Evaluating the reliability of the
stream tracer approach to
characterize stream-subsurface
water exchange
Harvey et al., 1996
Two Approaches to Characterize
Hyporheic Exchange
• Hydrometric approach
– Sub-reach-scale measurement of hydraulic heads
and hydraulic conductivity to compute streambed
fluxes
• Stream tracer approach
– Reach-scale modeling of in-stream solute tracer
injections to determine characteristic length and
time scales of exchange with storage zones
Steps Taken in Study
1. Conduct stream tracer experiment during both low and
high base flow and simultaneously measure hydraulic
gradients and tracer movement in the subsurface.
2. Use the transient storage model and statistical methods to
identify storage characteristics.
3. Compare detailed subsurface measurements with model
storage characteristics.
4. Assess the reliability of the stream tracer approach to
characterize hyporheic exchange under variable flow
conditions.
St. Kevin Gulch, Colorado
• Third order channel: smaller, headwater, upper
reaches of watershed (Strahler, 1952)
• Average slope: 0.07 (varies from 1% to >20%)
• Stream flow: sustained by inflow of
groundwater from permanently saturated areas
of the lower hill slope.
• Channel sediment: well-sorted sand and gravel,
distributed in patches that range from fine sand
to coarse sand to gravel.
• Alluvial sediment: poorly sorted fine and
coarse sand, gravel and cobbles ~2 m thick and
extends ~5 m on either side of stream.
Acid burn. AMD seeps from St.
Kevin Gulch near Leadville,
Colorado, an area mined for gold,
silver, lead, and zinc. Image
credit: Carol Russell/EPA
Methods
• 60 wells, piezometers, staff
gauges emplaced along a 36-m
study reach.
• LiCl tracer injected into stream at
a steady rate for four days.
– low base flow- 10 L/s (August
1990)
– high base flow-120 L/s (June
1991)
• During tracer injections stream
water samples were collected at
both endpoints and from a
subset of wells along the reach.
• Hydraulic heads were measured
in all wells and at all staff gauges
during the injections.
Subsurface Measurements of Hyporheic Exchange
• Using Water Balance Approach 12 m sub-reach
– Well-sorted gravel bar deposits adjacent to channel (6 wells)
– Poorly sorted alluvium at sides and beneath stream (8 wells)
• Reach-averaged streambed flux measured by closely spaced
hydraulic head measurements and divided into:
– Stream-hyporheic exchange- flow paths are short, concentric-shaped,
and enter and return from the subsurface within the stream reach.
– Stream-groundwater exchange - flow paths much longer and leave or
enter study reach once.
• % stream water at wells was calculated using standard mixing
models.
– Measuring the distribution of tracers in the stream and subsurface.
• Travel time for stream water to reach wells was determined by
observing the arrival of the chloride tracer.
Stream Tracer Experimentation and Modeling
• 1D model where storage of solute is simulated as a mass transfer between
the channel and a decoupled storage reservoirs in which mixing is
complete and instantaneous (Figure B)
• Stream Tracer Model Equation:
–
–
–
–
–
–
–
t and x = time and direction along the stream
C, Cs, CL = concentrations in the stream, storage zones, & groundwater
Q = in-stream volumetric flow rate
qL = groundwater in flow
D = longitudinal dispersion coefficient in the stream
A and As = stream and storage zone cross-sectional areas
α = stream water exchange rate with storage zones
Stream Tracer Experimentation and Modeling
• Stream flow discharge (Q)- determined by the dilution gaging
method.
• Groundwater inflow (qL)- estimated as the difference in stream flow
at endpoints divided by the reach length.
• Other parameters (A, D, α, and As) were determined by inverse
methods using the nonlinear, least squares regression approach.
• Simulations were fit to measured data in order to select the best-fit
values for the parameters.
Stream Tracer Experimentation and Modeling
During the four day injections of cl
tracer, concentrations were
measured at upstream (1329 m)
and downstream (1382 m)
endpoints.
After cutoff of the tracer,
concentrations:
• initially decreased rapidly in the
stream
• followed by a longer period
where tracer concentration
remained elevated above
background concentration levels
Assumptions Underlying the Use of the Stream Tracer Approach to
Simulate Hyporheic Exchange
The downstream change in stream flow in a channel without
tributaries that is closely connected with shallow ground water is
Q = stream flow discharge
X = downstream direction
qinL & qoutL = reach-averaged groundwater flux into and out of stream
qins &qouts = reach-averaged fluxes of stream water out or into hyporheic flow paths
Assumptions Underlying the Use of the Stream Tracer Approach to
Simulate Hyporheic Exchange
• Groundwater flow paths are determined by “Seepage runs”
estimate the net groundwater flux across the streambed (qinL – q
out ), and is determined by the difference in streamflow at
L
upstream and downstream reach ends.
• Hyporheic exchange fluxes are estimated by a series of equations
and substitutions that include the previous set of equations.
Primary assumptions associated with stream tracer approach
• Mass Transfer Process- Solute holding times in the hyporheic zone
are assumed to be distributed exponentially (the bulk response of
all hyporheic flow paths can be modeled as a simple first-order
mass transfer between channel and well mixed reservoir).
• Exchange parameters uniquely characterize hyporheic exchange
rather than mixing between the central channel and surface water
storage zones (bottom pools, recirculating eddies).
– Surface water storage processes are assumed to be accounted for by
the longitudinal dispersion coefficient.
Primary assumptions associated with stream tracer approach
• Required channel length must be met for proper mixing to occur in
the stream and allow the surface water storage to be properly
accounted for by the longitudinal dispersion coefficient.
• Lengths have been established and require:
–
–
–
–
Transverse dispersion coefficient
Channel depth
Shear velocity
Proportionality coefficient
Subsurface Results
• Hyporheic exchange 40 - 80% of total streambed water flux
• Hyporheic exchange greatest at low base flow
• Groundwater inflow to the stream increased at high base flow.
• 30% decrease in hyporheic exchange relative to low base flow.
• Decrease in hyporheic exchange attributed to higher inflow from
groundwater increasing the resistance of stream water to recharge
in the hyporheic zone.
Subsurface Results
Water table contours were used to map individual hyporheic flow paths that
ranged in length from cm to m in the 12-m sub-reach.
High base flow compared to Low
•
Length and distance of
penetration of individual
hyporheic flow paths reduced
•
Percent stream water
composition in hyporheic flow
paths less
•
Finding consistent with the slight
reduction of hyporheic exchange
at higher base flows determined
by water balance
Subsurface Results
Solute tracer arrival times indicated that timescales of hyporheic
exchanged ranged between minutes and tens of hours.
• Well-sorted gravel
sediment averaged 6
hour exchange times
• Poorly sorted alluvial
sediment averaged 84
hour exchange times
Stream Tracer Results
• Stream tracer modeling provided simulations that were a good
match to measured tracer concentrations in the stream.
• Best fit model results were similar to measurements for both high
and low base flow.
• Parameter uncertainty estimates were mostly below 20%.
Comparison of Stream Tracer Modeling Parameters with Subsurface
Measurements
Low base flow
• Best fit storage zone cross section area twice as large as best fit
stream cross sectional area which is similar to hydrometric
exchange measurements.
• Best fit storage zone residence time of 6 hours at low base flow was
consistent with hydrometric exchange measurements of travel time
to reach gravel bars.
Comparison of Stream Tracer Modeling Parameters with Subsurface
Measurements
High base flow
• Stream tracer and hydrometric exchange approaches were not
consistent.
• Stream tracer approach indicated orders of magnitude decreases in
storage zone area, while measurements suggested only a modest
reduction in storage zone area.
• Stream tracer approach indicated orders of magnitude decrease in
storage zone residence time, measurements showed a 30%
reduction in hyporheic flux.
Sensitivity of the Stream Tracer Model to Subsurface Storage
Processes
• Sensitivity is the partial derivative of modeled stream tracer
concentration with respect to a change in the value of a parameter.
• Model less sensitive at high base flow
Comparison of Stream Tracer Modeling Parameters with Subsurface
Measurements
• No objective means to assess whether the stream tracer approach
could uniquely distinguish surface and subsurface water storage
processes, but calculations indicated that the equilibrium phase for
mixing was not met in the study reach, which means mixing in the
stream could not be accounted for by adjusting the longitudinal
dispersion coefficient and that some overlap of surface and
subsurface storage was expected in the tracer signals.
• The tracer experiment at high base flow was likely more sensitive to
surface water storage processes, because measured hyporheic flow
paths were affected little by the change.
Stream tracer approach could not detect all timescales of streamhyporheic exchange fluxes.
• Model exhibited maximum sensitivity to exchange processes that occurred
quickly following the cut-off of the tracer.
• Window of time .2-2 best fit residence times.
• Same window of time when normalized travel times are plotted against
distance from the stream and linear trends are placed.
• Model sensitivity did not account for the longer deeper alluvium
timescale.
Stream tracer method compared to “Hydrometric-Based” Simulation.
• Models were compared against an independent low flow dataset.
Summary
• Advantage of stream tracer approach to detect hyporheic
exchange is the simplicity and efficiency at large scales
compared to subsurface observations.
• Hyporheic exchange persisted through two seasons of low
and high base flow.
• Differences is magnitude of hyporheic exchange accounted
for by groundwater inflow opposition of localized recharge
of stream water to hyporheic flow.
• Stream tracer approach estimated exchange with greater
reliability at low base flow.
Summary
• Greater sensitivity at low base flow resulted from interaction with a
larger proportion of the tracer with flow paths and a higher plateau
concentration.
• At high base flow the stream tracer approach is likely more
sensitive to surface water storage processes than to hyporheic
exchange.
• Regardless of base flow, stream tracer approach only accurately
characterized the fastest exchange times (between streams and
gravel bars).
• Without modification, (addition of multirate/multiclass storage) the
approach may not be sensitive to longer timescale interactions with
deeper alluvium.