THE IMPORTANCE OF
SURFACE WATER/ GROUNDWATER INTERACTIONS
ISSUE PAPER
FINAL
MARCH 30,1999
(WITH MAY 19,2000 EDITIONS)
KERIANNE M. GARDNER
Environmental Protection Agency
1998 EPA Intern Program Project
Office of Water/ Water Quality Unit
Region 10 EPA
1200 Sixth Avenue
Seattle, WA 98101
(206)553-0268
Publication number: EPA 910-R-99-013
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THE IMPORTANCE OF
SURFACE WATER/ GROUNDWATER INTERACTIONS
ISSUE PAPER
Publication number: EPA 910-R-99-013
ERRATA
THE MARCH 30,1999 VERSION WAS EDITED ON MAY 19,2000 TO REFLECT
THE FOLLOWING MINOR CHANGES:
PAGE?: The citation for Figure 9 did not show up due to the text box sizing.
The text box was adjusted, and the reference was cited in the "Works Cited"
section. Previously, the reference had been omitted from the "Works Cited."
Instead, the individual chapters were cited, with the main text title included
with those individual citations.
PAGES: The last sentence on the page, beginning "At a local scale...", was
moved to immediately follow the sentence ending "...moving slightly uphill."
Previously, the later sentence was located on the following line.
WHITE ETAL. 1998 was a misprint. The citation should read Winter et al.
1998. There were 2 instances of this occurring: P.5, Last paragraph and P. 8,
last paragraph.
KERIANNE M. GARDNER
Environmental Protection Agency
1998 EPA Intern Program Project
Office of Water/ Water Quality Unit
Region 10 EPA
1200 Sixth Avenue
Seattle, WA 98101
(206)553-0268
Publication number: EPA 910-R-99-013
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INTRODUCTION
Why is the connection between surface water and ground water relevant to the average
citizen, the scientist, and the policy-maker alike? When ground water mixes with surface water,
they impart their characteristics upon one another and unique gradients develop for each
parameter. Since ground water and surface water are essentially one resource, there is potential
for the surface water quality to affect ground water and vice versa (Naiman et al.1995; Squillace
et al. 1993) . These interactions could affect quality of life for fish or drinking water for humans.
While ground water scientists have long studied the physical aspects of ground water/
surface water interactions, it has been in fairly recent times that these interactions have been
looked at in relation to their ecological implications. With the coming of a more holistic
approach to environmental protection, surface water/ ground water (SW/GW) interactions have
been receiving heightened attention by ecologists and have begun to be looked at by policy
makers and watershed managers. It is generally understood in conceptual form that ground water
has the ability to enhance or detract from the surface water quality, yet little is known outside of
specialized scientific realms about the processes by which these two entities interact. In the past,
emphasis has been placed on studying the physical and chemical effects that ground water has on
surface water quality, but it is also important to look at the ecological role surface water/ ground
water interactions play. The study of surface water/ ground water interactions from an ecological
perspective is slowly picking up momentum, with new fields of science being named to focus on
these issues.
OBJECTIVE
The paper intends to enlighten a broad audience and nurture a general understanding of
the interaction between surface water (SW) and ground water (GW). Scientists, non-scientists,
community groups, programs within EPA, and other agencies all have many different viewpoints
and access to a variety of resources. This paper hopes to capture the ideas presented in a number
of previously published sources and establish a common language to discuss how the realms of
surface water and ground water interact, what the importance of these interactions are, and what
ecological functions are involved. The two target goals of this paper include: 1) Make a case for
the importance of understanding the ecological roles GW/SW interactions play, especially in
relation to temperature and salmon; 2) Lay groundwork for future pilot projects to explore means
of incorporating these interactions into water quality work. Ultimately, this paper aims to make
the case for the importance of expanding our understanding of this connection and convince
water quality managers of the need to explore ways to apply this understanding more directly in
water quality work.
BACKGROUND
In current EPA programs, there is a general tendency to isolate each medium into its
respective program (Naiman et al. 1995), thus ground water and surface water have been
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managed through separate programs. Though a slow transformation is under way to look at the
"big ecological picture" via the "watershed protection approach", there is still some difficulty in
comprehending how ground water and surface water connect programmatically. The "watershed
protection approach" is described as a holistic way to address the complex and persistent
problems in watersheds around the nation (Houghton 1993).
Nineteen ninety-three was a dynamic year for addressing surface water and ground water
holistically . A prominent scientist in the field of ground water ecology, Jack Stanford, briefed
Congress on the recent scientific findings and stressed the connectedness of ground water and
surface water (Houghton 1993). In a watershed field workshop led by Jack Stanford, ecotone
areas are described as "landscape hot spots of diversity" because of the diversity of habitat that
tends to occur within transition areas. Since SW/ GW interactions link terrestrial and aquatic
components, the processes involved in these interactions contribute to the overall diversity of
habitat (Stanford et al. 1994). Carol Browner, EPA Administrator, spoke of treating ground water
protection as an integral part of watershed management in the first series of hearings on the Clean
Water Act reauthorization, but stopped short of calling for a national ground water protection
goal (Houghton 1993). In the opinion of the Groundwater Monitor's contributing writer, EPA's
recognition of the connections highlighted by Jack Stanford is a step in the right direction
because the past Clean Water Act had ignored those connections (Houghton 1993). Nevertheless,
ground water is often viewed as a physically isolated resource in regulatory language (Job &
Simonsl994).
As mentioned before, the primary function of this issue paper is to focus attention on the
need for and value of integrating the surface water/ ground water (SW/GW) interaction zone, or
ecotone, into our water quality programs. We hope that this paper will lay the groundwork for
pilot projects to explore means to better quantify this interaction and account for it in such
programs as Total Maximum Daily Load (TMDL), Source Water Protection (SWP), Watershed
Assessment, and Restoration. The focus is on making these concepts understandable to a broader
audience of people which may include non-scientists, general scientists, policy writers, and
watershed managers.
How GROUND WATER WORKS
The hydrologic cycle includes 4 main pools of water: oceans, ice, ground water, and atmosphere.
Of these, ground water is the 2nd smallest but has great importance in terms of human use (see
figure 1 from Winter et al. 1998). In
ground water/ surface water interactions,
the ground water component is much
greater than the surface water, but has much
less visibility, and thus attracts less public
interest.
FIGURE 1: THE HYDROLOGIC CYCLE
(From Winter et al. 1998)
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Because ground water is unable to been seen, water movement beneath the land surface is
a difficult concept to visualize. Imagine a latticework below ground, with water flowing between
the open spaces. At the time of formation, some rocks contain many spaces, or voids, while
others are more solid. Usually rocks that are located near the earth's surface acquire more voids
as they undergo physical and chemical weathering. This weathering breaks rocks down into
various sized particles. These particles are deposited by gravity, wind, rain, or ice. Collections
of these deposited particles are called sediments. During deposition of sediments, the variety of
particle sizes allow openings, called pore spaces, to be left between the grains (Fetter 1994). It is
through this latticework of pore spaces that ground water flows.
The ability of ground water to flow through the lattice work is affected by a number of
properties. Within sediments, there are pathways that connect the pore
spaces and allow water to pass from one pore space to the next. First of
all, the range of particle sizes will affect the amount of pore space, or
porosity, present. Porosity can be measured as a percentage by dividing
the volume of void space by the total volume of soil and void space. If
there is a wide range of particle sizes, the smaller grains may fill the
space between the larger grains (See Figure 2). This will decrease the
porosity and thus reduce the amount of space for water to flow through.
FIGURE 2: INTERSTITIAL SPACES BETWEEN PARTICLES
(FROM FETTER 1994)
Next, the shape of grain sizes will affect the amount of pore
space available for water to travel through. Spherical shaped particles will be able to pack
together tighter than oblong or irregular shaped particles. Lastly, the material composition and
orientation of the particles will affect the porosity. Grain size is a property of material
composition that will affect porosity. Clay particles tend to adhere to one another and form
clumps of soil, called peds. These peds will act as larger sized particles and form abundant pore
space. If the clay soil is compacted, the structure of the peds will be broken down, the particles
will be smaller, and more closely packed. This change in composition will decrease the porosity
of a soil.
Saturates zone (ground water]
FIGURE 3: SATURATED AND
UNSATURATED ZONES
(FROM WINTER ET AL. 1998)
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This water located below the land surface is grouped into two primary zones. The first
zone is the unsaturatedzone, where the voids and spaces between particles (sometimes termed
"interstices") contain both air and water. The second is the saturated zone, where the voids are
completely filled with water. The water table is a discreet line located at the uppermost surface
of this zone, where the pore water pressure is equal to the atmospheric pressure (See figure 3).
The water table generally follows the topography of the land surface.
A soil's ability to store water, along with its ability to transmit water make up the most
important hydrologic properties. Hydraulic conductivity describes the rate at which water is able
to move through the soil or other medium. It is determined by the size, shape, and connectedness
of the pores, as well as the viscosity of the fluid. Temperature affects the viscosity of the water
flowing through the latticework of pores. The colder the water, the more viscous, or "thick"it is.
An aquifer is a rock unit that is able to hold and transmit water at rates fast enough to
supply reasonable amounts of water to wells. Aquifers may be near the land surface, with many
continuous layers of materials with a high ability to allow water to permeate through. These
types of aquifers are called "unconfmed aquifers" or water table aquifers (Fetter 1994). (See
Figure 4) A confining layer is a rock unit with very low, or no, ability to allow water to
permeate through it. Aquifers which are overlain by a confining layer are called confined, or
artesian, aquifers. Occasionally, a patch of material with a low ability to allow water to pass
through it will be located above the water table. This layer will intercept the water moving
through the unsaturated zone and form an area of saturated soil. This type of aquifer is called a
perched aquifer (Fetter 1994). Seeps and springs arise where the water table or the perched
aquifer intersect the surface topography, as in valleys or hillsides (Fetter 1994, McMastin 1996).
(See Figure 5).
UfttOftftncd aquifer
confining bed
FIGURE 4: AQUIFER TYPES (FISRWG 1998)
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FIGURE 5: SPRING FORMATION
DUE TO PERCHED AQUIFER
(FROM FETTER 1994)
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The ultimate source of ground water is precipitation, such as rain and (melted) snow,
which infiltrates through the unsaturated zone and eventually recharges the underlying aquifer.
Recharge is defined as "the accretion (or gradual growth in size due to external addition (Morris
1969)) of water to the upper surface of the saturated zone" (Winter et al. 1998). Aquifers may be
recharged by water that has seeped through the unsaturated zone, by ground water that has moved
laterally, or by water that has seeped upward from underlying sources. (See Figure 6). There is
substantial variation in quantity, distribution, and timing of precipitation. Therefore, ground water
recharge is a dynamic process, and the position of the water table varies seasonally and from year
to year (Winter et al. 1998).
water
table
FIGURE 6: PRECIPITATION
RECHARGING GROUND
WATER (FISRWG 1998)
How FLOW SYSTEMS WORK
One may imagine that ground water flows much the same as surface water, from an area of
higher elevation (source) to an area of lower elevation (mouth). In ground water ecosystems, this
is not always the case. Unlike surface water, ground water flows down "gradient". Within the
ground water environment, there are pressures and gradients which force ground water movement.
The measurement of hydraulic head is used to document hydraulic gradient, which is calculated as
a change in head per unit distance, within a ground water flow system (Fetter 1994). Hydraulic
head may be measured by using a piezometer. "Head" is a measure of the amount of pressure that
groundwater is subjected to. Water flows from an area of high pressure to an area of lower
pressure at a given point within an aquifer. A piezometer is a small diameter well with a section of
slotted pipe on the end (Fetter 1994). Further detail regarding methods of measurement will be
discussed in a later section.
Within the saturated zone, there are 3 scales of flow systems within a flow network: a
local flow system, which is the most near the surface; an intermediate flow system; and a regional
flow system, which is the deepest and travels the greatest distance (Brunke and Gonser 1997,
Winter et al 1998, Fetter 1994). The three scales of flow systems are differentiated by length and
duration of the ground water's residence below ground before surfacing and interacting with
surface water. (See figure 7).
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loot r.Vi.
Slut* RfM DinHiiiKiofSow Ian/ftw !fj
FIGURE 7: THREE GROUND WATER
FLOW PATHWAYS: REGIONAL,
INTERMEDIATE, AND LOCAL (FROM
TOTH 1963, PRINTED IN APPLIED
HYDROGEOLOGY FETTER 1994)
The largest is the regional/ continental scale. Flow at this scale is determined primarily by
geographic features of the landscape, such as mountainous regions, coastal areas, and glacial till
areas. At this scale, surface water would enter into the ground water system near the headwaters
and would not reemerge until it reached the mouth of the regional system. These ground waters
travel deep below the surface of the earth. The water moves very slowly through this scale,
because it is traveling primarily through bedrock, which has a very low ability to transport water.
It may take thousands of years for waters to move from recharge to discharge zone. This type of
ground water is most resilient and least likely to be impacted by human actions. It would take a
large scale geologic event, such as an earthquake, to cause change at this scale. Within this large
scale, smaller scales are nested.
The intermediate scale is distinguished by topography within a region. Ground water
flows at an intermediate depth and is in the system for an intermediate amount of time. It may
take one to 100 years for the ground water to travel through this pathway, depending upon the
distance the ground water travels before surfacing. In this pathway, precipitation is intercepted,
infiltrates into the saturated zone, and flows down gradient as ground water. In this pathway, the
ground water flows toward a stream or wetland area. This pathway is less resilient to changes
than the deeper regional flow pathway, and thus more likely to be affected by human actions.
Within each intermediate flow pathway, at least two local flow pathways are nested.
Local flow pathways are driven by more local topographic features, such as dips or
depressions in the landscape. In local flow pathways, ground water flows most near the surface
and resides within the flow path for the shortest duration of time, relative to the above flowpaths.
Usually, ground water is present in this flow path for less than a year. This pathway is most
susceptible to changes and is most likely to be impacted by human activities.
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In the intermediate and local flow pathways, flow between ground water and surface water
may occur in either direction. Influent is where surface water moves into ground water. Other
terms that describe this net flow of surface water to ground water include down-welling, stream-
fed aquifer, aquifer recharge, and infiltration. Effluent is where ground water moves into surface
water. Other terms that describe this type of net flow include upwelling, aquifer-fed stream,
aquifer discharge, and exfiltration (Brunke & Gonser 1997).
How RIVERS WORK: GROUND WATER IN A RIVERINE CONTEXT
Regional upland
FIGURES: GROUND
WATER IN A RIVERINE
CONTEXT
(WINTER ETAL 1998)
Direction of regional flow
The previous section outlined the classic view of how ground water flows. This section
aims to superimpose the classic way ground water flows upon the morphology of a riverine
system to see how ground water works in this setting (See figures 8 and 9).
At the large scale, surface water would enter into the ground water system near the
headwaters and would not reemerge until it reached the mouth of the regional system. Imagine a
basin area, from the headwaters to the outwash plain (See figure 9). The basin morphology
determines the large scale regional flow pathway, ground water would flow below ground,
without surfacing, from mountains to the outwash plain. This scale is not easily influenced by
human activities, nor does it have a profound or direct ecological impact upon the surface waters
of the riverine system, therefore it will not be focused on in this paper.
FIGURE 9: A CROSS-SECTION VIEW
OF FLOW SYSTEM FROM
HEADWATER TO OUTWASH PLAIN.
Surface and interstitial flows
indicated by horizontal and vertical
arrows, respectively. Thickness of
the arrow represents volume of
overland flow. Numbers refer to
segments in river continuum:
1. headwaters, 2. transition,
3. montane floodplain, 4. transition,
5. valley floodplain, 6. transition,
7. Coastal plain
(Gibertetal 1994)
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In a riverine system, the intermediate scale is distinguished by floodplain morphology
within a region. Ground water flows at an intermediate depth and is in the system for an
intermediate amount of time. Imagine a stream reach located in a canyon riverine system with
alluvial valleys. There would be alternating narrow canyons composed of bedrock and wider
alluvial valley floodplains composed of gravel. Ground water enters the intermediate pathway at
the top of an alluvial gravel floodplain and reemerges at the opposite end of the floodplain, where
the channel is again constricted by a bedrock canyon. Each episode of surface water exiting the
narrow canyon, infiltrating into the alluvial gravels as ground water, then reemerging as surface
water at the next constriction would constitute an intermediate flow pathway. It may take one to
100 years for the ground water to travel through this pathway, depending upon the size of the
floodplain, the hydraulic conductivity, the depth of the gravels within the floodplain, and the
number of bends, or sinuosity, in the channel. This pathway is less resilient to changes and more
likely to be affected by human actions than the large scale pathway. Within each intermediate
flow pathway, at least two local flow pathways are nested (Gibert et al 1997, Fetter 1994). (See
figure 10)
FIGURE 10: A) Local interaction due to bed morphology (pool and riffle).
B) Intermediate interaction due to channel morphology. (From Winter et al. 1998)
Local flow pathways in a riverine system are driven by stream bed morphology, such as
pool riffle sequences. Within a channel, the bed is composed of alternating pools and riffles
formed by the presence of sediment bars or large woody debris. Each episode of surface water
entering into the stream bed from a pool, traveling towards a riffle, and then exiting out of the
stream bed into the next pool would constitute a local flow pathway (Brunke & Gonser 1997,
Winter et al 1998, Pacific Rivers Council 1995).
Whether the stream is gaining or losing surface water to the ground water realm depends
upon the head in the aquifer relative to the elevation of the stream surface (See figure 4 in
previous section). In order for ground water to contribute to surface water, the head in the water
table must be higher than the elevation of the stream surface. For surface water to enter into
ground water, the converse must be true (Winter et al. 1998, Weston 1998). Therefore, at an
intermediate scale, water surface elevation is low during times of low flow, so ground water tends
to flow towards surface water, even if it must move slightly uphill. At a local scale, pressure that
builds in a deep pool behind a riffle will tend to push surface water into the interstitial space of the
gravel, thus causing the surface water to move into ground water (Weston 1998).
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ECOLOGICAL IMPORTANCE AND VALUE OF GROUND WATER/ SURFACE WATER CONNECTION
The surface water/ ground water ecotone forms a varied habitat that is important to both
aquatic and wildlife communities. Ecotone is a term used to describe the transition zone between
different habitat types (or "eco types"). Prior to the First International Workshop of Land/Water
Ecotone in 1988, ecotone was used as term to describe a terrestrial community transition, such as
shrubland to forest (Gibert et al. 1997). In the context of surface water and ground water
transition zone, the term "ecotone" encompasses water flow and living and non-living
components of surface water/ ground water interactions. In the ecotone concept, the Hyporheic
zone is contained within the land/ water ecotone and is comprised of upwelling and downwelling
ecotones. In literal terms, Hyporheic can be broken down into its Greek base words "Hypo" and
"rhe", which mean below and flow, respectively (Borror 1960). It is defined by the presence of
ground water that originated as surface water in the river. The hyporheic zone is functionally a
composite of both riverine and ground water ecosystems. (See figure 11).
FIGURE 11: THE HYPORHEIC ZONE
(from FISRWG 1998)
The Hyporheic zone provides a number of ecologically important services, including
thermal, temporal, and chemical buffering, "food service", habitat, flow augmentation, and
refugia.
PROVIDES BUFFERING CAPABILITIES (Chemical Temporal, and Thermal)
CHEMICAL BUFFERING/ CLEANING CAPACITY : When surface water recharges ground
water, there is opportunity for organic pollutants and detritus to become trapped in the
sediment. While trapped, sediment bacteria may catalyze reactions that could change the
chemicals into a less toxic form or could transform the chemicals into available nutrients.
In contaminated aquifers, many bacterial micro-organisms residing in ground water and
sediment interstices can carry out some degradation and denitrification (Gibert et al.1997).
• TEMPORAL (Reduce potential for flooding) The riverine aquifer that underlies the riparian
zone on a floodplain, commonly referred to as "Bank storage", provides a vital buffer
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during periods of rapidly rising water levels. Bank storage may be a slightly misleading
term in that one may think that water enters into the stream bank, is stored, and then the
same water is released at times of low flow. Water that enters into a bank or other
landform, such as a mid-channel gravel bar, will flow below ground and may reemerge at
a different location and time (Refer to the previous section "How Rivers Work"). With a
rapid rise of water level within the stream, water is forced to move from the stream into
the stream bank. (See Figure 12). These rises in water levels are usually caused by storm
precipitation, rapid snow melt, or release of water from an upstream reservoir. In a
situation where water rises above its bank, instead of simply running overland and
downstream, much of the precipitation may percolate into the ground to recharge aquifers.
These diversions allow the onslaught of water into streams to be delayed by days, weeks,
or months and thus mitigate the effects of flood peaks (Winter et al 1998, McMastin 1996,
Brunke & Gonser 1997).
Bank Storage
FIGURE 12: "BANK STORAGE", or recharge
of the riverine aquifer that underlies the
riparian floodplain.
(From Winter et al. 1998)
• THERMAL: Since ground water temperatures remain relatively constant, the water that
discharges into surface water tends to be cooler relative to surface water in the summer,
and warmer relative to surface water in the winter.
The influx of ground water into surface water plays an important role in the maintenance
of essential fish habitat for many salmonid species. (Winter et al. 1998, Williamson et al. 1998,
NMFS etal. 1998, Stanford 1994).
FORMATION AND MAINTENANCE OF HABITAT :
The Hyporheic zone creates valuable habitat for micro-organisms, macro-invertebrates,
fish, and wildlife. For example, surface water areas where ground water upwells is the common
denominator in spawning habitats chosen by adult chum salmon. They rely on the characteristics
of the Hyporheic zone, such as an adequate supply of dissolved oxygen and warmer winter
temperatures, to incubate their eggs (NMFS et al. 1998).
• Deposition of gravels within a floodplain form a hydraulically connected network of voids
as described in the section "How Ground Water Works". These structures within the
Hyporheic zone creates an environment which hosts rich and diversified faunal
populations (Brunke & Gonser 1997; Gibert et al. 1997; Stanford 1994).
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• The influx of surface water into the ground water environment often brings oxygen rich
water to a system that is often limited by oxygen availability. This allows a unique and
biologically diverse set of organisms to thrive in the surface water/ ground water
interaction zone (Naiman et al. 1995).
• These organisms range from microbiotic organisms to macro-invertebrate species. Some
surface dwelling organisms, such as specialized stoneflies and certain species offish,
spend at least a portion of their life cycle in the Hyporheic zone (Brunke & Gonser 1997,
Stanford 1994; Houghton 1993).
• Interstices between gravel coupled with relatively warmer ground water flowing into the
surface water system form ideal habitat conditions for fish to spawn in and for the eggs to
incubate in. The influx of relatively warmer ground water prevents smaller tributaries and
side sloughs from freezing during the winter (Brunke & Gonser 1997; Stanford 1994).
Relatively cooler ground water flowing into surface water during summer provides the
necessary lower temperatures required by some salmonid species for rearing and spawning
(Pacific Rivers Council 1995; Brunke & Gonser 1997).
• The combination of diverse organisms and warmer ground water flowing into surface
waters provide ideal winter feeding grounds for juvenile salmonid species (Pacific Rivers
Council 1995). The influx of warmer water prevents smaller tributaries and side sloughs
from freezing and the diversity of organisms in the Hyporheic zone provide adequate
nutrition. More detail into the food web relationships will be discussed in the "Food
Service" section.
Riparian habitat: Ground water's role in sustaining bank vegetation provides an indirect
effect on surface water quality by enhancing bank stability against erosive forces and
providing shade (Brunke & Gonser 1997; Williamson et al. 1998). Surface water/ ground
water interactions are also key components in wetland habitat formation (Gibert et al.
1994).
A number of wildlife species rely on habitats associated with surface water/ ground water
interactions due to the higher productivity of those areas. The influx of warm ground
water, relative to cold surface water, temperatures during the early spring provide an
important food source for wildlife. The influx of ground water prevents the freezing of
smaller tributaries, sloughs, and wetlands, thus allowing vegetation to green up earlier,
sustaining fish populations, and providing water fowl habitat (USFWS 1998).
"FOOD SERVICE"
The surface water/ ground water interaction zone plays a vital role in the aquatic food web.
(See figure 13). The macro invertebrates, termed benthic organisms, that reside in the Hyporheic
zone for all or a portion of their life cycle provide an important food source for fish species. The
fish may browse along the gravel interstices to access the macro invertebrates or they may wait for
the organisms to become dislodged from their interstitial habitat. The fish catch the macro
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organisms as they drift downstream (FISRWG 1998).
FIGURE 13: FOOD RELATIONSHIPS FOUND
IN STREAMS. Note the "Benthic
organisms", or those invertebrates that
live in the hyporheic zone for a portion of
or all of their life cycle. These organisms
provide an important food source for
many fish species. Thee fish may browse
directly on these species, or they may
catch them as they are dislodged and
move downstream (FISRWG 1998).
J
• Surface water flowing into the Hyporheic provides the basic food source (organic matter)
for microbial activity in ground water ecosystems. Sediment interstices act as an effective
retention site for particulate organic matter (Gibert et al 1994). Organic matter that has
been deposited onto the sediment layers is carried into the Hyporheic zone by surface
water that is moving into ground water (Stanford 1994). Decomposition of organic matter,
or detritus, by microbial organisms provides bioavailable nutrients for macro invertebrates
and other organisms who spend a portion of their life cycle in the Hyporheic zone, such as
specialized stone flies. Macro-invertebrates and stone flies emerging from the Hyporheic
zone will be an available food source for salmon and other aquatic organisms (Stanford &
Ward 1994, Gibert et al. 1997).
• Surface water moving into ground water is one of the ways in which microorganisms may
colonize ground water environments. Microorganisms migrate actively or passively via
percolation from surface, lateral migration from recharge areas, and introduction into
sediments during deposition (Gibert et al. 1997).
Aside from permanent and seasonal habitat, the surface water/ ground water interaction
zones provide temporary refugia.
REFUGIA:
With increased fragmentation and degradation of habitat, there are only a few healthy and
productive patches scattered throughout each stream system. Theses healthy spots are called
"Refugia", because they offer refuge for endangered aquatic species (Pacific Rivers Council
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1995). Hyporheic zones, areas of ground water input, and micro-habitats within flood plains, such
as pools, are locations associated with refugia (Berman 1998).
Interstitial area provides temporary shelter from high discharge areas, desiccation, and
extreme temperatures. They also provide protection from predation for immobile life
stages of insects and fish, such as pupae and eggs, respectively. (Brunke and Gonser
1997).
• Locations where cooler ground water flows into surface water streams offer localized areas
of cool temperatures for fish to take refuge from warmer surface water temperatures
(Pacific Rivers Council 1994).
AUGMENT FLOW :
• "Base flow" is ground water that discharges into surface water channels and maintains
flow through times of low flow (USEPA 1993; Stanford 1994). Stream flow, as well as
base flow varies with season, physiography and climate (Winter et al. 1998). According to
Paul Jehn, associate director of Idaho's Water Resources Institute, "nearly 100 percent of
the water (in the Snake River) comes from the ground... If you took ground water away,
there would be no river" (Houghton 1993).
PROBLEMS OF NOT ACKNOWLEDGING THE INTERCONNECTEDNESS OF GW AND SW
While streams and ground water are not always well connected, when they are, the
interaction zone may be extensive. Current research has suggested that surface water/ ground
water interactions may potentially occur up to 2 kilometers from the stream channel (Stanford
1994, Gibert et al 1997). Certain species spend all or a portion of their life cycle in the Hyporheic
zone. If a pollutant load becomes too high for ground water organisms to tolerate, the number,
diversity, and population viability may decline. Since ground water invertebrates and micro-
organisms are an important food source for fish, their decline could affect the viability of native
fish populations (Duncan & Fuentes, 1998). The presence or absence of certain ground water
species may indicate the location of surface water/ ground water interaction zones and a decline in
the diversity of ground water species may indicate a decline in water quality (Stanford et al. 1994,
Gibert etal. 1997).
The discovery that surface water and ground water has the potential to interact at such a
distance from the stream channel may present some potential problems with current regulations.
Taking this discovery into consideration, waste sites or lagoons located a mile away from the
stream channel have the potential to rapidly contaminate ground and river water. If the ground
water joins surface water bodies rapidly, as is common in hyporheic zones, there may be great
potential for water quality problems to develop (Duncan & Fuentes 1998). An example of this
was given by Paul Jehn: In an attempt to follow Best Management Practices (BMPs) to control
nonpoint source pollution, efforts intended to hold water in a lagoon and thus prevent nutrients
from running off into a stream actually allowed time for these nutrients to seep into the soil. The
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irony of this is that the same nutrients that infiltrated into the ground water would discharge into
the stream via a spring source a few miles away. This is an instance where regulations intended to
protect surface water quality could inadvertently affect ground water quality, and in turn affect
surface water quality (Houghton 1993).
Often when contaminated ground water discharges into surface water, the current is fast,
and the pollutants are rapidly washed down stream and diluted beyond traceable measures. If they
are volatile chemicals, they may vaporize into the atmosphere. However, if the pollutants have
heavier components, such as metals, the pollutant may accumulate in the sediments. Without
precise sediment bioassays performed at the exact discharge areas, these pollutants may not be
able to be measured. That is not to say that they wouldn't have any effect on incubating salmon
eggs or invertebrate ground water organisms (Duncan & Fuentes 1998). Surface water
contamination may also occur during periods of low flow, when polluted ground water provides
significant stream flow (USEPA 1993). Since ground water may not be protected to the extent
that surface water is, the regulations for surface water may fail at times of low flow as a result of
under-protected ground water (Gibert et al 1997, Squillace et al. 1993).
Another problem associated with sediment is in relation to agricultural practices. Large
alluvial gravel filled valleys are effective ground water transport and storage areas that frequently
interact with surface water (Brunke & Gonser 1997, Winter et al. 1998, Pacific Rivers Council
1995). They are often desirable areas for grazing and agriculture. While these areas have the
ability to introduce the cooling effects of ground water to a surface water system, they are also
easily degraded by mismanagement practices. Active bank erosion as result of excessive
trampling of stream banks by grazing ungulates may compact soil, which decreases interstitial
spaces between soil particles, thereby diminishing the amount of water stored or transported in the
stream bank. Compaction of soil particles could force ground water to prematurely rise to the
surface, which would allow solar radiation to warm the water, and would prevent it from passing
its cooling effects onto the stream (Gibert et al. 1994).
In association with the loss of bank structure and stability, ground water pathways become
impaired, and a disconnection between the ground water zone and the surface water zone results.
Since ground water has potential to add to the flow volume of the stream, flow may decrease.
With a decrease in flow, the amount of sediment load that the water body will be able to carry will
also decrease. This decreased carrying capacity coupled with an increase in sediment load has the
potential to widen the channel, clog the gravel interstices, and cause channelization, or the
formation of multiple channels where only one had existed previously. This widening and/ or
division of the waterbody will expose more surface area of the water to the warming radiation of
the sun. The increase in temperatures could be detrimental to the ability offish species to
maintain a sustainable population. The clogged interstial spaces between gravel in the Hyporheic
zone could suffocate salmonid spawning beds and hinder food availability.
The placement of wells could also be affected by surface water ground water interactions.
If a well is installed relatively close to a stream, and pumps at a moderate rate, some of the ground
water that usually discharges to the stream will be diverted up into the well. If the pumping rate is
increased, there is a possibility that the flow of ground water may be reversed and instead the well
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may pull up surface water, thus risking contaminating ground water and drinking water (Winter et
al. 1998). Aside from the risk of surface water contaminating ground water, surface water
volumes could be reduced. As described above, decreased flow volumes could
cause problems related to increased sediment deposition.
As exemplified in the above paragraphs, there are a number of potential problems that may
arise as result of interaction between surface water and ground water. An important concern in
the future will be to determine adequate policies to address these issues.
SW/ GW INTERACTIONS IN POLICY
There tends to be an "Out of sight, out of mind" view of ground water in regard to surface
water quality which is made evident in that there is no "Clean Ground water Act". While ground
water is protected in relation to drinking water, in areas where drinking water is not an issue,
ground water quality is not monitored. According to LaJuana Wilcher, former EPA Assistant
Administrator for Water, "the line between surface water protection and ground water protection
has been drawn in people's minds...in spite of the fact that up to 40 percent of U.S. surface water
comes from ground water" (Ground water Monitor May 5, 1992).
At the first Ground water Ecology Conference in 1992, Jack Stanford suggested that the
reauthorization of the Clean Water Act should redefine ground water as "a continuum of
hydrologic units that contain differing and dynamic food webs that in many, many ways determine
the quality of this country's surface water" (Ground Water Monitor May 5, 1992). From
providing an important food source to fish and aquatic life to playing a large role in metals
loading in sediment layers, the contribution of ground water has the potential to do great good or
to do great harm to surface water quality. There is a need to look at how these interaction will be
managed from a policy standpoint in the future.
While there have been a few court cases regarding surface water/ ground water
interactions, there is still a long road ahead of policy makers. The fact that ground water ecology
is such a young science and many of the theories relating to this subject are still being studied will
slow the implementation of policy. Nevertheless, the study of surface water/ ground water
interactions has potential to greatly improve the current methods of evaluating surface water
quality.
MEASURING INTERACTION
Ground water scientists have conducted and continue to conduct extensive research in the
development of technical tools to measure and predict the presence of surface water/ ground water
connections. While progress has been made, the methods of measurement are extremely complex,
require extensive technical knowledge, and are resource intensive. Having said that, I do not wish
to scare off interest in this subject area, but consider this a forewarning as to the complexities of
this issue. Currently, there is not one simple, agreed upon way to measure the connections, and
there are many theories as to the best methods out there. The scope of this paper is not at the
depth necessary to adequately describe the methods involved in performing the actual
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measurements. However, it can steer you in the right direction, by naming some current tools,
methods, and resources where more information may be found.
Tools for identification range from inexpensive to outrageously resource intensive. The
tools themselves may be moderate to highly complex to use. It is the associated method, location
of measurements, and quantity of data that will increase resource needs. First, a topographic map
& aerial photo, where braided channels, ancient stream channels, and dense vegetation may
indicate a surface water/ ground water interaction zone. Next, vegetation type, where species such
as cottonwood and the presence of algae along shallow edges of waterways, may point to a surface
water/ ground water ecotone (Williamson et al 1998; Keeland et al.1997).
Various probes may be used to measure changes within the channel, which may indicate
the points of surface water/ ground water interaction. Temperature probes may be used to
determine change in temperature, which indicate influence of ground water on surface water.
Hyporheic probes may be used to measure interstitial flow rates and change in gradient, and a
piezometer may be used to measure change in hydraulic head, which indicate the potential for
ground and surface water to interact (Lee & Cherry 1978; USEPA 1993; Brunke & Gonser 1997;
Duncan & Fuentes 1998; Weston 1998).
Research is currently being conducted to test the feasibility of using specific micro- and
macro-organisms to indicate the distinction between the surface water/ ground water and true
ground water (Stanford et al. 1994).
RESEARCH NEEDS
By better understanding and accounting for surface water/ ground water interactions and
the ecological function that those interactions serve in the watershed, we may enhance the
effectiveness of our programs to protect and restore water quality.
The importance of conserving, restoring, or protecting the Hyporheic zone is driven by the
potential destruction of important biological communities and ecological processes due to
pollution and by human demand for potable water sources (Naiman et al. 1995). The steps
necessary to turn around the downward trend of water quality will require more than just focusing
on improving the polluted reach. Instead, focus should first be on understanding how specific
freshwater ecosystems work.
The research area of primary priority should be devoted to providing sound scientific
information with emphasis on ecological restoration and rehabilitation. In order to successfully
restore a stream, an understanding of how freshwater ecosystems function, what the causes of
disturbance are, and how specific freshwater respond to and recover from disturbances will be
needed (Naiman et al. 1995). At this time, there are a number of ongoing research projects and a
few published papers that cover restoration of a natural flow regime as a means of restoring
Hyporheic flow. One method currently being researched, the Hydrogeomorphic (HGM) Wetland
Classification System, advocates identifying wetlands based on their position in the landscape
(Hashisaki 1996). By determining site specific ranges of values for individual functional
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indicators, land managers and policy makers may eventually be able to answer questions such as
"Is the system working? Is it clean enough? Is it sustainable?" (Naiman et al. 1995).
Further research in the development of a range of scientifically based tools and techniques
will help to increase success of restoration efforts (Stanford et al. 1996). Research will also help
identify bioindicators of water quality, such as stoneflies, and other ecological indicators which
may be used to delineate ecotone boundaries, to predict the base flow quality of the stream, and to
monitor the progress of restoration (Job & Simons 1994; Naiman 1992). Supporting further
research in this area may lead to a more feasible way to quantify the effects of base flow on stream
temperatures, and thus better methods to monitor restoration efforts.
These research ideas presented are not brand new concepts, but build on existing concepts.
The research should complement existing programs and encourage a broader ecological
perspective on water quality issues.
TOTAL MAXIMUM DAILY LOAD (TMDL) PROGRAM AND SW/ GW INTERACTIONS
The investment made to support the research needs highlighted above will promote a
greater understanding of surface water/ ground water interactions in an ecological context and will
result in more effective scientific tools and methods. These tools and increased knowledge will
play a large role in the development of long-term comprehensive water quality management plans
(or Total Maximum Daily Loads- TMDLs).
Taking surface water/ ground water interactions into account can greatly help in the
development of sustainable restoration plans for waters that are not currently meeting Water
Quality Standards. With a broader ecological perspective, the focus will be on restoring the
system to properly functioning conditions, which will save money in the long run, since there will
not be the high maintenance costs associated with temporary solutions.
There are current research programs that show that the addition of the ground water
component has the potential to cool the stream or provide refugia for fish, and thus mitigate the
effects of increasing temperatures. A study evaluated subsurface flow patterns adjacent to a
beaver dam and pond. That study found that water would enter the subsurface flow pathways at a
relatively high temperature, and after flowing slowly through the subsurface, it emerged on the
other side of the dam at considerably lower temperatures. The water was tracked by it's thermal
pattern through wells, and it was determined that it took 2-3 months for the water to move from
the pool, through the subsurface, and back into the surface water on the other side of the dam.
There was a marked temperature decrease from this single subsurface interaction. It may be
possible that the cooling effects could be cumulative, and thus provide an even greater decline in
temperature (Lowry & Bescheta 1994).
The ability to quantify ground water temperature impacts may provide incentive for
stakeholders to implement protective measures around healthy riparian corridors and restore
impaired riparian corridors. Restoration efforts to return streams and river systems to Properly
Functioning Conditions (PFC), where channels are reconnected to the floodplain and allowed to
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reach a post-disturbance equilibrium, could benefit from the ability to quantify the interaction of
ground water with surface water (FISTWG 1998).
With a better understanding of surface water/ ground water interactions, reasoning behind
management actions may be presented in a more clear manner to stakeholders and with better
tools, progress could be tracked. With the ability to efficiently quantify the effect of surface water/
ground water interaction, recommendations for stream restoration made in the development of
Total Maximum Daily Loads(TMDLs) may be monitored for effectiveness. Eventually, research
of this concept could lead to the ability to predict the effectiveness of regulation criteria (Naiman
etal. 1995).
CONCLUSION
This paper draws from a number of previously published sources to illustrate how ground
water flows, how ground water works within a riverine system, and what the ecological
importance of the surface water/ ground water interaction is within the environment. It also
discusses the problems that arise when we fail to acknowledge the interconnectedness of surface
and ground water. While the subject of surface water/ ground water interaction from an
ecological viewpoint is a relatively young science, one can see that it has great implications within
our ecological and programmatic systems. There is much to be learned still, as the measurement
and interpretation of interaction areas are complex. However, with attention and perseverance,
developing our understanding of surface water/ ground water interactions has the ability to
enhance our efforts to protect watersheds and improve conditions for the lives dependant upon
healthy watersheds.
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