Environmental Engineering Reference
In-Depth Information
of the experiment. Ideally, all water inflows to
the stream (including injected, surface water,
and groundwater) rapidly and uniformly mix
in the stream; however, concentrations should
be measured at several points in a cross section
of the stream over time at selected downstream
stations to verify steady conditions. As with
the stream water-budget methods discussed
in
Section 4.2
, the tracer injection method is
most useful for small streams where ground-
water inflow over a reach constitutes a signifi-
cant percentage of streamflow (in the stream
water-budget method, 5% was the minimum
suggested percentage).
The tracer injection method can be applied
in any stream that satisfies the above require-
ments. The method has proved particularly
useful, however, in mountain streams where
steep slopes provide a favorable mixing envi-
ronment. Traditional current-meter methods
of measuring stream discharge are not well
suited for these streams because of the steep
slopes and the potential for hyporheic flow
within cobble-lined streambeds. Hyporheic
flow is the transient movement of surface
water into and out of the subsurface. The trac-
er-injection method is usually little affected
by hyporheic flow, but large hyporheic zones
or surface storage features can complicate
efforts to quantify groundwater discharge
rates (Harvey
et al
.,
1996
).
The tracer injection method has been widely
used in studies of acid-mine drainage in moun-
tainous regions of the western United States
(Bencala
et al
.,
1987
; Kimball
et al
.,
1994
,
2004
,
2007
; Walton-Day
et al
.,
2005
). The injected tracer
is used to determine groundwater and surface-
water inflows along an extended stream reach;
simultaneous measurements of stream water
chemistry permit calculation of contaminant
loads (such as metals) contributed from each
subreach. Studies such as these can enhance
the effectiveness of remediation efforts by iden-
tifying subreaches that contribute the greatest
contaminant loads and whose clean-up can best
improve stream water quality.
The stream tracer-injection method provides
a measure of base flow. Translating a base flow
estimate to an estimate of recharge is subject
to the caveats discussed in
Section 4.1.2
. In par-
ticular, it may be difficult to determine the area
of the aquifer that contributes discharge to a
particular stream reach. Recharge estimates
from this method are for a single point in time.
If hydrologic conditions within a watershed
vary throughout the year, it may be desirable
to apply the method at different times of the
year. Careful planning and intensive sampling
over a period of a few days are required, but a
large number of locations can be sampled, thus
providing very detailed information on stream/
aquifer interaction. Walton-Day
et al
. (
2005
)
obtained samples from 53 locations along and
adjacent to a 9.5-km reach of Lake Fork Creek
near Leadville, Colorado, during a tracer test
that lasted 30 h.
4.7 Discussion
The methods described in this chapter are
similar in that they require data on stream-
flow, stream stage, or surface-water chemistry.
However, there are fundamental differences
among the methods, and an understanding of
the type of recharge and the time and space
scales over which methods are applicable is
important. Stream water-budget methods
and seepage measurements usually provide
estimates of focused recharge or exchange of
groundwater and surface water. Streamflow
duration curves and hydrograph analysis tech-
niques provide estimates of base flow that is
associated with diffuse recharge occurring
across a watershed.
Estimates of groundwater and surface-water
exchange rates determined from stream water-
budget methods are integrated over the reach
of the stream where flow measurements are
conducted; however, the estimates represent
exchange rates only for a particular point in
time. Seepage meters provide estimates of
exchange rates over a very small area of stream-
bed. Therefore, seepage rates determined with
meters represent point estimates in both space
and time. Seepage measurements at multiple
locations and times can be used to determine
variability in exchange rates.
