RIVER RESEARCH AND APPLICATIONS
River Res. Applic. 22: 937–946 (2006)
Published online 31 August 2006 in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/rra.947
EFFECTS OF FLUCTUATING RIVER FLOW ON GROUNDWATER/SURFACE
WATER MIXING IN THE HYPORHEIC ZONE OF A REGULATED, LARGE
COBBLE BED RIVER
EVAN V. ARNTZEN,* DAVID R. GEIST and P. EVAN DRESEL
Pacific Northwest National Laboratory, P.O. Box 999, MS K6-85, Richland, WA 99354 USA
ABSTRACT
Physicochemical relationships in the boundary zone between groundwater and surface water (i.e. the hyporheic zone) are
controlled by surface water hydrology and the hydrogeologic properties of the riverbed. We studied how sediment
permeability and river discharge altered the vertical hydraulic gradient (VHG) and water quality of the hyporheic zone
within the Hanford Reach of the Columbia River. The Columbia River at Hanford is a large, cobble-bed river where water
level fluctuates up to 2 m daily because of hydropower generation. Concomitant with river stage recordings, continuous
readings were made of water temperature, specific conductance, dissolved oxygen and water level of the hyporheic zone.
The water level data were used to calculate VHG between the river and hyporheic zone. Sediment permeability was
estimated using slug tests conducted in piezometers installed into the river bed. The response of water quality measurements
and VHG to surface water fluctuations varied widely among study sites, ranging from no apparent response to covariance
with river discharge. At some sites, a hysteretic relationship between river discharge and VHG was indicated by a time lag in
the response of VHG to changes in river stage. The magnitude, rate of change and hysteresis of the VHG response varied the
most at the least permeable location (hydraulic conductivity (K) ¼ 2.9 104 cms1) and the least at the most permeable
location (K ¼ 8.8 103 cms1). Our study provides empirical evidence that sediment properties and river discharge both
control the water quality of the hyporheic zone. Regulated rivers, like the Columbia River at Hanford, that undergo large,
frequent discharge fluctuations represent an ideal environment in which to study hydrogeologic processes over relatively
short time periods (i.e. days to weeks) that would require much longer periods (i.e. months to years) to evaluate in
unregulated systems. Copyright # 2006 John Wiley & Sons Ltd.
key words: hysteresis; hyporheic; groundwater/surface water interaction; hydraulic gradient; specific discharge; spawning gravels;
permeability; hydraulic conductivity
INTRODUCTION
The hyporheic zone is the subsurface region of streams and rivers where groundwater and surface water interact
(Valett et al., 1993). Water flow into and out of the hyporheic zone is largely influenced by advective exchange with
the river, a process generally controlled by channel morphology, pressure head of overlying surface water and the
permeability of riverbed sediments (Landon et al., 2001; Worman et al., 2002; Cardenas and Zlotnik, 2003; Rose,
2003). Consequently, the magnitude of river flow and the underlying geology can significantly affect chemical,
physical and biological gradients within the hyporheic zone, which ultimately will affect the structure and function
of aquatic ecosystems (Stanford and Ward, 1993; Curry et al., 1994; Wroblicky et al., 1998; Soulsby et al., 2001;
Alexander and Caissie, 2002). The need for additional information describing the role of river stage and riverbed
permeability in controlling flow into and out of the hyporheic zone has been well recognized (Vervier et al., 1992;
Curry et al., 1994; National Research Council, 2003). However, most of the previous work has been conducted in
systems where river levels change slowly.
*Correspondence to: E. V. Arntzen, Pacific Northwest National Laboratory, P. O. Box 999, MS K6-85 Richland, WA 99354, USA.
E-mail: [email protected]
Contract/grant sponsor: The Bonneville Power Administration; contract/grant number: 1994-06900.
Copyright # 2006 John Wiley & Sons Ltd.
Received 15 June 2005
Revised 14 February 2006
Accepted 5 March 2006
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E. V. ARNTZEN, D. R. GEIST AND P. E. DRESEL
In watersheds that are not influenced by hydroelectric development, fluctuations in river discharge and the
subsequent hyporheic response to these fluctuations often occur seasonally (Soulsby et al., 2001; Alexander and
Caissie, 2002). For example, watersheds in north temperate climates receive their precipitation primarily as snow
during the winter. Peak river flow occurs each spring in the form of snowmelt, resulting in higher river flows that
cause hydraulic gradient changes during which the direction of water flow in the hyporheic zone is altered (Fraser
et al., 1996; Wroblicky et al., 1998). Physicochemical changes during gradient shifts are often different when
stream discharge is rising from when it is declining, and the data can exhibit a noticeable hysteresis (Soulsby et al.,
2001; Alexander and Caissie, 2002). However, understanding the physicochemical response of the hyporheic zone
to changes in river discharge requires long-term studies.
In rivers regulated by dams, there is an opportunity to observe the hyporheic response to changing river discharge
over much shorter time periods (Curry et al., 1994; Peterson and Connelly, 2001). For example, in river systems
influenced by hydroelectric dams operated to produce electric power in response to short-term electrical demand
(termed load-following), the change in river flow can occur over time periods measured in hours. The short time
scale and large magnitude of river discharge fluctuations associated with load-following allows for the rapid
collection of physicochemical data during gradient changes. This information can be used to address fundamental
gaps in our knowledge of hyporheic zone structure and function (Williams, 1993; Curry et al., 1994) and provide
data that could be used to help verify and improve advective exchange models (Wörman et al., 2002).
We conducted this study in the Hanford Reach of the Columbia River, an 80-km stretch of a large river that often
experiences rapid discharge fluctuations in response to load-following power production operations at Priest Rapids
Dam (situated at the upstream terminus of the Reach). Discharge from Priest Rapids Dam varies hundreds of cubic
meters per second (m3s1) both seasonally and hourly as water management is maximized for power production.
Discharge fluctuations cause the river stage to change up to 2 m per day, which alters the vertical hydraulic gradient
(VHG) within the hyporheic zone and affects the magnitude and direction of hyporheic water flow (Figure 1). The
objective of our study was to determine the time scale at which river discharge fluctuations changed water quality
and the VHG within the hyporheic zone of the Hanford Reach. Included in our evaluation was an assessment of how
variation in riverbed permeability affected the VHG at different study locations in the Reach.
STUDY SITE
The Columbia River flows through the Pasco Basin of the Columbia Plateau in southeastern Washington State.
Bordered by Miocene basalt ridges to the north and south, the Hanford Reach of the Columbia River is a gaining
stream that flows through an unconfined aquifer consisting of Miocene to Pliocene fluvial deposits of the Ringold
Formation, and Pleistocene flood gravels of the Hanford Formation (Hartman and Dresel, 1998). During most of the
year, there are ubiquitous differences between the dissolved oxygen, specific conductance and temperature of the
river water and the groundwater (Dirkes et al., 1999). Riverbed permeability varies widely in the Hanford Reach,
with hydraulic conductivity values ranging from 2.8 105 cms1 to 4.3 102 cms1 (Arntzen, 2002).
METHODS
Water quality data were obtained from one piezometer near rkm 604. This site was adjacent to a groundwater plume
that generally flowed toward the river, especially at low river stage (Hartman et al., 2001). Previous groundwater
modelling efforts in the unconfined aquifer adjacent to this location used relatively high permeability estimates
(hydraulic conductivity ¼ 9.3 103 cms1) to estimate groundwater flow rates (Hartman and Dresel, 1998). Rkm
604 was selected because it was close to groundwater which had different physicochemical properties than the
adjacent river water. The differences increased the likelihood that hyporheic water chemistry would change
measurably in response to river stage fluctuations, especially during periods of relatively extreme fluctuations in
river discharge. The location was sampled 4–6 February 2000 during a period when the hourly river discharge
ranged from 2179 m3s1 to 5886 m3s1.
The piezometer (5 cm inside diameter [i.d.]) was constructed of a galvanized shaft and a ductile iron drive point
threaded onto a 30.5 cm-long stainless steel screen and installed such that the top of the screen was 1.2 m below the
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
GROUNDWATER/SURFACE WATER MIXING IN THE HYPORHEIC ZONE
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Figure 1. A conceptual model represents how groundwater/surface water interaction might differ in zones of relatively high riverbed
permeability compared to those with relatively low permeability
riverbed. The top of the piezometer was threaded so that galvanized extensions could be attached and the
piezometer could be purged. During installation, a 2.54-cm-diameter steel drive rod was inserted into the
piezometer and a pneumatic hammer was used to pound the rod and piezometer into the sediment (Geist et al.,
1998). Following the installation, the drive rod was removed and the piezometer was developed by pouring and
pumping approximately 2.5 L of river water through the piezometer to ensure that the screen was not plugged.
During 4–6 February 2000, a Hydrolab Minisonde data logger, capable of recording specific conductance,
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
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E. V. ARNTZEN, D. R. GEIST AND P. E. DRESEL
temperature, pressure head and dissolved oxygen, was deployed with its sensors positioned at the top of the
screened interval. The Minisonde was programmed to record the data every 30 min. We compared Minisonde data
to representative surface water chemistry results obtained from the U.S. Geological Survey National Stream
Quality Accounting Network (2005) and to representative groundwater chemistry data obtained from wells
screened to the unconfined aquifer as part of the U.S. Department of Energy Hanford Site groundwater monitoring
program (HEIS, 2000). VHG was determined as the ratio of the difference in head pressure between river and
hyporheic zone to the distance from the riverbed to the top of the piezometer screen:
VHG ¼
dh
½head inside piezometer river head
¼
dl distance from riverbed to top of piezometer screen
(1)
Regression analyses were performed to compare water quality data (specific conductance, temperature and
dissolved oxygen) to the river stage at rkm 604 during 4–6 February 2000. A least squares linear regression was
performed between each variable and river stage. Analysis of variance was used to determine whether each
regression was significant (a ¼ 0.05).
To determine how changes in riverbed permeability affected hyporheic water flow during discharge fluctuations,
we measured the hydraulic conductivity and continuously monitored the VHG at three Hanford Reach locations
(rkm 602, 582 and 577) where spawning habitat studies for fall Chinook salmon (Oncorhynchus tshawytscha) were
being conducted. The composition of the riverbed varied between the locations. At rkm 602, the dominant substrate
(by weight) was cobble (>64 to 128 mm) in a matrix consisting mostly of fine sand (>0.062 to 0.5 mm). At rkm
602, the median grain size—D50, the size for which 50% of the sample (by weight) is finer or coarser—was
57.7 mm. At rkm 582, the dominant substrate was coarse gravel (>16 to 64 mm) in a matrix consisting mostly of
fine sand (>0.062 to 0.5 mm). The D50 at rkm 582 was 35.5 mm. At rkm 577, the dominant substrate was coarse
gravel (>16 to 64 mm) in a matrix consisting mostly of fine sand. However, there was a silt component at rkm 577
much larger than at the other two locations. The D50 at rkm 577 was 22.3 mm.
The hydraulic conductivity of the riverbed at each site was determined by conducting slug tests in nine
galvanized piezometers at each of the three locations (Butler, 1998). In October 2001, 54 slug tests were conducted
at the three sites. The piezometers had an inside diameter of 3.2 cm and were installed such that the tops of the
screens were 30 cm below the riverbed. Piezometers were installed and developed using the same technique as for
the initial water quality investigation. To perform the test, an airtight pressure-regulating wellhead assembly was
threaded to the top of each piezometer. The assembly consisted of a 5.0-cm ball valve coupled to a 20.0-cm section
of schedule-40 PVC pipe containing a small valve stem for pressurizing. A pressure transducer (Instrumentation
NW Model 9800) (Instrumentation NW is located in Kirkland, WA, USA) was lowered into the piezometer to
measure changes in hydraulic head during the test. A modified rubber stopper was used to seal the transducer cable
entry into the well assembly. The system was pressurized with a portable battery-powered air compressor (Black
and Decker VersaPak cordless inflator), causing the water level in the piezometer to depress downward. The change
in water level was measured by the transducer and recorded by an electronic data logger (Campbell CR10X). When
the water level in the well was sufficiently depressed, the air compressor was shut off and the ball valve
simultaneously opened, marking the beginning of the slug test. When the pressure was released, the data logger
recorded the pressure response (rising water level) with respect to time. Based on preliminary results and on past
research in similar sedimentary environments, intragravel flow was assumed to be laminar (Reynolds numbers less
than unity; Vaux, 1968). All the slug tests were overdamped and were therefore analysed using the Bouwer and Rice
method (Bouwer and Rice, 1976; Bouwer, 1989; Butler, 1998; Weight & Wittman, 1999). Hydraulic conductivity
was determined via the equation
rc2 lnðRe =RÞ l
H0
K¼
ln
(2)
2Le
t
Ht
where K is hydraulic conductivity (cms1), rc is the radius of the well casing (cm), Re/R is the dimensionless ratio of
radius of gravel envelope to distance away from the well over which the average value of K is being measured
(obtained as outlined in Fetter, 1994), Le is length of the screen or open section of the well (cm), H0 is the drawdown
at time t ¼ 0, Ht is the drawdown at time t ¼ t and t ¼ time from H0. Slug tests were summarized using the geometric
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
GROUNDWATER/SURFACE WATER MIXING IN THE HYPORHEIC ZONE
941
mean hydraulic conductivity (Butler, 1998). Standard error in hydraulic conductivity measurements was computed
as the sum of the standard error associated with linear regressions using the Bouwer and Rice method and the
standard deviation of replicate samples.
VHG was monitored continuously at the three locations 16–19 November 2001 during a period when the hourly
river discharge ranged from 1075 m3s1 to 4839 m3s1. At each of the three sites, one levelogger (Solinst, Inc.,
Model 3001 LT) (Solinst Canada Ltd. is located in Georgetown, Ontario, Canada) was deployed in a piezometer
that was screened to the hyporheic zone and monitored total pressure (cm H2O). In addition, a second levelogger
was deployed in a piezometer screened to the overlying river column that measured total pressure (cm H2O). The
hyporheic loggers were positioned with their sensors at the tops of the piezometer screens, which were installed to a
depth of 30 cm below the riverbed. At each site, both data loggers were programmed to record data in 15-min
intervals. The Solinst loggers recorded total pressure differences with a precision of 0.7 cm H2O pressure. The
atmospheric pressure adjacent to the river was logged at 15-min intervals and subtracted from the total pressure
readings of each logger so that changes in pressure reflected only changes due to river stage.
Figure 2. Relationship between river stage (closed circles) and the temperature (8C), specific conductance (% GW assuming pure GW specific
conductance ¼ 658 mScm1 and pure river water specific conductance ¼ 142 mScm1) and dissolved oxygen (mgL1) of hyporheic zone
(DL ¼ 1.2 m) collected every 30 min during 4–6 February 2000 at rkm 604 near the 100 D Area
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
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E. V. ARNTZEN, D. R. GEIST AND P. E. DRESEL
RESULTS
During January–March 2000, Columbia River water in the Hanford Reach near rkm 626 ranged from 4.5 to 4.78C,
142–150 mScm1 specific conductance and 13.2–13.8 mgL1 DO (U.S. Geological Survey, 2005). During the same
time period, groundwater samples from several wells screened to the unconfined aquifer adjacent to the study site at
rkm 604 ranged from 14.3 to 21.38C, 182–1526 mScm1 specific conductance and 0.1–2.4 mgL1 DO (HEIS,
2000). Data collected from the hyporheic zone at rkm 604 during 4–6 February 2000 indicated the water there was a
mixture of groundwater and river water. Hyporheic temperature ranged from 6.4 to 8.18C, specific conductance
114–458 mScm1 and dissolved oxygen 4.3–9.4 mgL1. Hyporheic temperature, specific conductance and
dissolved oxygen co-varied with river stage and responded relatively quickly to stage changes (Figure 2). Scatter
plots of dissolved oxygen, temperature and electrical conductivity logged by the Minisonde placed in the hyporheic
zone showed a linear trend with river stage (r values ranged from 0.5 to 0.9, p for all of the relationships was
significant [p 0.05]). There was also a hysteresis between hyporheic zone temperature and river stage (Figure 3).
At rkm 582, the VHG between the hyporheic zone and river remained fairly constant over a range of river stages
(Figure 4A). The VHG at rkm 602 and 577, however, varied depending on whether the river stage was relatively
high or relatively low (Figure 4B,C). During low stage at these two sites, the VHG was positive, indicating the
potential for upward flow from the hyporheic zone into the river. During high stage, the VHG was negative,
suggesting the potential for downward flow from the river into the bed. Variance of the VHG appeared to be a
function of the rate and magnitude of river stage fluctuations. The VHG showed the greatest variation at the least
permeable location (rkm 577) and showed the least variation at the most permeable location (rkm 582; Figure 4).
DISCUSSION
Although all the sites experienced similar fluctuations in river stage over short (hourly) time periods, the empirical
VHG data varied among sites (Figure 4). The variance in VHG appeared to be related to the hydraulic conductivity at
each location. It is reasonable to assume differences in hydraulic conductivity could cause the observed variability,
Figure 3. Arrows show the direction of passing time, helping to indicate that hyporheic temperature decreased during river stage increases and
increased during stage decreases. Circles show data collected every 30 min. The temperature response of the hyporheic zone was different when
the stage was rising than when it was falling, suggesting a hysteretic relationship
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Figure 4. The relationship between VHG and river stage for rkm 582, 602 and 577 varied during 16–19 November 2001. The nature of the
response appeared to be related to the hydraulic conductivity of the riverbed, which was highest at rkm 582 (8.8 103 cms1), and lowest at rkm
577 (2.9 104 cms1)
given that highly permeable riverbed sediments allow surface water to penetrate the hyporheic zone more freely than
less permeable sediments, producing greater changes in VHG where the permeability is lower (Vaux, 1968; White,
1993; Wroblicky et al., 1998). The expected change in VHG can be numerically simulated using Darcy’s equation to
model VHG as a function of hydraulic conductivity (K), assuming that hydraulic properties are isotropic at the
locations studied and that hyporheic flow satisfied all requirements of the Darcy equation:
Q
dh
(3)
q ¼ ¼ K
A
dl
where q is specific discharge, Q is volumetric discharge, K is hydraulic conductivity, dh/dl is the gradient of
hydraulic head (VHG) and A is cross-sectional area. Differences in VHG responses between high and low
permeability locations can be explained by rearranging Darcy’s equation to solve for VHG:
VHG ¼ q
K
(4)
Equation (4) shows that the VHG varies inversely with the hydraulic conductivity. To demonstrate how the
magnitude of the VHG would be expected to increase in zones of lower permeability, we simulated specific
discharge and used hydraulic conductivity values from the empirical measurements at rkm 582, 602 and 577 in
Equation (4) to determine the effects on VHG (Figure 5). Specific discharge was assumed to be a direct function of
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
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E. V. ARNTZEN, D. R. GEIST AND P. E. DRESEL
Figure 5. VHG was simulated using Darcy’s equation with varying hydraulic conductivity while keeping the relationship between specific
discharge and river stage the same at all three sites. Hydraulic conductivity values determined from the empirical data were used for the
simulations (A, B and C)
diurnal, sinusoidal river stage fluctuations, with a range similar to past studies in the Hanford Reach (Geist, 2000).
The same relationship between specific discharge and river stage was used to simulate all three locations. To best
approximate the empirical data using our modelled data, it was necessary to account for the hysteresis present in the
empirical data between river stage and VHG (especially at rkm 577 where K was lowest). We postulate that the
hysteresis was caused by a time lag between change in river stage and the pressure response of the hyporheic zone
and that the duration of the lag was a function of the magnitude of the river stage change and the hydraulic
conductivity of the hyporheic zone. This relationship has implications for the use of VHG as a measure of habitat
quality. For example, if the VHG were being used as an indicator of habitat quality in a location with relatively low
permeability, VHG results could vary widely depending on whether the river discharge was increasing or
decreasing. We approximated the hysteresis in the model by incorporating an iteratively determined 1.5 hr time lag
between VHG and river stage. In general, the predicted VHG response and the measured VHG response varied
similarly as a function of hydraulic conductivity, in terms of both VHG magnitude and the hysteresis between VHG
and river stage. This was especially true at rkm 577, where hydraulic conductivity was low and there was a
relatively large hysteresis, and at rkm 582, where hydraulic conductivity was high and there was very little
hysteresis. At rkm 602, there was a greater lag in the empirical VHG response than predicted by the model, possibly
indicating that hydraulic conductivity estimates there are less accurate than at rkm 577 and rkm 582. The results
suggest that the relationship between heterogeneity of riverbed sediments and the flux of hyporheic water can be
monitored over time and predicted, allowing for improved hyporheic models in hydro-systems with large, frequent
fluctuations in river discharge.
Although the general relationship between permeability and the movement of surface water into the riverbed is
well established, surface water movement is also complicated by additional factors, including channel morphology,
depth of alluvium, location of groundwater discharge zones and the magnitude and duration of surface water head
changes (Vaux, 1962; Stanford and Ward, 1993; Wondzell and Swanson, 1996; Packman and Bencala, 2000;
Alexander and Caissie, 2002). These additional factors complicate investigations into the nature of groundwater/
surface water interaction, especially when sampling must occur across large spatial scales. Our study sites were
Copyright # 2006 John Wiley & Sons Ltd.
River Res. Applic. 22: 937–946 (2006)
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distributed within a 25-km reach of the Columbia River, and water depths precluded us from working near the
thalweg of the channel. Although we selected locations with similar channel morphology, water depths, channel
velocities and surface water fluctuations, little is known about the depth of alluvium and precise locations of zones
of groundwater discharge into the river within most of our study site (Lee et al., 1997; Peterson et al., 1998; Geist,
2000). Future efforts would benefit from minimizing the uncertainty caused by the effects of scale. Variables such
as hydraulic conductivity and VHG would be better tested within a microcosm of the large-scale study site, that is at
the channel bedform scale (pool-riffle sequence), or in areas where groundwater discharge is known to be present or
absent at the expected flow regime, or compared where hydraulic properties of the sediments were expected to
differ by several orders of magnitude (e.g. fine silt vs. coarse sand).
Past research in the hyporheic zone of smaller, unregulated streams has shown that the VHG in the hyporheic
zone can reverse itself with changes in seasonal stream discharge, changing the direction of water flow and altering
physical properties and solute concentrations of the hyporheic zone (Williams, 1993; Soulsby et al., 2001;
Alexander and Caissie, 2002). However, the long time scales over which such observations are made prohibit rapid
collection of data describing physicochemical characteristics of the hyporheic zone during gradient reversals.
Several seasons of data collection would be required to describe mixing scenarios within a suite of hydraulic
conditions. One benefit of studying the physicochemical characteristics of the hyporheic zone of a large, regulated
river is that daily river stage fluctuations of greater than 2 m are common (Curry et al., 1994; Geist, 2000; Peterson
and Connelly, 2001; Arntzen, 2002). River stage changes of such a great magnitude and short duration allow
hyporheic zone properties to be studied on a scale of days rather than months, providing for the relatively rapid
construction of large, fine-scale data sets that can be used to model the interaction. Additional studies will help
bridge the gap between bench-scale models and hyporheic processes in large rivers and allow models to incorporate
the non-linear physicochemical hyporheic response to changes in river discharge.
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