfall.
As result of prolonged overpumping, the water table has fallen to as much as
20 m beneath the land surface. This depletion has caused the total
disappearance in this area of the Guardiana River since 1984 in addition to
the progressive desiccation of the Tabias. Phreatophytic vegetation has been
lost totally and the highly organic soil is undergoing a slow process of
spontaneous combustion. In 1986 and 1987, two large fires burned over one
third of the national park (Llamas* et al., 1992).
In 1987, under pressure from Spanish and international ecological groups,
the aquifer officially was declared "overexploited" in accordance with the
1985 Spanish Water Law. This required the preparation of a management
plan and the creation of a local Users Committee to attempt to mitigate
these impacts. About $10 million (US) from the. Spanish government was
used to attempt to regenerate the wetland. The primary approach was to
supply water from other sources including land applying pumped ground
water, importation of surface water via aqueduct and impoundment of
surface water to retain it for the park (Llamas et al., 1992). However, the
reallocation of water resources for regeneration was met with considerable
local opposition and ultimately only 10% of the water deficit was made
available during the first three year trial period.
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According to some researchers, the changes in the vegetation and fauna are
so significant that the area appears to be an "ecological desert" even though
there sometimes is water on the land surface. It has been suggested that the
government's investment in regeneration'would be spent more wisely by
promoting reconversion to crops requiring less water (Llamas et al., 1992).
UNITED STATES
ARIZONA
Introduction
In many areas in Arizona, particularly in the southern part of the state,
ground water pumping has increased dramatically since the 1940s to meet
the needs of irrigated agriculture, industry and rapid population growth.
Ground water supplied 48% of Arizona's large water demand in 1985 and
agriculture accounted for 87% of the total water use (U.S. Geological Survey,
1990).
The imbalance between the quantity of water consumed in Arizona and the
long-term dependable suppy is a major problem. Overall about two thirds of
the ground water pumped in recent years was withdrawn from storage (U.S.
Geological Survey, 1990). Annual pumpage rates may exceed natural
recharge rates by more than 500.times in some areas (Schumann et al.,
1986). Ground .water levels have declined by 50 to 300 ft in several basins
throughout the state including the Gila, San Simon, Avra Valley, and others
and by 400 ft or more in several additional basins such as the Santa Cruz and
Salt (Schumann et al., 1986).
As transition zones between aquatic habitat and surrounding terrestrial
habitat, riparian and wetland ecosystems are the most biologically
productive areas in the arid Southwest. Over 85% of wildlife species are
dependent upon wetland and riparian areas for some aspect of their
existence. These areas serve as important breeding areas, refuges from the
desert heat, important corridors for animal movement through the
surrounding desert, and critical sites for forage production (Davis, 1993).
The highest known breeding bird densities in the United States have been
recorded in the desert riparian habitat (Carothers et al., 1974).
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Historically many of Arizona's rivers and streams were perennial and
supported large expanses of wet meadows, marshes, swamps, and dense
mesquite bosques (woodlands) (Davis,. 1993). These wetlands and riparian
areas have been impacted extensively and severely in the last century. Only
about 15% of the riparian areas which were present in the early 1800s still
remain and the percentage is even lower when only Sonoran Desert riparian
areas are considered (McNatt et al., 1980). Impacts result from ground
water pumping as well as other causes such as arroyo cutting, livestock
grazing, land clearing for agricultural and urban development, and
hydrological modification of surface flows (Stromberg, 1994). It is
frequently difficult to identify which factors are responsible for riparian
damage or loss.
Gallery forests of Fremont cottonwood (Populus fremontii) and Gooddirig
willow (Salix gooddingii) historically covered hundreds of kilometers along
the floodplains of many of Arizona's rivers and similar low-elevation rivers
in California, Utah, and northern Mexico (Stromberg, 1993c). Today these
Sonoran riparian cottonwood-willow forests are among the most threatened
forest types in the United States (Swift, 1984). Excessive ground water
pumping is one of several causes for the decline of these forests.
Similarly, mesquite (Prosopis velutina) woodlands were the most abundant
riparian type in the southwestern United States (Klopatek et al., 1979). In
locations where ground water is less than 50 ft such as the alluvial
floodplain aquifers which were historically common in Arizona, mesquite
can grqw to tall dense canopy forests called bosques. Deep root systems and
symbiotic associations with nitrogen-fixing bacteria contribute to the high
productivity of this species. In turn, bosques support diverse and abundant
ecosystems including one of the highest densities of breeding birds of any
southwestern habitat. Again, as a result of ground water decline and several
other land uses such as those listed above, these bosques now are reduced to
relatively small isolated remnants, virtually none of which remain in
pristine condition. Attempts to restore degraded mesquite bosques have had
limited success (Stromberg, 1993b).
Other ecosystem impacts which commonly occur with the loss of mesquite
flowers and fruits include fewer insects and insectivores. Avian abundance
and diversity decline with reduced canopy volume. The activity of nitrogen-
fixing bacteria is decreased thereby reducing the soil nutrient pool
(Stromberg et al., 1992). Furthermore, as has been observed in many other
drainages, death of riparian vegetation results in increased flood flows and
increased erosion and channel widening (Groeneveld and Griepentrog, 1985).
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Another ramification of the extensive ground water depletion is widespread
land subsidence which is manifested as generally lowered land elevation as
well as sink holes and earth fissures. Subsidence has affected .more than
3,000 mi2 in southern Arizona alone (Schumann et al., 1986). Subsidence
was detected initially in 1948 in the lower Santa Cruz basin and since then
subsidence of up to 12.5 ft has been measured in many southern drainage
basins:
In addition to the economic costs of impacts to man-made structures,
subsidence and fissures can result in costly environmental impacts as well.
Fissures transect natural drainage patterns and can capture large volumes of
surface runoff which may deprive downstream alluvial aquifers of recharge
vital to riparian communities. Accelerated erosion along fissures forms
gullies which exacerbates the impact on natural drainage channels over time.
Accelerated erosion also occurs in natural drainages along the periphery of
subsiding basins where the gradient between the basin floor and *fne
surrounding mountains is increased. In constrast, subsidence decreases the
gradient of streams and rivers which traverse subsiding basins, thereby
reducing surface water flow rates and increasing sediment deposition and
flooding. The Combination of these effects can have major impacts on the
natural hydrology and ecology of an. area. These effects are most pronounced
in the Salt River and lower Santa Cruz River basins (Schumann et al., 1986).
Santa Cruz River Basin
The 13,790 mi2 .watershed of the Santa Cruz River is located in southern
Arizona and northern Mexico. The Santa Cruz River rises in the mountains of
southeastern Arizona and after a short loop south into Sonora, Mexico, it
flows generally northwest. It is. an intermittent desert drainage containing
interrupted perennial and effluent dominated reaches and regions of
subsurface flow. Perennial flow is absent except in short reaches. Primary
drainage is to the Gila River near Phoenix in south central Arizona.
The floodplain of the Santa Cruz is alluviated deeply. These sands and
gravels generally are unconsolidated and are major'water bearing units.
Some wells drawing from these deposits yield over 1,000 gpm (Stromberg,
1994). At the time of early settlement, this unconfined alluvial aquifer
supported extensive riparian communities including many gallery forests.
Numerous marshes and springs were present where structural features
forced underflow to the surface and these areas also supported wetland
ecosystems. A combination of factors .primarily including ground water
39
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pumpage and arroyo cutting have eliminated or drastically reduced most of
these wetland features (Hendrickson and Minckley, 1984).
The natural vegetation of the low floodplain of the Santa Cruz River is
dominated by Fremont cottonwood (Populus fremontii) and Goodding willow
(Sa//x gooddingii) and dense bosques (woodlands) of velvet mesquite
(Prosopis velutina), netleaf hackberry (Celtis reticulata) and Mexican elder
(Sambucus mexicana) on the river terraces. Other riparian vegetation
associations include cienegas (marshes), sacaton grasslands, and shrublancls
of seepwillow (Baccharis salicifolia), rabbit brush (Chrysothamnus
nauseosus), and burro brush (Hymenoclea spp.) (Stromberg, 1994).
The Santa Cruz basin has been inhabited and cultivated continuously since
the seventeenth century which has resulted in extensive hydrological arid
ecological changes (Hendrickson and Minckley, 1984). Early irrigation
required diversion of surface flows from the river. For the last few decades
most of the water needs of the extensive irrigated croplands, the mining
industry, and the rapidly growing population have been met by ground water
(Stromberg, 1994). Total pumpage in this hydrologic basin has increased
greatly and often has exceeded that of any other basin in southern Arizona by
nearly an order of magnitude (Hendrickson and Minckley, 1984). Ground
water levels have declined throughout the basin with the maximum decline
of 460 ft measured in the lower Santa Cruz basin and 150 ft in the upper
Santa Cruz basin (Schumann et al., 1986).
The following examples of the impacts in riparian ecosystems In the Santa
Cruz basin serve to illustrate the effects of ground water decline. In
general, the extent and severity "of ecological-impacts increases downstream
in the drainage. In many cases, impacts are regional in extent and native
riparian vegetation has been eliminated totally.
i
In the lower Santa Cruz basin, ground water is the only source of water for
municipal, industrial, and agricultural use (Schumann et al., 1986). Water
and land uses in the relatively recent past practically have elimated riparian
vegetation from the lower reaches of the river (Stromberg, 1994).
In the upper Santa Cruz basin irrigation and mining consume the first and
second largest quantities of ground water respectively. During the past
several decades, ground water pumping by mines and pecan growers has
caused massive .ground water declines in Pima County. This has resulted in
total elimination of riparian habitat from central portions of the river
(Stromberg, 1994). Obligate phreatophytic species have been replaced by
scrub species such as desert broom, burro weed and burro brush. Mesquite
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bosques and sacaton grasslands historically supported by subflows
essentially are no longer present.
Ground water pumpage in the Avra Valley, a major tributary to the Santa
Cruz, has been extensive and water tables have undergone major declines
(White et al., 1966). Farther upstream, recent ground water withdrawals
from the floodplain aquifer in Santa Cruz County have caused localized water
table declines and reduced abundance of cottonwood-willow forests.
Ground water pumping from the floodplain aquifer for the growing
populations of Nogales, Arizona (population 20,000) and Nogales, Sonora
(population 250,000) is creating cones of depression which have caused low
growth rate, low tree density and low canopy cover of Fremont cottonwoods
and other riparian populations (Stromberg et al., 1993a). In areas where
ground water depths are greater than about 25 ft, cottonwoods and willows
have been lost. Effluent released into the Santa Cruz channel from the
Nogales International Wastewater Treatment Plant is increasing recharge to
the alluvial aquifer to the benefit of riparian vegetation.
In comparison, where ground water levels re-main shallow, riparian
communities are healthy. At a site near the Mexican border where a shallow
bedrock layer serves to minimize ground water level deline (7 to 10 ft),
populations of cottonwoods and willows are in .relatively good ecological
condition arid survival of seedlings is high. .Similar geological conditions
exist farther downstream at the Guevavi Narrows and again riparian
communities are able to thrive (Stromberg, 1994). > "" ' '
Tanque Verde Creek
Tanque Verde Creek is an ephemeral river in the Sonoran Desert near Tucson.
It flows for 16 miles from its headwaters in the Rincon Mountains to its
confluence with Pantano Wash in south central Arizona, in some areas, the
alluvial floodplain aquifer supports large mesquite (Prosopis velutina)
bosques. Increased ground water pumping in this area has had a severe
impact on these ground water dependent riparian woodlands (Stromberg et
al., 1992). ......
Regional ground water decline in this area already had resulted in sublethal
stress to mesquite bosques in the early 1980s as measured by low stem
water potential, reduced leaflet size, and canopy mortality of .over 45%
(Stromberg et al., 1992). Depth to ground water ranged from 1 to 46 ft in
1986. When the City of
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1988, ground water began declining at an unprecedented rate of 12 to 21
ft/year, reaching depths of up to 105 ft in 1990 (Stromberg -et al., 1992).
Water depths of this range are typically lethal to mesquite, particularly in
coarse alluvium with low water retention capacity. Trees farther upstream
m areas where ground water levels remained at about 10 to 16 ft were in
good ecological condition with tall stature and no canopy dieback
(Stromberg, 1994).
San Pedro River Basin
Roughly parallel to, and 40 to 70 miles to the east of, the Santa Cruz River,
the San Pedro River flows northwest from Sonora, Mexico for about 150
miles to join the Gila River at Winkelman. The 16,635 km2 drainage basin is
similar to others in the Basin and Range province. Thick floodplain alluvium
of gravel, sand, and silt overlays basin fill materials throughout the
drainage. Irrigation and municipal wells in the valley obtain water from
both the basin fill aquifer and the overlying alluvial floodplain aquifer
(McGlothlin et al., 1988; Stromberg, 1994).
The San Pedro River is largely perennial in the upper portion of the basin and
intermittent in most other reaches. Baseflow in the river and the water
table beneath the riparian zone are maintained almost entirely by inflow
from the regional basin fill. aquifer. At times of low flow, the entire flow is
diverted in lower reaches (Stromberg, 1994).
Irrigated agriculture began in the latter part of the nineteenth century and
increased steadily until the lateM960s. Ground water pumpage roughly
parallels growth in irrigated acreage (Hendrickson and Minckley, 1984).
As recently as a century ago, the San Pedro River was unincised and marshy
along much of its length (Hendrickson and Minckley, 1984). Ground water
withdrawal for agriculture, mining and municipal supplies, as well as dam
construction, overgrazing and clearing of riparian vegetation for pasture
drastically have altered the hydrology of this area (Richter and Richter,
1992). Surface flows have been reduced or eliminated in some areas. Ground
water levels have declined and springs, wetlands, and" cienegas have been
reduced to isolated remnants, several of which are located in the headwaters
of tributary valleys such as the Aravaipa and Babocomari. Cienega
vegetation has been replaced widely by riparian, scrub species (Stromberg,
1994).
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The most severe water level declines in the upper San Pedro basin have
occurred in wells near the expanding population and agricultural centers of
Sierra Vista and Hauchuca City. Water levels in this area declined
approximately 1.4 ft/year between.._1966 and 1986 (Stromberg, 1994),
Ground water pumping in this area also has reduced the baseflow in the river
(McGlothlin et al., 1988). Riparian mesquite bosques along the river have
experienced, sublethal stress,as..a result of ground water decline (Stromberg,
1993b). ;
Portions of the upper San Pedro River in the far southeast corner of the state
contain some of the healthiest remaining desert riparian ecosystems in the
southwest United States. Included are cienega plant associations, mesquite
bosques, and cottonwood-willow forests as well as the most extensive
remaining sacaton grasslands in Arizona. Also indicative of the health of
this ecosystem is the low abundance of the exotic saltcedar (Stromberg,
1994).
About 40 perennial miles of the river in this area and its associated riparian
zone were acquired in 1986 by the Bureau of Land Management to be managed
as the San Pedro Riparian National Conservation Area. The primary
management objective is the protection of the remaining riparian habitats.
Cattle grazing and sand and gravel mining are restricted and agricultural
lands have been retired from farming.
Although ground water pumping is not allowed in the Riparian Conservation
Area itself, there is concern that the cone of depression from increased
ground water overdraft in the vicinity of nearby Sierra Vista may extend to
the Riparian Conservation Area to the detriment of native vegetation
(Richter and Richter, 1992). Also there is high potential for future
development elsewhere within this portion of the basin (Stromberg, 1994).
Ecological modelling predicts that ground water decline of only 3 ft would
result in the loss of many marsh species including the Huachuca water umbel
(Lilaeopsis schaffneriana var. recurva) which is a Federal Endangered
Species Category 1 listing candidate and only recently has been rediscovered
along the San Pedro River (Stromberg, 1994). in, some areas in the
Conservation Area, seasonal ground water fluxes of this magnitude already
have eliminated obligate wetland species and-threaten facultative wetland
plants. Other impacts which have been noted in some areas are low
survivorship of cottonwood and willow seedlings, increased establishment
of saltcedar and riparian scrub species such as burro brush and rabbit brush,
and sublethal stress in sacaton, grasslands and mesquite bosques (Stromberg,
1994).
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Gila River Basin
The Gila River is the major drainage of southern Arizona. The basin extends
eastward into the mountains of New Mexico, crosses the full width of the
state and joins the Colorado at the western state border. Topographically
the basin is typical of .those in the Basin and. Range physiographic province.
It is a comparatively flat, wide, sediment filled valley between narrow
rugged mountain ranges. Basin fill materials and terrace and floodplain
alluvium have been significant ground water bearing units in the past (Culler
et al.f 1970).
Prior to the twentieth century, there were extensive marshes, swamps, and
floodplains along much of the river. Dominant vegetation included cattail
(Typha domingensis), bulrush (Scirpus olneyi), giant reed (Arundo donax),
commonreed (Phragmites communis), arrowweed (Pluchea sericea), and many
cottonwood and willow trees. The dense vegetation of these well-developed
riparian communities often reached 10 to 15 ft in height and supported large
and diverse wildlife populations. The river gradient was shallow and the
floodplain was so level that marshy lagoons formed in places along the main
channel (Rea, 1983).
By the end of the nineteenth century the perennial flow of surface water was
reduced drastically and only in a few locations did surface flow continue
until the 1950s. Deprivation of surface water recharge coupled with channel
incision and ground water exploitation caused the floodplain water table to
decline. As a result, the entire riparian community of willows and
cottonwoods was eliminated and replaced in many areas by exotic saltcedars
(Tamarix spp.) (Rea, 1983). '
One locality in this drainage provides an example of the extent of ecological
impacts which have resulted from the major hydrological alterations which
have occurred in this drainage. As late, as 1940 an extensive bosque known
as the New York Thicket existed at the confluence of the Santa Cruz and Gila
Rivers and three major washes (Vekol, Green and Santa Rosa). The bosque
was up to 6 miles wide in areas with mesquite and screwbean mesquite
(Prosopls pubescens) reaching heights up to 40 ft. By the late 1970s ground
water pumping had caused the water table to decline to about 100 ft below
the land surface resulting in the death of 90% of the mesquite (Rea, 1983).
Although mesquite have deep roots and tolerate moderate water stress,
lowering the ground waterJevel below about 50 ft results in death of
riparian mesquite trees or in conversion of mesquite from a dense tree
community to a sparse shrub community (Stromberg, 1994).
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Regional habitat loss such as that .described above has had a major impact on
wildlife in this area. Twenty-nine species of birds have been completely
extirpated from the middle Gila and lower Santa Cruz Rivers and several
other avian species have declined noticeably in population size because of
habitat deterioration. Loss of nesting sites and food sources are major
factors affecting species numbers (Rea, 1983). '
Casa Grande National Monument
Casa Grande National Monument is situated 1.5 miles south of the Gila River
and approximately 50 miles west of its confluence with the Salt River in
south central Arizona. The 480 acre site is located in the floodplain of the
Gila in Final County. Designated a national monument in 1918, it has had
restricted access to people and livestock for over 70 years (Judd et al
1971).
Ground water withdrawal for nearby agricultural development began in the
early twentieth century and increased in the 1940s, causing the water table
to decline from about 6 ft to about 43 ft (Stromberg, 1993b). An extensive
bosque of large mesquite trees which utilized alluvial ground water survived
during this initial period of decline. However, all of the trees died when
pumping increased and the water table dropped about 3 ft per year to depths
of 40 to 150 ft below the land surface (Judd et al., 1971). Continued
increases in pumpjng in the latter half of this century had lowered the water
table to as much as 650 ft as of 1970 and the site now is dominated by
upland desert shrubs (Stromberg, 1994). The area is littered with large
deformed stumps of dead mesquite trees (Judd et al., 197.1).
Verde River Basin
The Verde River watershed lies in the Central Mountains physiographic
province in central Arizona arid drains 17,218 km?. The river rises in the
-mountains to the west of Flagstaff and flows south to join the Salt River
east of Phoenix. The watershed is semiarid and has an average annual
rainfall of 30 cm (Stromberg, 1993a).
The floodplain of the lower Verde River contains thick deposits of alluvial
silt, sand, and gravel that support stands of mesquite (Prosopis juliflora),
interspersed with arrow-weed (Pluchea sericea), seepwillow (Baccharis
glutinosa), and saltcedar (Tamarix pentandra). Fremont cottonwoods
(Populus fremontii) and Goodding willow (Salix gooddingii) are common m
45
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the floodplain and occur in stands along the existing channel or along
sections of old river channels (McNatt et at., 1980). ..-._.
Where it is healthy, this riparian habitat supports a large and diverse fauna
including big game (mule deer and javelina), waterfowl and wading birds, arid
small mammals including beaver, muskrat, and rabbits. Higher than average
densities of coyotes, bobcats, skunks, and raccoons utilize this area. Over
160 species of birds are known to frequent this riparian habitat including
two endangered species, the bald eagle and the Yum a clapper rail. A portion
of the Verde River riparian area has been identified as having the highest
bird population density in North America (McNatt et al., 1980).
The riparian ecosystems in the Verde River floodplain are experiencing
considerable impacts due to water table decline. In central Arizona, ground
water pumping for agriculture reduces water available to the Verde River
riparian zone and its tributaries (Stromberg, 1994). Pumpage from the Big
Chino Valley aquifer also could pose a threat to the upper Verde River
riparian ecosystems (Stromberg, 1994).
In the lower Verde drainage the riparian community is "on the verge of
collapse" as a result of a combination. of natural and anthropogenic factors
(McNatt et al., 1980). Prior to 1977 less than 4% of the cottonwood trees
were dead along the Fort McDowell reach of the lower Verde. Since 1977, 46
to 84% have died. In addition, many of the remaining trees are approaching
maturity and very few seedlings are regenerating successfully in the
floodplain.
The cause of this mortality is a combination of hydrological factors.
Limited water releases from Bartlett Dam, located upstream of this reach,
have deprived the alluvial aquifer of recharge needed to maintain the
floodplain water table. Further, natural floods which assist in cottonwood
regeneration have been eliminated. Lastly, as of 1983, the City of Phoenix
operated an infiltration gallery and 14 wells in the area of greatest
mortality. These well fields withdraw 20,000 acre-ft of alluvial ground
water per year for municipal use and are capable of lowering the water table
over 10 ft. In 1977, the year of greatest cottonwood loss, drought
conditions resulted in almost no releases from Bartlett Dam and
considerable water table decline.
In comparison, Sycamore Creek, a nearby tributary to the Verde, is not
controlled or pumped but did experience similar drought conditions in 1977.
Riparian vegetation including cottonwoods in this drainage does not exhibit
increased mortality or impaired seedling regeneration (McNatt et al., 1980).
46
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Loss of cottonwoods is very detrimental to populations of nesting birds in
this area. Reduction of cottonwood densities from 46 trees/acre to "10
trees/acre resulted in over a 50% reduction in the number of nesting bird
pairs per 100 acres (McNatt et al., 1980). These data are derived from a
study of intentional phreatophyte removal for water conservation but are a
reasonable estimate of avian impacts from cottonwood mortality due to
water level decline as well.
Salt River Basin
The Salt River drainage is a major tributary to the Gila and passes through
the growing metropolitan area of Phoenix. Ground water pumping in the
basin has increased steadily over the past 50 years. About two-thirds of the
total ground water withdrawal in Arizona occurred in the combined areas of
the Salt and lower Santa Cruz basins (Schumann et al., 1986). Ground water
level decline has exceeded 300 ft in many parts of the Phoenix, Tempe, Mesa
and Scottsdale areas. Subsidence of 3 to E> ft has been measured in an area
of over 500 mi2. Fissures resulting from this subsidence capture surface
water flow in some locations (Schumann et al., 1986), creating the potential
for detrimental ecological impacts associated with surface dewatering.
From the time of earliest record of European,explorers until the 1920s, the
Salt River was a perennial stream lined with cottonwoods and willows.
Large sections of the channel consisted of sand bars which were exposed at
low water flow and were colonized by seepwillow and arrowweed. By the
1950s the river was a conduit only for flood flow as upstream irrigation
dams impounded all of the normal discharge. Native vegetation was
eliminated and many miles of impenetrable thickets of exotic saltcedar
(Tamarix chinensis] overgrew the dry channel banks. The saltcedar was
sustained by a sufficiently shallow water-table of approximately 23 ft (Graf,
1982). , Without river recharge, intensified ground water pumping by Tempe,
Scotttsdale and Mesa in south central Arizona caused ground water levels to
decline to over 220 ft in the 1960s. This resulted in the elimination of the
saltcedar thickets, Despite ground water level rises of more than 115 ft
after recent floods, the ground water remains too deep and fluctuates too
erratically to sustain riparian vegetation except for a few ribbons of
tamarisk growth supported by irrigation return flows and sewage effluent in
some locations (Graf, 1982).
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CALIFORNIA
Introduction
California consistently leads all states in volume of surface and ground
water withdrawals. The state has retained this position for 40 years
primarily because of the large volume used by irrigated agriculture (U.S.
Geological Survey, 1990). During the past 100 years, the population and the
.associated industrial and agricultural demand increasingly have gravitated
toward the more arid areas in the southern portion of the state. Massive
projects have,been undertaken to export surface and ground water from areas
of relative abundance to areas were it is needed. However, available water
supplies are insufficient to meet current needs without substantial
depletion of ground water from storage. Furthermore, population projections
predict an average annual increase of more than 330,000 people for the
remainder of this century (U.S. Geological Survey, 1990). Periodic drought
further exacerbates this water shortfall.
Ground water overdraft is an increasingly critical .problem in many areas of
the state. Statewide, ground water pumping exceeds recharge by an average
of 2.0 million acre-ft/year. Eleven basins in California have been identified
by the Department of Water Resources as being subject to critical conditions
of overdraft, based on problems of salt water intrusion, deterioration in
quality, land subsidence, and prohibitive pumping costs (Fort et al., 1993).
California has a wide range of water rights laws. Ground water regulation is
undertaken primarily at the local- level and overdraft generally is permitted
unless curtailed through adjudication of a basin or administration by a local
management entity (Fort et al., 1993). Prevention or minimization of
overdraft is difficult under this system.
Owens Valley
Owens Valley is a long narrow closed basin located in east-central
California. The 3,300 mi2 valley is bounded longitudinally by the Sierra
Nevada on the west and the White and Inyo Mountains on the east. One major
river, the Owens River, flows south through the valley. Numerous tributaries
drain the east face of the Sierra Nevada and have formed extensive alluvial
fans along the west side of the valley (Hollet et al., 1991). Historically,
springs and wetlands were found throughout the valley (Rogers et al., 1987).
48
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The valley is filled with uncdnsolidated to moderately consolidated
sedimentary deposits and intercalated volcanic flows and ash. Nearly all the
recoverable ground water in the valley is found in these valley-fill materials
(Rogers et ah, 1987). An unconfined aquifer is present throughout the valley.
Depth to water ranges from the land surface to more than 15 ft below the
surface of the valley (Sorenson et a!., 1989). The saturated thickness of this
unit ranges from 30 to 100 ft. An underlying confined water bearing unit
also is present throughout most of the valley. The degree of confinement is
negligible in many areas as the clay beds of the intermediate confining layer
are discontinuous. Virtually all, of the, ground water in the Owens Valley
aquifer system is derived from precipitation that falls in the Sierra Nevada
and infiltrates through the alluvial fans (Hollet et al., 1991).
As a result of the rain shadow effect caused by the Sierra Nevada, the
climate in the Owens Valley is semiarid to arid. Most of the land in the
valley is covered by native vegetation. The communities which occupy the
greatest land area are 1) shallow ground water alkaline meadow, 2) shallow
ground water alkaline scrub, 3) dryland alkaline scrub, and 4) dryland
nonalkaline scrub (Sorenson et al., 1991). The three shrubs Nevada saltbush
(Atriplex torreyi), greasewood (Sarcobatus vermiculatus), and rabbitbrush
(Chrysothamnus nauseosus ssp. viridulus) in combination with two, grass
species saltgfass (Distichlis spicata ssp. stricta) and alkali sacaton
(Sporobplus airoides) comprise more than 90% of the vegetation growing on
shallow ground water zones of the Owens Valley floor (Groeneveld, 1989).
Because of the availability of shallow ground water, the valley floor
supports about 73,000 acres of phreatophytic vegetation (Dileanis and
Groeneveld, 1989).
* *
In the early 1900s the City of Los Angeles recognized the value of the
relatively abundant surface and ground water supplies in the Owens Valley
and acquired much of the land. A 233 mile aqueduct was completed in 1913
to divert water from the Owens River and modest quantities of ground water
(generally less than 10,000 acre-ft/year). Prior to this diversion the Owens
River flowed into the 100 mi2 Owens Lake. Evaporation now exceeds inflow
and, except in very wet years, Ihe lake is dry (Hollet et al., 1991).
In 1970 a second aqueduct was completed which increased the average
capacity for exporting water by 50% (Rogers et al., 1987). The majority of
the additional export has been ground water. Ground water export accounts
for over 50% of the discharge from the valley's aquifer system (Hollet et al.,
1991) and evapotranspiration by phreatophytic vegetation accounts for the
majority of the remainder (Duel!, 1990). 7
49
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Extensive export of surface and ground water from this arid region has
resulted in widespread impacts to the shallow ground water vegetation
(Rogers et al., 1987). In the early 1970s, phreatophytic plants covered about
the same acreage and conditions were similar to those observed between
1912 and 1921. In 1981, a loss of 20 to 100% of the plant cover on about
26,000 acres was noted. This reduction was postulated to be a response to
the increased pumpage of ground water and changes in surface water use.
Considerable public concern was expressed regarding these environmental
impacts and the related loss of recreational activities and wildlife habitats
(Hollet et al., 1991).
Historically, springs between the communities of Big Pine and Independence
in central Owens Valley discharged the largest quantities of water. Included
are Fish, Big Seeley, Little Seeley, Mines, Little Black Rock and Big Black
Rock Springs. A direct and immediate effect was measured in the quantity
of spring flow when nearby deep wells were pumped for export. Springs
ceased to flow with continued pumping and flowed again when pumping
stopped or was minimal (Rogers et al., 1987). As this occurred, aquatic and
riparian habitat was lost. Effects were particularly severe on the four
native fish species in the Owens River system. Two species, the Owens
pupfish (Cyprinodon radiosus) and the Owens chub (Gila bicolor snyderi), are
listed as endangered and one, the Owens dace (Rinichthys osculus), is
threatened.
Fish Slough is a remnant of a once widespread shallow aquatic/riparian
wetland in central Owens Valley. It supports a variety of rare plant species
and the endangered Owens pupfish (Pister and Kerbavaz, 1984). Three
springs provide the flow in this slough. Declining ground water levels have
reduced spring flow since 1971 and resulted in a reduction of riparian
wetland acreage. The area currently is designated as the Owens Valley
Native Fish Sanctuary which protects the refuge under an interagency
management plan. However, further agreements to avoid additional ground
water drawdown are needed (Pister and Kerbavaz, 1984).
i
Little Black Rock Spring also is located in central Owens Valley about 9
miles north of Independence and is used to support a local fish hatchery. In
1971, when the discharge from jme spring began declining, a nearby well was
pumped to replenish the water supply for the hatchery. Soon after pumping
began, the spring ceased to flow. Additional surface water was used to meet
the needs of hatchery (Perkins et al., 1984). Not only was the attempt to
mitigate the loss of spring flow unsuccessful in terms of supplying adequate
volume to sustain phreatophytic vegetation, the surface water was lower in
alkalinity, salinity and nutrients. The result was a significant loss in marsh
50
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area as well as a change in species composition. The area once inhabited by
marsh vegetation has been invaded , by more drought tolerant perennials
(Perkins et ai., 1984). -
Fish Springs Lake and the Springfield are two other wetland environments
which have undergone similar changes as a result, of ground water pumping.
The lake is now ephemeral and artesian flow at the springs has ceased.
Marsh vegetation has been lost entirely from these areas and plant diversity
ultimately has been reduced to include only those annuals which can survive
on infrequent precipitation.
As part of ongoing litigation between the City of Los Angeles and-the local
Jnyo County government, a ground water monitoring program has been
designed to curtail pumping when potential, ecological impacts are predicted.
Provisions for well shutdown have been included as the basis for a
permanent agreement for ground water management to preserve the existing
vegetation cover (Groeneveld, 1989). It is essential to prevent further
vegetation loss because attempts at revegetation have been unsuccessful.
Poor soil aeration limits the invasion of xeric shrub species from the nearby
alluvial fans (Groeneveld, 1989).
Carmel River Valley
The Carmel River drains an area of 255 mi2 in the northern Santa Lucia
Mountains of the central California coast range. The upper 21 miles flow
through steep canyons with little alluvium and the lower 15 miles flow
through an alluvial valley known as the Carmel Valley. The^ alluvial fill is
typically 15 to 30 m thick and consists of sand and gravel with some silt and
clay interbeds. Stream flow in the Carmel River is in response to
precipitation with high flows during the rainy season from November to
April. The river stage declines in late spring and summer: In its upper
reaches the Carmel River is perennial but in the alluvial valley flow is
intermittent, typically drying up in late summer (Kondolf and Curry, 1986).
The alluvial valley fill is a generally unconfined aquifer, although localized
areas of confinement exist. Seasonal fluctuations of the water table result
from recharge of the aquifer by winter stream flow and subsequent decline
of the water table by drainage of bank storage and, more importantly, by (
ground water withdrawals primarily during the dry season (Kondolf et al.,
1987). /
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Because of the low water retention capacity of the poorly developed and
coarse textured floodplain soils, plants in the riparian zone are-either xeric
or phreatophytic in habit due to the long summer dry period. In undisturbed
locations, the mature riparian forest is composed of approximately 60% red
willow (Salix laevigata), 30% black cottonwpod (Populus trichocarpa), and
10% California sycamore (Platanus racemosa) and white alder (Alnus
rhombifolia) combined (Groeneveld and Griepentrog, 1985).
The upper watershed is settled sparsely. Extensive commercial and
residential development has occurred in the last three decades in the lower
Carmel Valley, especially near the river mouth on the Monterey Peninsula.
Prior to the 1960s, the Carmel River supplied most of the water for the
peninsula. Because little surface water storage capacity is available, the
increasing demand for municipal water supply has been met by ground water
withdrawn from the alluvial aquifer. Production from the aquifer reached a
relative peak in 1976 and then decreased in 1977 because the aquifer was
depleted locally (Kondolf and Curry, 1986). However, throughout the 1980s,
additional wells have been drilled in the Carmel Valley for increased export
of ground water to the Monterey Peninsula downstream (Kondolf et al., =1987).
This extensive ground water withdrawal has altered the riparian ecosystems
as well as the general hydrology and geomorphology of the Carmel River
itself (Groeneveld and Griepentrog, 1985). Beginning in the late 1960s
residents noticed that trees in the vicinity of the municipal wells were
dying. Analysis of aerial photographs taken at intervals between 1956 and
1980 confirmed progressive loss of riparian forest cover over that period
(Groeneveld and Griepentrog, 1985). Downstream of the reach which was
impacted by ground water pumping phreatophytes remained healthy (Kondolf
and Curry, 1986).
During two years of drought in 1976 and 1977, the alluvial aquifer received
almost none of its usual annual recharge from river flow. Water table
drawdown of over 10 m was measured along 4'km of the river in the vicinity
of the well fields. Downstream of the pumped reaches, drawdown was
minimal. The low water retention capacity of the coarse alluvium resulted
in a rapid decline of the water table. This prevented phreatophytes from
extending their root systems to follow the declining water table even though
poplars, of which the black cottonwood is a member, have been observed to
achieve daily root growth rates of up to 5 cm (Groeneveld and Griepentrog,
1985). Coupled with negligible summer precipitation, the riparian
vegetation was eliminated rapidly.
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Pumping of the municipal" wells was sufficient to have a major effect on
several other aspects of the regional hydrology. Drawdown_Jn the pumped
reach reversed the hydraulic gradient in the upstream direction in this area.
Stream flow became influent to the banks in the reach of major pumping
which had previously been a gaining reach of the river. In fact by mid-
August 1982, the Carmel River entirely dried up in the pumped area but re-
emerged downstream of the pumped reach (Kondolf et al., 1987). The loss of
bank storage to sustain the base flow in the summer months was extremely
detrimental to the summer downstream migration of steel head trout smolts
(Sa/mo gairdneri) as well as the rest of the riverine aquatic life (Kondolf et
al., 1987). ,
Channel geomorphology has also been impacted by the loss of the stabilizing
effect of the roots of riparian vegetation. The channel of the Lower Carmel
River had been essentially stable from 1939 to 1977, Since. 1978-the reach
in which the majority of the phreatophyte dieoff occurred has experienced
extensive bank erosion. Aerial photographs depict the channel widening from
60 ft in 1976 to over 400 ft in 1982 (Groeneveld and Griepentrog, 1985).
The flows that produced this erosion were not unusual events (two five year
recurrence interval flows) but the -impact on the river channel was
equivalent to that of the 100 year recurrence interval event which occurred
in 1911. Downstream reaches unaffected by pumping maintained healthy
bank vegetation and experienced no major erosion (Kondolf and Curry, 1986).
Recent bank erosion along the Carmel River has caused property losses in
excess of $1.5 million. Further losses can be expected because the present
channel is near the threshold of meandering and braided characteristics
rendering it inherently unstable (Kondolf and Curry, 1986).
Lower Colorado River (Needles and BIythe)
Indirect documentation was found on the impact of anthropogenic lowering
Of the riparian water table in the vicinity of Needles and BIythe. These two
communities are near the Arizona - California border on the lower Colorado
.River. The climate is extremely arid. The river supports a shallow alluvial
aquifer with depths to ground water ranging from 2 to 4 m throughout the
growing season (Busch et al., 1992). The natural riparian vegetation is
phreatophytic forest species including Fremont cottonwood (Populus
fremontii), Goodding willow (Sa//x gooddingii) and the Eurasian native
saltcedar (Tamarix ramosissima). The first two species , are obligate
phreatophytes and as such are highly vulnerable to impact if ground water
pumping sev_ers their connection to their water supply. This has been
observed in ths lower Colorado floodplain. With the reduction or elimination
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of the mature canopy species, saltcedar can competitively exclude native
species and dominate the plant community. Similarly, understory shrubs
such as screwbean mesquite (Prosopis pubescens) and arroweed (Tessaria
ser/cea) have become dominant (Busch et al., 1992).
Central Valley
The Central Valley of California occupies about 12% of the total land area of
the state. In the past century, it has become one of the most hydrologically
and ecologically altered regions in the country. Ground water pumping has
played a major although not exclusive role in these changes.
Central Valley is a long alluvium-filled structural trough occupying
approximately 20,000 mi2 of relatively flat land lying between the Coast
Ranges on the west and the Sierra Nevada to the east. The climate is arid to
s.emiarid with precipitation decreasing to the south. The northern half of the
basin is drained by the Sacramento River and the southern portion is the
drainage basin of the San Joaquin River. The area surrounding'the confluence
of the two rivers is called the Delta. The most southern portion of the
valley, the Tulare Basin, has no perennial surface outlet. However, there is
considerable underflow of ground water from the Tulare Basin to the San
Joaquin drainage basin (Katibah, 1984).
Lenses of gravel, sand, silt, and clay of predominantly fluvial origin fill the
entire valley. Most lenses are not widespread with the exception of the
Corcoran Clay Member which forms an extensive confining bed between the
overlying semiconfined aquifer and the underlying confined water bearing
zone. The degree of vertical leakage is highly variable (Bertoldi, 1992).
In 1850 the Central Valley contained an estimated 4 million acres of
wetlands (Peters, 1989). Prior to the construction of over 100 dams on the
two main rivers and their tributaries, seasonal flooding formed vast flood
basins and large shallow seasonal lakes which supported marsh vegetation.
Dense riparian forests of Fremont cottonwood, California sycamore, and
willow and associated intermediate and undergrowth species utilized the
riverine silt of the natural levees. Along the Sacramento River, the levees
frequently prevented mountain streams from reaching the main river
resulting in a network of distributaries ending in sinks of tule marshes.
Because of the lack of surface drainage in the Tulare Basin, seasonal lakes
with abundant marsh vegetation were common (Katibah, 1984). The Delta
was an area of convoluted inlets and islands supporting wetland ecosystems
(Conniff, 1993).
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Because these wetlands occurred in an otherwise arid area, they long have
been important wintering areas for Pacific Flyway waterfowl. About 60% of
the ducks, geese, and swans of this flyway use the valley wetlands during
the winter (Peters, 1989). .
Beginning around 1850, fertile soil, flat land, and abundant surface water
provided the incentive for the development of one of the nation's most
productive agricultural areas. Today more than $15 billion worth of crops
are produced annually on the approximately 7.3 million acres of irrigated
agricultural land in the valley (Bertoldi, 1992).
To support this lucrative industry, massive flood control and water diversion
projects have been constructed and hundreds of thousands of acres of
wetlands have been drained in the past 100 years. Two large aqueduct
projects were built in the middle of this century to convey water from the
northern part of the basin to the San Joaquin Valley.
Ground water resources have been developed simultaneously with the peak of
development occurring in the 1960s and 1970s. Over 100,000 irrigation
wells have been constructed. Pumpage increased from 362,000 acre-ft in
1912 to about 15 million acre-ft in 1977 (Bertoldi, 1992). Ground water
levels have been altered significantly throughout the valley. Most long-term
declines have been less, than 100 ft except, in the southern part of the San
Joaquin Valley were heads have declined from 100 to 400 ft (Bertoldi, 1992).
In many areas horizontal hydraulic gradients have been reversed and
downward gradients have been created as the deeper aquifer is frequently
the most heavily pumped. As a result of -increased pumping costs, ground
water withdrawals have leveled off or declined since 1967 and additional
water needs are being met with increased surface water delivery. The
recovery of the potentiometric surface from 1967 to 1984 in the most
heavily pumped areas averages approximately one half of the previous
drawdown (Llamas et al., 1992).
Of the 4 million predevelopment acres of wetlands in the valley, less than
400,000 remain today. It is difficult to determine the role which ground
water pumping has played in this 90% decline because other hydrologic
modifications such as surface water impoundment for flood control and
irrigation also have been extensive. Today the majority of the remaining
wetlands in the Central Valley are managed for waterfowl habitat and sport
hunting (Peters, 1989). Recent inventories of remaining riparian vegetation
indicate that most of the acreage is in a disturbed or degraded condition
(Katibah, 1984). : v , ' .
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Ground water pumping in the Central Valley also has resulted in the largest
volume of anthropogenically induced land subsidence in the world (Bertoldi,
1992). Subsidence has been most extreme in the heavily pumped San Joaquin
Valley with about 5,200 mi2 having subsided more than 1 ft and localized
regions of 20 to 30 ft. The loss of 20 million acre-ft of aquifer storage is
about half of all the manmade surface water storage capacity in the state
(Conniff, 1993). Islands in the Delta area which were originally at or
slightly above sea level are now 10 to 20 ft below sea level (Bertoldi, 1992),
undoubtedly resulting in impacts to all levels of the wetland ecosystems.
FLORIDA
Introduction
Wetland vegetation throughout extensive portions of Florida has died or is in
a state of premature decline due to excessive ground water withdrawals
(Bacchus, 1994). Many of Florida's freshwater wetlands are associated with
shallow unconfined aquifers perched above confined regional aquifers.
Ground water withdrawal from shallow saturated zones can cause extensive
impacts in these wetlands. In addition, where confinement is intermittent,
pumping of underlying aquifers has resulted in increased downward recharge,
thus draining wetland ecosystems of the water which sustains them.
Prevalent fractures in the extensive karst regions in Florida promote this
induced recharge (Bacchus, 1995).
Many land use activities have the potential to alter the hydrology of
wetlands in Florida. Included are ditching for drainage for agriculture or
other development, cattle grazing, silviculture, mining, dredging and filling
operations, and other land uses. Occasionally it may be difficult to separate
the impacts of these activities from the impacts of ground water
withdrawal for consumptive uses. However, numerous instances of
ecological impacts from ground water withdrawal are sufficiently well
documented around the state for this activity to be considered a significant
threat to wetlands in Florida. Furthermore, population growth in many areas
of the state continues to increase the demand for ground water. Maximum
pumping for irrigation_of crops, lawns and golf courses often occurs during
the summer, the "season of greatest vulnerability for wetlands (Ormiston ef
al., 1994).
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Dade County ' '.
Municipal well fields in Dade County; in southern Florida are increasingly
susceptible to contamination from salt", water intrusion and urban land uses
in the Miami metropolitan area. ; To minimize the risk of contamination, the
Northwest Well Field was developed;in 1983 in an inland well field
.protection zone. This 65 mi2 site is comprised primarily of undeveloped
wetlands and rock pits. The hydrologic effects of this well field are of
major interest because of the increasing preference for siting municipal
supply wells in such inland areas (Sonenshein and Hofstetter, 1990).
The Northwest Well Field is located in the Everglades, a large wetland area
which covers most of southern Florida. The hydrology of the Everglades has
been altered drastically throughout this century for flood protection and land
development. Many canals and levees have been constructed.
The Northwest Well Field is operated by the, Metro-bade Water and Sewer
Authority. The system consists of 15 supply wells with a total capacity of
165 mgd and an average production of 132 mgd for the period 1984 through
1987. Water is withdrawn from the Biscayne aquifer, which is an unconfined
aquifer composed primarily of a very porous sandy limestone with
interbedded sandstone (Sonenshein and Hofstetter, 1990).
' ! ..*.,-' ,
Continuous water level data beginning in 1960 are available at five
observation wells. Two additional observation wells were installed, one in
1970 and another in 1982. .Monitoring data show that water levels in the
center of the well field have declined 6 to 7 ft during both the wet and dry
seasons since pumping began. Water levels in a 10 mi2 wetland area
surrounding the well field have been below the land surface 100% of the time
since pumping began. Sonenshein and Hofstetter (1990) concluded that this
dewatered area was no longer considered a wetland. In another 10 mi2
wetland area surrounding this core area, water levels have been below trie
land surface as much as 99% of the time since pumping began. Water levels
at four monitoring wells outside the cone of depression of the well field
showed no effect from pumping. While no references describing the nature
and extent of ecological changes in this area were found, impacts
undoubtedly are extreme given the extensive alteration ,of the hydrology.
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Jensen Beach Peninsula, Martin County
Jensen Beach Peninsula is a long narrow spit along the southeastern coast of
Florida. The North Martin County well field is located on the peninsula in the
vicinity of significant wetland areas. Average daily withdrawal is 1.65 mgd.
A long-term ground water monitoring network has been established
primarily to monitor salt water intrusion. Data from inland locations show a
gradual decline in water levels resulting from a variety of possible causes
including increasing well field withdrawals, surface drainage modifications
or below average precipitation over the monitoring period. Ecological
investigations in the area have shown cause for concern that increased
pumping may adversely affect wetland vegetation (Shupe and Gleason, 1989).
Loxahatchee River Basin, Palm Beach and Martin Counties
The Loxahatchee River Basin is an area of considerable wetlands near the
southeast coast of Florida. Historically, water was retained in the wetlands
throughout the basin during the wet season. During the dry season, this
water provided fresh water discharge to the Northwest Fork Loxahatchee
River (Birkitt and Gray, 1989). These ecosystems have undergone extensive
hydrological alteration as a result of several factors, the individual effects
of which are difficult to distinguish. Channelization and drainage for
development and ground water pumping in municipal well fields have reduced
water storage in the basin. This has resulted in excessive flows in the wet
season and very low to non-existent flows in the dry season.
'
The reduced dry season flows have resulted in severe alteration of the
natural hydroperiod of the basin wetlands. Water levels -in surface water
bodies have been lowered as well. Of particular concern is the fact that
decreased discharge in the Northwest Fork Loxahatchee River has allowed
salt water to intrude farther into the drainage, damaging wetland vegetation.
Cypress trees increasingly are being replaced by mangroves in a significant
portion of the river basin including the reach which has been designated a
federal Wild and Scenic River and in Jonathan Dickinson State Park (Birkitt
and Gray, 1989).
^
As of 1989, the Northern Palm Beach County Water Control District was
developing a major Water Resources Plan to reallocate water in the basin in
an attempt to restore a more natural hydrologic regime. Specific objectives
were to enhance the quality of the wetlands, to restore historic hydroperiods
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and water flow patterns, and to increase ground water recharge to provide a
sustainable long-term municipal water supply.
When fully implemented, the Water Resources Plan Was designed to create
30,000 acre-ft of surface water storage capacity to store water during the
wet season and redistribute it during the dry season. The creation of these
reservoirs was expected to destroy 2,358 acres of wet prairie or marsh
habitat and 330 acres of low pine flatwoods. However 4,855 acres of open
water habitat and 650 acres of newly created wet prairie and freshwater
marsh habitat were expected to replace formerly degraded wetland areas. In
addition, 4,200 acres of wetlands were expected to be restored (Birkitt and
Gray, 1989).
Pasco, Pinellas, and Hillsborough Counties:
Tampa - St. Petersburg Area
Wetlands such as marshes, wet prairies, pondcypress domes (Taxodium
ascendens), and swamps are common in southwest Florida and typically
comprise 20 to 30% of the landscape (Rochow, 1993). They serve as nesting
and feeding sites for Florida sandhill cranes, bald eagles, wood storks (a
federal endangered species), and an array of amphibians, as well as provide
habitat for other wildlife (Bacchus, 1995).
; , - ''
Throughout most of southwest Florida ground water is the primary source of
drinking water as well as water for agricultural and for industrial uses. To
meet the needs of the rapidly growing Tampa - St. Petersburg area, ground
water pumping has increased extensively in recent years (Rochow and
Rhinesmith, 1991). At least 75%.of the public water supply is derived from
concentrated regional well fields (Southwest Florida. Water Management
District, 1996).
The Southwest Florida Water Management District has maintained an
extensive program of hydrological and biological monitoring in regional well
fields for approximately twenty years (Rochow, 1994). Observed impacts
from pumping include lowering of lake levels, reduction in stream flow, and
destruction of wetland habitat. In some areas along the coast, lowered
ground water levels have caused water quality degradation as a result of
saline intrusion (Southwest Florida Water Management District, 1996).
Rochow and Rhinesmith (1991) report that detrimental irnpacts observed in
wetlands include replacement of aquatic plant species by upland species,
decreased abundance of wetland-dependent wildlife, increased wetland
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susceptibility to fire damage, increased soil subsidence, and excess wetland
tree mortality.
Based on this monitoring, the District has found that water table drawdowns
ranging from 0.5 to 3.0 ft can be expected within a distance of approximately
one mile from most production wells (Rochow, 1993). The District also has
determined that any ground water development which lowers the water table
in the surficial aquifer by as little as 1 ft can adversely impact wetlands.
Consequently, in 1989 the District enacted a rule requiring detailed
environmental review for any water use permits where drawdown was
predicted to exceed 1 ft (Rochow and Rhinesmith, 1991). Long-term site-
specific ecological monitoring is required of permitees as the full impact of
ground water decline may not become evident for one to two decades after
pumping begins (Rochow, 1994).
Studies conducted at specific well fields under the jurisdiction of the
Southwest Florida Water Management District provide examples of the
wetland impacts which result from excessive ground water withdrawal
(Southwest Florida Water Management District, 1996). The Starkey well
field consists of 14 production wells located about 30 miles north of St.
Petersburg in Pasco County. The well field boundaries encompass abut 8,200
acres of predominantly natural and undisturbed pine flatwoods, cypress
domes, marshes, wet prairies, and sandhills (Rochow and Rhinesmith, 1991).
Pumping began in the 1970s at a rate of 1 to 3 mgd and increased beginning
in 1983 to an average of 13 mgd in 1989. At this rate of withdrawal, the
water table was predicted to decline at least 1 ft in an area of about 4 ml2
in the central part of the well field.
Beginning shortly after the increase in pumping in the mid-1980s, the
following impacts were observed at several sites in the Starkey well 'field:
1) extensive invasion of upland weedy species including dog fennel and
broomsedge, 2) destructive fires, 3) abnormally high tree fall, and 4)
excessive soil subsidence/fissuring (Rochow, 1994). Subsidence results
primarily from the oxidation of the highly organic soil. This contributes to
the. loss of tree species, particularly cypress, by reducing root support.
Abnormal hydroperiods also render trees more susceptible to pathogens.
Although present at nearby unaltered control wetlands, fish were absent and
amphibians were absent or in very low abundance at sites associated with
the well field. The limited food resources at these lower trophic levels was
thought to curtail the utilization of these sites by wetland reptiles and
wading birds (Rochow and Rhinesmith, 1991).
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Impacts of pumping at the the-Eldridge-Wilde well field also have been
extensively studied. This well field is located about 26 miles north of St.
Petersburg in Pinellas and Hillsborough Counties and consists of 3,500 acres
of pine flatwoods, cypress domes, and marshes as well as improved pastures
and citrus groves. Production began in 1956 and averaged about 16 mgd in
1965, after which it increased steadily to an average of 29 mgd since 1972.
Water table decline of up to 3 ft is predicted for about 50% of the area of the
well field. Vegetation, changes indicative of severely altered hydroperiods
were observed on aerial photographs beginning in the mid-1970s (Rochow
and Rhinesmith, 1991). Cypress,have been almost completely eliminated
from some monitoring sites. Recent assessments estimate that 85% of the
wetlands on well field land have been moderately to severely affected in
terms of subsidence, loss of canopy species, and invasion of upland species
(Rochow, 1994).
Similar impacts have been documented at the Cypress Creek well field in
Pasco Cpurity. Pumping began in 1976 and reached an annual rate of 30 mgd
which has been sustained since 1979. A water table decline of 3 ft has been
modelled for about 60% of the site (about 6 mi2) and the, 1 ft drawdown
contour encompasses about 12 mi2. Biological monitoring since 1975 has
documented minor to severe impacts to wetlands in this area. Although
some hydrologic modifications undoubtedly have occurred as a result of
agricultural and residential development in this area, detailed long-term
monitoring has documented that some of the observed ecological impacts
result from ground water pumping. Wetlands with a predicted water table
drawdown of 1 ft or greater have hydroperiods about 50% shorter than
control sites outside the area of water table drawdown. This coincided with
12 to 28% mortality of tree species such as-pop ash and cypress within the 1
ft drawdown area as well as other impacts similar to those mentioned above
at other well fields (Rochow, 1994).
Other nearby well field sites monitored by the Southwest Florida Water
Management District and its permitees show similar results. At the Cross
Bar well field the remains of 400 dead gopher tortoises were found at Big
.Fish Lake which was dry during 1991 in spite of above average local rainfall
for that year. Extensive replacement of wetland plant species such as
floating hearts and waterlily by drought tolerant species such as dog fennel
and maidencane was also noted. Trends of wetland alteration beginning in
the late 1970s at the Morris Bridge well field were found to have stabilized
when pumpage was reduced by 40% from 1986 to 1993 (Rochow, 1994).
Lastly, in addition to several tens of thousands of acres of wetlands which
have been irreversibly altered in this area, ground water pumping has
contributed to the failure of more than 1,000 private wells which have been
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repaired or redrilled at a cost of $3.5 to $4 million, has drained lakes and
streams, and has increased the rate of sinkhole formation in the-area (House
Committee on Natural Resources, 1994).
Because these types of wetlands and hydrologic conditions are not confined
to southwest Florida, but extend throughout the Southeastern Coastal Plain
physiographic province, it is likely that wetland impacts such as those
which have been documented carefully by the Southwest Florida Water
Management District and its permitees are occurring elsewhere, either on a
local or potentially regional scale (Bacchus, 1994; Bacchus, 1995). For
example, depressional wetlands typical of southwest Florida occur
throughout Florida, southern Georgia, North and South Carolina, Alabama,
Mississippi and a portion of Louisiana. The range of pondcypress wetlands,
which are extremely vulnerable to alteration of hydroperiod, is
approximately coincident with the limits of the Floridan aquifer in Alabama,
Georgia and South Carolina and continues throughout North Carolina,
Mississippi and the southeastern portion of Louisiana (Bacchus, 1995).
IDAHO-
Bruneau River, Owyhee County
Geothermal discharge at Hot Creek and 128 small flowing thermal springs
and seeps along a 5.3 mile length of the Bruneau River in southwestern Idaho
has decreased significantly over the last 25 years threatening the organisms
which inhabit the springs and the outflow (Federal Register, 1993). Ground
water in this area flows northward through volcanic rocks from areas of
recharge along the Jarbidge and Owyhee Mountains and is discharged as
spring flow or leaves the area as underflow. Prior to development, natural
recharge and discharge from the regional geothermal aquifer underlying the
600 mi2 Bruneau area were estimated to be approximately in balance at
22,800 acre-ft/year. .
Ground water development began in the late 19th century and discharge to
wells increased throughout this century, primarily to meet the growing
demand from irrigated agriculture. Maximum discharge was reached in the
early 1980s at 49,900 acre-ft resulting in annual deficit pumping of over
26,000 acre-ft. In large part due to the Conservation Reserve Program
administered by the U.S. Soil Conservation Service, pumping has declined to
the 1991 level of withdrawal of 34,700 acre-ft in spite of a prolonged
drought throughout the hie 1980s. Pumping has caused water levels in the
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volcanic portion of the geothermal aquifer to decline more than 30 ft in
much of the Bruneau area (Federal Register, 1993). , -.._.',.
As a result, geothermal spring discharge also has declined. For example, in
1965 spring discharge at the Indian Bathtub spring was 2,400 gpm and
declined to about 130 to 162 gpm during the summer of 1978 (Young et al.,
1979). 'By 1985 the spring had ceased to flow during the irrigation season.
A similar trend of reduction of discharge was noted at other springs in the
area (Federal Register, 1993).
The Bruneau Hot Springsnail (Pyrgulopsis bruneauensis) is found only in the
springflow of Hot Creek and the springs and seeps of this area. Most of the
springs are small and highly vulnerable to impacts from water level decline.
As a result of habitat threats from declining spring flow, the springsnail
was determined by the U.S. Fish and Wildlife Service to meet the criteria of
an endangered species in 1993. Population estimates made in 1982 and 1992
show a 50% reduction in the number of individuals in many springs. In some
local areas such as the Indian Bathtub spring the species has been totally
eliminated as a result of spring flow decline and sedimentation. Common
aquatic associates of the springsnail -which are also at risk include three
mollusks, the creeping water bug (Ambrysus mormon minor) (also endemic to
the Hot Creek thermal spring complex), and the skiff beetle (Federal
Register, 1993). .
If water levels in the geothermal aquifer continue to decline, the U.S. Fish
and Wildlife Service anticipates that all remaining thermal springs
containing Bruneau Hot Springsnails eventually could cease to flow, causing
the extinction of the endemic species (Federal Register, 1993).
INDIANA
Indiana Dunes National Lakeshore
The Indiana Dunes National Lakeshore is a federally protected natural area
managed by the National Park Service. It is a 12 mile segment of lakeshore
and dunes located on Lake Michigan in northwest Indiana. In the middle
1970s the Bailly Nuclear Generator was constructed on a 7 acre site about
800 ft west of the Lakeshore boundary. During construction, ground water
levels under the nuclear site were drawn down 20 to 30 ft for about 18
months. With this drawdown, the westernmost ponds in the Lakeshore were
predicted to dry up almost completely. Less than 0.5 ,;ft 5! water was
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expected to remain in about 1% of the pond under average conditions.
Sustained dewatering would obviously be damaging to the aquatic and
surrounding riparian ecosystems (Marie, 1976).
NEVADA
Introduction
Nevada is one of the nation's fastest growing states. Its population
increased 50% from 1980 to 1990. Nevada is also the most arid state in the
nation (U.S. Geological Survey, 1995a). Because surface water resources are
scarce to nonexistent in many areas and most often fully appropriated, new
development frequently relies on ground water.
The combination of these factors creates an ongoing need to balance the
allocation of scarce water resources between humans and aquatic and
wetland ecosystems. These isolated habitats are typically critical refuges
for the plant and animal species which are entirely dependent upon them.
Ash Meadows and Devils Hole, Nye County
Ash Meadows is a unique and biologically rich hydric ecosystem in the
Amargosa Desert in southwestern Nevada. Within the 162 km2 area, over 30
springs and seeps discharge a total of 17,000 to 20,000 acre-ft of water
annually, sustaining an oasis in* the otherwise arid region. Many of the
springs are large with headwater pools of 6 to 10 m in diameter. The area is
a discharge point for several thousand square miles of a regional flow
system developed in carbonate rocks (Westenburg, 1993).
Like much of the Great Basin, the Ash Meadows area was much wetter during
the late Pleistocene, supporting Lake Ash Meadows as an ephemeral feature.
As the climate of the post-pluvial period became drier, Ash Meadows
developed as an isolated remnant environment. For this reason, the flora and
fauna of Ash Meadows represent a unique and threatened biota, including
two extinct and over 25 endemic species. The eight plants, two insects, ten
mollusks, five fishes and one mammal species constitute the highest amount
of biological endemism of any area in the United States (Williams, 1984). As
of 1984, eight species were candidates for listing as federal endangered
species and two others officially had been designated as endangered species,
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the Devils Hole pupfish (Cyprinodon' disibolis) and the Amargosa pupfish
(Cyprinodon nevadensis pectoralis) (Williams, 1984).
Dense to. moderate growths of mesquite and saltbush occur at the springs and
along the outlet channels. Extensive saltgrass marshes cover the poorly
drained flatlands which receive the spring discharge (Dudley and Larson,
1976). The large spring discharge supports waterfowl and other migratory
birds.
Several factors have contributed to extensive alteration of this ecosystem.
Carson Slough, the major discharge channel for the region was drained for
mining peat and clay from 1910 to 1930. During the mid-1940s several
exotic predators were introduced to the detriment of the native fish species.
Most recently, further alteration of the natural hydrology has occurred.
Prior to 1960, the small number of local residents used natural spring flow
for irrigation downslope. Beginning in 1961, development of the ground
water resource increased steadily and about 40 wells had been drilled as of
1976. In 1967, a large ranching corporation began acquiring extensive
acreage and water rights to develop about 12,000 irrigated acres in crops
for cattle feed (Dudley and Larson, 1976). Pumping for the ranching
operation began in May 1969.
Beginning in 1969, spring discharge throughout the area began to decline and
reached, a record low in 1972. Annual overdraft in 1971 was estimated to. be
1,500 acre-ft Discharge was reduced by as much-as 50% in some springs
(Dudley and Larson, 1976).
Of particular concern was the'2.5 ft decline in water level in Devils^ Hole, a
warm pool in a collapsed depression in the limestone hills of a 40 acre
disjunct part of Death Valley National Monument. This pool is the sole known
natural habitat of the Devils, Hole pupfish (Cyprinodon diabolis), a federally
designated endangered species (Williams, 1984). The pupfish feeds and
reproduces on a slightly submerged rock ledge which is highly susceptible to
exposure under conditions of ground water decline. In light of declining
spring discharge, in 1976 the United States Supreme Court established a
minimum water level to ensure the survival of the pupfish. Pumping rates
were reduced and eventually all pumping for irrigation ceased in 1982.
Water levels recovered quickly (Westenburg, 1993). In 1984, the Ash
Meadows National Wildlife Refuge was created to protect remaining endemic
species (Graham, 1992).
-Subsequent to the termination of pumping for irrigation for the ranching
operation, a large tract was sold to a developer. In the mid-19eSs plans
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were completed for development of over 33,000 homes in the area, requiring
additional ground water withdrawal (Williams, 1984). It is clear that the
feasibility of this development must be assessed carefully to avoid future
impacts on the spring ecosystems.
Las Vegas Valley and Las Vegas Wash, Clark County
Increased ground water pumping to meet the demand of the burgeoning
population of the Las Vegas valley as well as for crop and landscape
irrigation needs has contributed to a complex alteration of the hydrology of
this region in the last fifty years. This frequently has been to the detriment
of the wetland ecosystems for which this city in the Mojave Desert was
named. Ironically, Las Vegas means "the meadows" (Graham, 1992).
Water use in Las Vegas is about 350 gallons per day per person, about twice
the national average and almost three times that of other arid cities such as
Los Angeles and Tucson (Egan, 1994). While undoubtedly inflated by the large
tourist population, water consumption is still high in this city which
receives only 4 inches of precipitation per year. The majority of the water
usage is for irrigation of urban landscapes (Katzer and Brothers, 1988).
The demand for water in Las Vegas Valley has increased from that obtained
from a few domestic wells in the early part of the century to an annual use
of more than 200,000 acre-ft. About one third of the demand is supplied by
ground water and the rest is delivered from Lake Mead in the Lower Colorado
River system (Burbey, 1993). Fine to coarse grained alluvial fan and
floodplain deposits comprise the principal aquifers within the valley. In
many areas the water table is within 1 to 10 ft below the land surface
(Emme and Prudic, 1991). Along principal drainage courses such as Las
Vegas Wash, hydrophytic vegetation such as reeds and cattails abound.
Phreatophytic species such as mesquite, saltgrass, and saltcedar are also
abundant in these areas.
Pumping of ground water has led to water level decline of up to 300 ft in
parts of the valley (Katzer and Brothers, 1988). Artesian springs which
were scattered throughout the valley around the turn of the century have
ceased flowing and their associated marsh ecosystems have been lost. In
addition to wetland impacts, induced recharge from the overlying shallow
aquifer, which is high in natural and anthropogenically introduced solutes,
threatens the quality of the drinking water supply. Dewatering of fine-
grained sediments has caused subsidence over 400 mi2 in the valley (Katzer
and Brothers, 1988).
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When used to irrigate lawns, golf courses and crops, ground water withdrawn
from the deeper aquifer typically recharges the shallow aquifer in the
central part of the valley. This water ultimately discharges into ephemeral
channels, primarily Las Vegas Wash. Excess irrigation recharge and treated
sewage effluent have combined to change the previously ephemeral wash into
a perennial stream. As a result, the wash has experienced considerable
erosion and headcutting in recent years. The channel level has been lowered
as much as 15 ft, isolating adjacent wetlands dependent on bank storage. As
a result of headcutting, hydrophyte-dominated vegetation has been
eliminated in lower reaches and has re-established in areas farther
upstream where ground water levels are sufficiently shallow. Continued
headcutting will ultimately threaten these riparian areas as well (Burbey,
1993); ".
Other irnpacts from the loss of wetland vegetation along the major washes
include diminished habitat for bird species such as phainopeplas, cactus
wrens and crissal thrashers (Graham, 1992). These species depended
primarily on the dense mesquite stands the washes previously supported. In
addition, the endemic fish species, the Las Vegas dace (Rhinichthys deaconi),
became extinct when a creek in what is now downtown Las" Vegas was
dewatered by declining ground water levels (Graham, 1992).
' ' -i
As the population of Las Vegas continues to increase at a rate of almost
1,000, new residents each week, additional sources of water are being
sought. In addition to pursuing increased allocation of surface water from
the Lower Colorado and the Virgin Rivers, Las Vegas is considering a vast
new ground water project. 'Utilizing 1,200 miles of pipelines to convey
ground water pumped from more than 20,000 mi2 in central Nevada, the $2
to $5 billion project would be one of the biggest and most expensive water
projects undertaken in the West (Egan, 1994). The environmental
consequences of this pumping, including potential impacts on ground water
flow and springs in 4 counties, are being evaluated.
Humboldt River Basin
Within 'the Humboldt River Basin in north central Nevada is a large gold
deposit called the Carlin Trend. Advances in ore extraction technologies
have allowed the profitable development of over 15 mines in this area.
These mines have or are proposing to dewater local aquifers to enable ore
extraction from deep pits. Water table suppression is desired at depths as
great as 1,700 ft and pumping rates necessary to accomplish this range from
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a few hundred to over 70,000 gpm. This magnitude of ground water pumping
is unprecedented worldwide (Miller, 1993; Manning, 1994).
The cumulative impacts of all the projected ground water pumping
operations on the Humboldt River Basin have never been evaluated thoroughly
(U.S. Geological Survey, 1995b). Ecological impacts are expected during the
active dewatering phase as well as after dewatering ceases and ground
water levels recover.
During the mining operations, many companies are disposing of pumped
ground water by discharging it directly to the Humboldt River and its
tributaries. This will provide additional flow temporarily for irrigators and
riparian habitat along the river. Some of the pumped ground water is
reinjected or applied to the land and eventually reaches the Humboidt River
as well. As a result, wetlands and springs have been created which are
dependent on this temporary water source. If not actively sustained after
the mining phase of these operations is completed, these habitats will be
lost.
The major impact is expected after pumping has stopped and the affected
aquifers re-equilibrate with respect to the natural regional hydrology
(Meyers, 1994). When pumping is discontinued, the cones of depression
around the pits and the pits themselves will fill by diverting ground water,
by reducing surface water discharge, or by reversing hydraulic gradients and
drawing water from the Humboldt system. It is estimated that in excess of
3,000,000 acre-ft will be drawn into the cones of depression and pit lakes
from both ground and surface water sources. (The relative contribution from
each source is unknown (Miller, -1993; Miller, personal communication)). In
comparison, the annual flow of the Humboldt River at Winnemucca is only
around 150,000 acre-ft. Predictive models suggest that pit refilling could
eliminate most surface baseflow from two Humboldt River tributaries,
Maggie and Boulder Creeks, and reduce the flow in the Humboldt River by 6 to
66% (Meyers, 1994). Additional mines and proposed expansions of several of
the existing mines are not included in this estimate and would exacerbate
further these impacts.
i , v ,
Detailed descriptions of the anticipated ecological impacts of the post-
mining phase have not been provided in the Environmental Impact Statements
for these projects. It is evident that many wetland and riparian habitats in
this arid basin would be impacted to some extent. Rough estimates for just
two of the pits project the loss of about 2,000 acres of springs, seeps, and
wetlands. Several creeks in the area have been identified preliminarily by
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the U.S. Fish and Wildlife Service as potential recovery sites for. the
federally listed threatened Lahorttan cutthroat trout.
Honey Lake Valley, Washoe County
Developers in the Reno area in .west central Nevada have sought to utilize
ground water to meet increased water demand, lh'1991, Washoe County was
granted rights to pump 13,000 acre-ft of ground water from the Honey Lake
valley and import it to the Lemmon and Spanish Springs valleys via a 39 mile
pipeline.
It was anticipated that this pumping would eliminate or degrade 185 to 485
acres of springs, seeps, and wet meadows as the water table declined to 61
to 78 ft in the area of withdrawal (Bureau of Land Management, 1993b). Even,
with proposed mitigation measures such as pumping additional ground water
to sustain wetlands, the impact in the draft Environmental Impact Statement
was considered "significant and adverse". As. of 1994, the project was on
hold pending resolution of legal matters unrelated to water or wetland
issues (Hill, personal communication).
OKLAHOMA
Beaver River, Texas County , "
The Ogallaja Formation underlying most of the Oklahoma Panhandle is part of
the High Plains regional aquifer system which extends 174,000 mi2 from
southern South Dakota to northwestern Texas. This aquifer (also known as
the Ogallala aquifer) is primarily a water table aquifer deriving recharge
from precipitation (Wahl and Wahl, 1988). The average annual precipitation
in the Oklahoma Panhandle is about 17 inches, average lake evaporation is 62
inches, and average annual evapotranspiration is about 16 inches. Thus
annual recharge to the aquifer in this region is low.
The primary land use in the Panhandle is agriculture, with the land about
evenly divided between farming and ranching. The introduction of the
center-pivot sprinkler system in the early 1960s resulted in a ca'pid increase
in the use of ground water for irrigation. In 1963 there were about 450
large capacity" wells in the Panhandle. The number had risen to 2,500 by
1984 (Wahl and Wahl, 1988).
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«-«E
Because the rate of withdrawal from the High Plains aquifer is much greater
than the rate of recharge, widespread water level declines have occurred
(U.S. Geological Survey, 1990). In western Oklahoma declines of 25 to 100 ft
are common. This has resulted in a substantial reduction in discharge in the
Beaver River which drains most of the Panhandle. The 10-year moving
average discharge was relatively stable from 1950 to 1965 at about 25 to
30 cfs. By 1986, the 10-year moving average discharge had decreased to
about 7 cfs. Prior to 1971, the river was generally perennial, ceasing to
flow less than 15% of the year. By the mid-1980s the river was dry about
85% of the year. Changes in river discharge are not correlated with changes
in precipitation or surface water diversions (Wahl and Wahl, 1988).
The long annual periods without surface discharge and the associated
depletion of bank storage undoubtedly have been detrimental to riparian
ecosystems in this semiarid area. Ground water overdraft resulting in
reduced stream discharge and adverse impacts to riparian communities also
has occurred in other states where ground water from the High Plains
aquifer is utilized in excess of recharge (Wahl and Wahl, 1988).
SOUTH CAROLINA
Savannah River Site, Barnwell County
The Savannah River Site is a nuclear production facility operated by the U.S.
Department of Energy (DOE). The 300 mi2 site was purchased by DOE in 1951
and the P Reactor began operating in 1954. "Throughout its operation, ground
water has been withdrawn from 6 production wells within a 1 mile radius of
the P Reactor. Although the primary source of cooling water for the reactor
is the nearby Savannah River, some ground water has been used for nuclear
production. Since 1985, ground water withdrawal has been reduced
drastically and 4 of the 6 wells have been abandoned (Bacchus, 1994).
The Savannah River Site contains numerous seepage wetlands which are
typical of the Southeastern Coastal Plain physiographic province. Shallow
ground water which flows laterally in response to a small gradient is
essential to sustain these wetlands. Highly permeable sandy soils underlain
by lower permeability clays sustain this lateral flow.
The aquitard separating the surficial aquifer from the underlying confined
systems is locally permeable and discontinuous in extent. Ground water
pumping for the site increased downward leakage from th-T-STirficial aquifer
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through the calcareous semiconfining unit to recharge the confined system.
As a result, the stage and duration of the hydroperiod of the wetlands was
altered significantly (Bacchus, 1994).
Aerial photographs taken between 1943 and 1992 were analyzed to
investigate changes in vegetation cover (Bacchus, 1994). Observed changes
in vegetation are at least in part a result of these perturbations in the
ground water hydrology (Bacchus, 1994). Forested areas in their natural
state contain dense canopies of pondcypress (Taxodium ascendens) and pond
pine (Pinus serotina). Vegetation changes included the loss of approximately
2,825 acres of forested wetlands coupled with an increase of approximately
2,183 acres of forested uplands and 141 acres of shrub wetlands including a
predominance of fetterbush (Lyonia lucida). Only a small portion of the loss
in forested wetlands can be attributed to conversion of wetlands along the
perimeter to planted loblolly pine by the U.S. Forest Service after the site
was purchased by DOE.
Ground reconnaissance revealed that the portion of the wetland complex
closest to the ground water withdrawals exhibited standing dead and dying
trees, canopy dieback-of pondcypress, and tree pathogens. Under natural
conditions, cypress are relatively free of disease and pests. However,
cypress associated with excessive subsurface drainage are susceptible to
internal fungal pathogens and bark and leaf beetles. In addition,
encroachment of wetland tree , species more tolerant of abnormal
hydroperiods such as tulip poplar, sweet gum, wax myrtle, and American
holly was observed suggesting that ground water levels had been lowered.
Lastly, subsidence of surface soil of approximately 30 cm had occurred
exposing cypress roots (Bacchus, 1994). Because cypress roots are adapted
to anaerobic conditions, exposure is potentially lethal (Bacchus, 1995).
TEXAS
Harris (Houston) and Galveston Counties
Rapid population and industrial growth since World ,War II in the Houston -
Galveston area have combined with agricultural demand to increase the need
for additional water_supplies. Prior to 1954 when Houston began augmenting
its water supply with surface water, it was the largest city in the nation
utilizing only ground water for1 public supply. Over 80% of the ground water
demand in some of the outlying areas of this region is for rice irrigation
(Neighbors, 1981).' > ,
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As a result of large amounts of water having been pumped from the ground,
water levels in the artesian aquifers have declined by as muph as 200 ft in
the Chicot aquifer and 325 ft in the Evangeline aquifer. Associated with
these water level declines is extensive subsidence of the land surface.
Subsidence was first noticed in this area in 1938. Between 1940 and 1952
subsidence averaged 0.4 ft/yr. After the utilization of ground water was
curtailed beginning in the mid-1950s, subsidence rates declined to about 0.1
ft/yr. Cumulative subsidence of over 9 ft has occurred near the Houston Ship
Channel and subsidence of at least 1 ft has affected over 4,500 mi2
(Neighbors, 1981).
Land surface subsidence has become critical to parts of the Houston -
Galveston area causing permanent inundation or increased exposure to
flooding. Since 1943, several thousand acres of bayfront land have been
permanently submerged and if uncontrolled, subsidence would have increased
that total to 20,000 acres. In Harris County, 945 mi2 have been heavily
Impacted by permanent submergence and increased flooding (Neighbors,
1981). Presumably much of this nearshore environment has or had some
wetland values which have been irrevocably lost due to subsidence.
Although subsidence is not reversible, the rate of subsidence can be slowed
or stopped with reduction or cessation of ground water extraction. Several
decades of supplementing ground water with surface water to meet the
Houston - Galveston region's growing demand have resulted in reduced rates
of compaction and subsidence. However, current projections indicate that
all readily available water supplies will be exhausted during the remainder
of this century (Neighbors, 1981)'. Conservation or increased ground water
pumping and renewed loss of nearshore acreage, will be among the few
alternatives remaining.
Balcones Fault Zone Edwards Aquifer
The Balcones Fault Zone Edwards Aquifer is located in south central Texas
and parallels the Balcones Escarpment.. It consists of massive limestone
deposits averaging 400 to 500 ft thick (Longley, 1992). It is the sole source
of water for San Antonio's population of about 1,000,000 (U.S. Geological
Survey, 1990).
Water levels in the aquifer and the numerous prolific springs which are
supported by the aquifer are at risk of serious decline. The average recharge
to the San Antonio portion of this aquifer is 637,000 acre-ft per year.
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Annual recharge is highly variable and directly related to annual precipation.
Pumping is also directly related to annual rainfall. Pumpage in 1989 was
542,000 acre-ft. During the summer of this year, aquifer levels decreased
by more than 1 ft per day, an occurrence which has become commonplace in
the spring and summer of recent years (Longley, 1992). In 1991, the "largest
well in the world" was drilled in this area (Swanson, 1991). Capable of
flowing under artesian pressure at a rate of 27,000 to 35,000 gpm, it
potentially could produce an amount equal to 111% of the annual recharge to
the aquifer in 1956, the low year of record (Longley, 1992).
The Balcones Fault Zone Edwards Aquifer supports a diverse assemblage of
over 40 species of aquatic organisms. Some species are subterranean and
others inhabit the many springs. Several species are extremely limited in
distribution. For example, the Texas blind salamander (Typhlolmolge
rathbuni) is found only in a limited area around the San Marcos springs. This
highly adapted amphibian was the first species to be placed oh the federal
Endangered Species list. Eleven other species of invertebrates, salamanders,
and fish are being considered for federal listing (Longley, 1992).
This unique community is at risk of being impacted severely by declining
water levels in the aquifer (Longley, 1992). In addition to reduced spring
flow, aquifer overdraft promotes the encroachment of highly saline water
which threatens the fresh water biota of this system. A test well located
less than 100 yards from major San Marcos springs was found to be highly
saline. Without immediate improved water management, this ecosystem is
likejy to be impacted severely (Longley, 1992).
WISCONSIN
Yahara River, Dane and Columbia Counties
The City of Madison is situated on a series of lakes in south central
Wisconsin which are part of the Yahara River system. The river, lakes, and
ground water form interdependent parts of the regional hydrologic system.
The ground water bearing units in the 323 mi2 drainage basin of the upper
Yahara consist of two aquifers. The lower sandstone unit is confined by an
overlying heterogeneous sandstone layer (Fetter, 1977). There is a
considerable amount of leakage between these two units
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The majority of the municipal and industrial water supply comes from deep
wells drawing from the confined aquifer. These wells are the primary
source of ground water discharge from the basin. As 'a result of pumping, the
water level has been lowered as much as 70 ft in the confined aquifer and as
much as 20 ft in the upper unit. Downward leakage from the upper aquifer
has been enhanced. An estimated 5.6 billion ft3 of the upper aquifer have
been dewatered. The water table decline in the upper aquifer has created a
gradient toward the well field areas causing ground water which formerly
flowed into the lakes and streams to be intercepted (Fetter, 1977).
Prior to 1958 treated sewage effluent was discharged into the Yahara River.
This effluent was a significant component of the annual discharge of the
Yahara River, particularly in the summer months of low flow. Beginning in
1958 the effluent was exported from the river basin as part of a water
quality improvement program for the lower Madison lakes.
The loss of ground water discharge and the effluent in the Yahara River
resulted in a 50% reduction of streamffow during periods of low flow. The
river now is predicted to be dry for periods in some years (Fetter, 1977).
Undoubtedly the combination of increased water table depth, loss of bank
storage and reduction of stream flow to the point of an occasional dry river
bed will have a detrimental impact on the riparian ecosystem.
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Section 7
Evaluation of the Adequacy of Existing Information
on Ecological Impacts of Ground Water Overdraft
Introduction
Based on the literature and data reviewed for this study, the investigation
and documentation of ecological impacts of ground water overdraft is
clearly not a major focus of attention for most researchers and regulators at
this time. Compilations of more than 6,300 citations published primarily in
the last decade on ground water hydrogeology, hydrology, and wetland
ecology were reviewed for this study (examples include van der Leeden,
1991; Atkinson et aL, 1986; Emery, in preparation; Hood, 1988; Jalink,
personal communication; Fjsk, 1989; Stanford and Simons, 1992; Stanford
and Valett, 1994). Fewer than 30 papers dealt directly with the ecological
impact of ground water overdraft. Applicable citations were also
, infrequenty found in government publications and refereed journals.
As was described in Section 6, ground water overdraft can be extensive in
many diverse areas. In some regions, research is occurring to accurately
evaluate the nature and extent of ecological impacts of ground water
overdraft. Notable examples include several basins in. Arizona, California
and Florida. Other researchers present sufficient data to document the need
for further study to understand this important aspect of wetland hydrology.
However, this topic definitely is 'addressed inadequatey in the technical
literature at this time.
i ' ' . ' ~; .
Clearly, investigation of the ecological impacts of ground water overdraft is
an emerging issue. Historically, studies .on ground water decline have
focused primarily on the economic impact of water shortage and the costs of
developing new water supplies. Impacts of ground water decline which
affect people's use of the land and water such as saline intrusion and land
subsidence have been well documented. .Investigation of these impacts has
centered on the resulting geologic hazards, structural damage, and economic
impacts of abatement with little or no attention being focused on the actual
or potential ecological impacts of these processes (for example Katzer and
Brothers, 1988; Atkinson et al., 1986; Schumann et al., 1985).
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Another example of the shortcoming of the available literature in addressing
the ecological impacts of ground water overdraft ist that major- changes in
surface hydrology are frequently documented without noting the unavoidable
impacts on associated riparian and aquatic ecosystems. For example,
abundant literature is available on springs, lakes, and rivers which are
almost or completely dewatered without noting the associated ecological
consequences (for example Wahl and Wahl, 1988; Fetter, 1977). Therefore it
is undoubtedly the case that the extent of ecological impacts resulting from
ground water overdraft exceeds those instances currently documented in the
available literature.
Technical Perspective
A major reason for the paucity of information on this subject is the
technical complexity of performing meaningful studies. Site-specific
studies are needed. Adequate data must be available on surface and ground
water hydrology as well as sufficient ecological data to measure valid
trends and physiological effects of sublethal stress. Studies must be of
sufficient duration to identify long-term and cumulative impacts.
Most wetlands and riparian areas are subjected to a variety of anthropogenic
ar\d natural perturbations. In some cases it may be difficult to distinguish
ecological impacts due to. ground water pumping from other causes of
impacts. For example, water level decline in riparian aquifers can result
from pumping as well as upstream surface water impoundment, stream
channel incision, and natural drought cycles (Swift, 1984; Scurlock, 1987).
Likewise, livestock grazing, watershed degradation, recreation, and the
introduction of exotic species may result in vegetation changes similar to
those associated with ground water overdraft (Stromberg, 1993c;
Stromberg, 1994; Hendrickson and Minckley, 1984). Accurate identification
of causal relationships requires extensive knowledge of the history of land
uses and carefully designed investigations.
Another, reason for the dearth of studies on this subject is that the current
scientific foundation for understanding wetland hydrology is weak. Until
recently, there have been relatively few studies done on the relationship
between ground water and wetlands (Carter and Novistzki, 1988). According
to Winter (1988), "Most hydrologic information relative to wetlands has been
based largely on theoretical studies of generalized settings, on scattered
field studies, and on hydrologic intuition". Calculation of a water budget
may be fraught with inaccuracies (Siegel, 1988). Particularly problematic is
the quantification of evaporation and transpiration from wetlands as well as
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the seasonal and annual changes in ground water flux in wetlands (Winter,
1988). Without an accurate understanding of the spatial and temporal
changes in water movement in wetlands and riparian areas, determination of
the magnitude of impact of ground water pumping is difficult.
Adequate surface and ground water monitoring data are essential but often
are unavailable and difficult to obtain. For example, the fact that springs,
wetlands, and seepage faces may be supported by shallow, localized
unconfined aquifers can complicate efforts to obtain sufficient data in some
locations (Winter, 1988). Such systems may be characterized, by
considerable seasonal and annual fluctuation in water level independent of
pumping. For example, Gerla (1992) noted that in some intermittent
wetlands, the water table rose over ten times the amount predicted by
infiltration due to physical processes in the capillary fringe. Furthermore,
while of essential ecological significance, hydrologic systems such, as these
may be unimportant as major water supplies for human needs and of low
economic value. Consequently, adequate long-term water level data may not
be readily available.
Ecological .monitoring for impacts from-ground water overdraft also may be
problematic. To be of use in preventing irreversible damage to wetlands and
riparian areas, investigations must focus on sublethal stress responses
rather than mortality. According to Bacchus (1995), "Field methods for
monitoring wetlands were designed primarily to estimate dominance and
(community composition) rather than to detect and measure responses of
natural systems to imposed stresses. Although standard approaches may
provide some insight into stress responses, such interpretations are
difficult because the majority of monitoring is short-term and there is a
dearth of data on stress responses in wetland species." Factors such as
changes in species composition and density, reproductive success,
susceptibility to pathogens, and introduction of new species should be
considered (Stromberg, 1994; Bacchus, 1995).
Inadequate information on several key aspects of wetland ecology further
hinders researchers' ability to develop predictive models of the impacts of
ground water drawdown. In many instances, it is not known whether certain
plant species are obligate or facultative phreatophytes in riparian settings.
Consequently the impact of water level decline beneath the depth of root
penetration may be difficult to evaluate (Busch et al., 1992). Furthermore,
studies of wetland ecosystems are rarely designed to identify the earliest
indicators of stress resulting from water level decline, Lastly,
ecophysiological studies have focused more heavily on the effects of
Hooding on root tissue, leaving greater uncertainty regarding the
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physiological tolerance of short-term and long-term desiccation (Busch et
al., 1992).
Regulatory Policy Perspective . ' -
It is not within the scope of this report to thoroughly explore the adequacy
of current regulatory policy in identifying and addressing instances in which
ground water pumping results in ecological impacts in wetlands. However,
several key factors are readily apparent in reviewing the literature available
on this subject. Foremost is the fact that federal agencies and most states
have very limited regulatory controls on ecological impacts from ground
water overdraft (Smith, 1989). While the ecological values associated with
maintaining surface water flows are well recognized, the concept is riot
reflected in ground water management in most instances (Fort et al., 1993).
At present, most approaches to management of ground water quantity are for
the^purpose of fulfilling senior water rights and not for the protection of
associated ecosystems. In some states, utilization of ground water is
unlimited as long as the water is put to beneficial use (Smith, 1989; Parfit,
1993). Limitations to this broad right may be technically difficult to invoke.
For example, in Texas, ground water drawdown may be limited if the ground
water meets the definition of an "underground stream" (Longiey, 1992).
Furthermore, the ongoing evolution of the regulatory definition of wetlands
complicates the issue (Lehr, 1991). For example, a decision by the U.S.
Supreme Court was required to include wetland areas saturated by ground
water (as opposed to surface water) under protection by the Clean Water Act
(Want, 1988).
Other regulatory provisions which provide some measure of protection to
wetland ecosystems may not be applicable to impacts from ground water
drawdown. Permits issued under Section 404 of the Federal Clean Water Act
are required for surface water diversions which involve fill or excavation of
waters of the United States including ditching, channelization and discharge
of dre'dged materials. However, permits generally are not required for
dewatering wetlands and fiparian areas through ground water pumping. This
is particularly ironic as the areal extent of ecological impact may be far
greater from dewatering than from placing fill for a project such as a road
crossing (Bacchus, 1994; Hill, personal communication). Furthermore,
because the permits are issued for individual wetland sites, the permitting
process is not conducive to the evaluation of cumulative impacts in an entire
hydrological system (National Research Council, 1991). Likewise,
Environmental Impact Statements may be inadequate in evaluating the
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cumulative impact of pumping operations on an aquifer a'nd also greatly may
underestimate the actual drawdown which occurs (Bureau of Land
Management, 1994). Finally, mining reclamation plans may" include
revegetation but not the replenishment of ground water pumped during, pit
excavation (Ross, 1992).
Improved regulation of ground water quantity to manage the impacts of
water level decline including rising pumping costs has riot been readily
accepted. For example, in parts of the High Plains dependent oh the Ogallala
aquifer, water level decline is extensive and rising pumping costs threaten
the profitability of agricultural water users. However, in a broad survey of
water users conducted in this area, regulatory controls to manage this
shared water resource were found to be unpopular and voluntary use of
conservation measures in-response to economic necessity was favored
(Kromm and White, 1986). However, while market mechanisms "have been
successful in some instances, they may be ineffective in prevention of
impacts to natural resources of limited monetary value such as some
wetlands (Smith, 1989).
Another impediment to improved management of depleted ground water1
resources is the multijurisdictional nature of many ground water systems.
Where aquifers cross state or local boundaries, individual pumpers have
little incentive to unilaterally restrict withdrawals of ground water.
Without cooperative agreements, a "use it or lose it" mentality may prevail.
The need for cooperation between states is large. In a survey to determine
the extent of interstate competition for ground water resources in the
western continental United States, interstate competition was found
somewhere on the borders of all states except Oregon (Smith, 1989).
Water use planning may be further complicated by the absence of a detailed
understanding of local hydrogeology. Unlike surface water users, individual
ground water pumpers may not recognize their co-dependence on a shared
water resource. Consequently, the effects of overexploitation may not be
recognized until they are considerable (Llamas et al., 1992).
Legal Perspective
There is no single legal approach to ground water overdraft in water law in
the various states. State laws often do not explicitly address overdraft so
policies may have to be inferred from other management mechanisms such as
special management districts or provisions for well interference. Among
the Western states, many states have a de facto policy of permitting
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unlimited ground water mining (Fort et al., 1993). Furthermore, there is
frequently poor integration between the legal framework for regulating the
utilization of surface water and ground water. While these two resources
are often very different in terms of their development and distribution, they
are hydrologically highly interconnected and in some instances must be
managed as a unified system (Llamas et al., 1992). A noteworthy example of
a statutory initiative in this regard is the Arizona Riparian Protection Act of
1992. The act directs the Arizona Department of Water Resources to
"evaluate the effects of ground water pumping and surface water
appropriations on riparian areas in the state" (Stromberg, 1992).
One of the primary challenges with regard to improved management of ground
water overdraft stems from the fact that legal controls on ground water
quantity frequently are in the private domain. The present use of ground
water has resulted largely from the accumulation of individual private
operations rather than larger scale, often publicly funded, impoundment and
distribution systems more commonly associated with 'utilization of surface
water (Llamas et al., 1992). Water rights often are attached to property
rights provided pumped water is used for a beneficial purpose. This has led
to the practice of water farming in some areas. For example, tens of
thousands of acres of irrigated farmland in rural Arizona have recently been
purchased-by municipal water purveyors. Their intention is to retire the land
from agriculture and export the ground water pumped on the property to
rapidly growing nearby cities (Checchio and Nunn, 1988). If not carefully
managed, such export may result in environmental impacts in the area of
origin. It is ironic that state statutory provisions intended to curtail
declining ground water levels in urban areas have provided the incentive for
this extensive water farming (Checchio and Nunn, 1988).
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Section 8
Recommendations
, . - v
The primary recommendations from this study fall into two categories which
will be discussed below. First, under certain circumstances, the potential
for ecological impacts from ground water overdraft is significant. However,
this phenomenon commonly is not investigated in most ground water or
wetland studies. Therefore, it is recommended that the issue receive more
research and regulatory attention in a broad array of contexts. Second, one
primary focus of this attention should be on an enhanced predictive
capability for early detection of "high risk" hydrologic settings and "high
risk" wetland and riparian communities and species. With care to avoid
erroneous oversimplification, this information should be used to improve
planning and policy regarding water use and wetland protection.
As was described in Sections.6 and 7, the existing literature is sufficient to
conclude'that adverse ecological impacts from ground water overdraft have
occurred in many diverse ecosystems. The potential exists for similar
impacts to be occurring in many ecosystems which have not yet been
investigated. Studies to identify and quantify consequences from pumping
should be conducted more frequently and more comprehensively. The
potential for wetland and riparian .impacts should be considered in regional
water use planning programs where water level decline is a concern
Similarly, this issue should be adequately addressed in regional hydrological
studies and wetland surveys. Investigations regarding other impacts
associated with ground water overdraft such as saline intrusion arid
subsidence should be coupled with or should include an assessment of the
potential for ecological impacts when wetlands are involved. Permitting
processes pursuant to federal, state, and local environmental protection and
natural resource management statutes should include ah assessment of the
potential for impact from subsurface drainage for any development projects
which may affect wetlands and riparian areas. In particular, a more rigorous
and comprehensive approach .should be employed in environmental impact
statements. To the maximum extent feasible, emphasis should be on
cumulative impacts of regional ground water resource development rather
than on individual wells or well fields or individual development projects.
Because hydrologic settings and wetland ecosystems are individually unique
and complex, site-specific studies will be essential. Therefore, achieving
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the preceding recommendation will- be time-consuming and costly. To
maximize the efficiency with which impacts can be identified, or ideally
prevented, additional research effort should be focused on developing
predictive capabilities for identifying "high risk" hydrologic settings and
"high risk" ecosystems and species. This approach can then be used as a
starting point to improve planning and policy. A goal of these efforts should
be prevention instead of restoration. Restoration of wetlands is technically
limited at this time and often politically difficult and costly. Therefore, use
of a more proactive predictive approach is likely to achieve better .results at
a lower cost.
As a starting point, examples of high risk hydrologic settings might include
locations with shallow ground water, rapidly drained soil, discontinuous
confining layers, or a large seasonal difference in precipitation which
results in ground water dependence during the dry season. For plant species
which may be particularly sensitive to hydroperiod perturbations, optimal
rates of ground water drawdown should be investigated to allow maximal
use of the resource with minimal ecological impact. Soil type and
meteorological factors would have to considered (Mahoney and Rood, 1992).
Another predictive tool which should be developed is the species specific
indicators of earliest water stress. Impacts must be identified before
structural degradation occur in plants. In some cases this will involve a
better understanding of the physiological effects of water deprivation on
plant tissues. In other cases, research will be required to identify the
source of water* utilized by plant species, which may vary seasonally, by
location or by age of the plant. Understanding seasonal dependence on ground
water, precipitation and surface 'water is essential to establish safe limits
on drawdown. Tracer studies using the stable isotopes deuterium and
oxygen-18 to distinguish facultative versus obligate phreatophytes appear to
have promise (Dawson and Ehleringer, 1991; Busch et al., 1992; Dysart,
1988) as opposed to earlier studies in which plants were extracted from the
soil (for example Gary, 1963).
In addition to an improved understanding of the response of individuals and
species to water level decline, it is important to consider that wetlands and
riparian areas are among the most spatially and temporally complex natural
systems on earth (Richter and Richter, 1992). Riparian systems respond to
water stress by undergoing changes that occur in a hierarchical fashion.
Whereas low levels of stress cause changes at the level of the individual,
high levels of stress result in ecosystem and community level changes
(Stromberg, 1992). Few studies have quantified relationships between
water stress and riparian ecosystem response, particularly in arid regions.
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Although highly complex, enhanced predictive capabilities are needed at
these levels as well. Linking ground water and ecological models is one
potential approach (Richter and Richter, 1992).
Predictive information of this type should be used more widely in ground
water management programs. A major objective should be to create a
management approach in which there is feedback between the occurrence of
hydroperiod perturbations, water stress tolerance limits, and allowed
pumping rates. For wells sited, in highly sensitive areas, permit conditions
could require pumping to be slowed or stopped when plant limiting soil
moisture or water level depths were reached (Groeneveld, 1989; Stromberg
et al., 1993bj. The long-term monitoring of ground water levels and
ecological impacts required by the Soutwest Florida Water Management
District is a useful example which could be utilized more widely.
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Section 9
Conclusions
A review of the existing literature on the environmental impacts resulting
from ground water overdraft revealed that localized wetlands and riparian
areas throughout the United States have undergone changes consistent with a
loss or reduction of the ground water discharge which previously had been
available. While many instances are localized, in some cases these impacts
have occurred on a regional basis throughout entire drainage basins or
aquifer systems. <
The locations in which diseernable impacts have been documented in this
study are not hydrologically or ecologically unique. It is likely that similar
impacts are occurring in other locations which have not been investigated,
particularly in the arid west where ground water is frequently the only
perennial water source available. It is probable that the existing body of
literature significantly underestimates the extent of ecological impacts
which are a result of ground water overdraft.
As the demand for ground water continues to grow and the remaining wetland
and riparian areas are subjected to an increasing array of developmental
pressures, the importance of this issue is clear. Left unaddressed, it is
probable that ecological impacts of ground water overdraft increasingly will
undermine other environmental efforts such as wetlands restoration and
endangered species protection.
Efforts to assess and rectify impacts from ground water overdraft must be
approached with a full understanding of the complexity of hydrologic
systems and wetland ecosystems. Many human activities and natural
processes can result in ecosystem changes. Causal"relationships are often
difficult to identify. Detailed,, site-specific, and often long-term studies
are needed to determine pumping rates which can be sustained without
undesirable impacts; A better predictive capability for optimal rates of
drawdown and early indicators of ecological stress will be needed to assist
water resource planners in being more responsive to this issue.
A deliberate "effort should be made to incorporate this issue into existing
environmental assessment and water resource planning processes. To the-
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maximum extent possible, emphasis should be on cumulative ecological
impacts and regional water allocation.
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Section 10
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