Environmental Engineering Reference
In-Depth Information
WT, are applied evenly over a test plot that is
typically one to several square meters in surface
area. The plot is usually irrigated, although not
in all studies. After a set period of time or after
a certain amount of water has been applied,
the soil is carefully removed in layers of prede-
termined thickness (e.g. 200 mm) and detailed
photographs are used to document tracer dis-
tribution in each exposed layer. The process
is repeated layer by layer until a final depth is
reached. The process produces a quasi-three-di-
mensional picture of tracer distribution in the
subsurface. Dye tracer experiments can provide
useful qualitative information on the import-
ance of preferential flow paths, but they cannot
be used by themselves to determine quantita-
tive flow rates for individual flow paths.
Many other chemicals and isotopes have
been used as applied tracers. These include sta-
ble isotopes (McConville
et al
.,
2001
), lithium
(Rosqvist and Destouni,
2000
),
15
N (Nielsen
et al
.,
1997
), and microspheres (Burkhardt
et al
.,
2008
).
Kung
et al
. (
2000
) used three fluorobenzoic
acids as tracers in a study of preferential flow
through agricultural soils; water samples for
tracer analysis were obtained from tile drains
in that study.
Studies described in this section all used soil
cores for tracer analyses. Fixed-depth instru-
m e n t s (
S e c t i o n 7. 2 .1
) can also be used to track the
movement of applied tracers within the unsatu-
rated zone. Fixed-depth instruments allow for
high-frequency data collection. Analysis of
fixed-depth instrument data is concerned with
determining tracer arrival time at specific
depths (as opposed to determining tracer pen-
etration depth at specific times, which is the
aim of soil core sampling). The profile method
(Equation (
7.1
)) and peak-displacement method
(Equation (
7. 3
)) can both be applied with fixed-
depth instrument data. For both methods, the
depth of penetration,
z
T
, is known, and Δ
t
, the
time between tracer application and arrival,
must be determined. Tracer arrival time,
t
T
(
z
1
),
can be set to the arrival time of the peak con-
centration or the arrival time of the center of
tracer mass:
∞
For the peak-displacement method, velocity is
determined as distance traveled divided by the
travel time. For example, if tracer concentration
data were collected at depths
z
1
and
z
2
, velocity
is calculated as:
tz
(7.15)
v
=−
(
z z tz
) /[
(
)
−
(
)]
2
1
T2
T1
and drainage can be calculated as velocity times
volumetric water content.
7.3 Groundwater tracers
Approaches for using groundwater tracer data
to estimate recharge are theoretically similar
to those based on unsaturated-zone data. There
are some differences, however, in terms of
underlying assumptions and implementation.
Unsaturated-zone tracers provide a point esti-
mate, in terms of space, of drainage or poten-
tial recharge; groundwater samples provide an
estimate of actual recharge that is integrated
over some larger but often poorly defined
area. Unsaturated-zone tracer analyses rely
on the assumption of vertical downward flow,
an assumption whose validity is seldom ques-
tioned. Analyses of groundwater tracer data
generally require no assumptions with regard
to flow within the unsaturated zone (thus, esti-
mates are usually not affected by the presence
of preferential flow paths in the unsaturated
zone). However, age-dating methods require
assumptions with regard to the travel paths
that groundwater takes from the water table to
a sampling point. Many tracers used in unsatu-
rated-zone studies are also used in ground-
water studies, including chloride,
3
H, and
36
Cl.
Other tracers are specific to saturated-zone
applications (e.g. dissolved gas tracers, such as
chlorofluorocarbons (CFCs), SF
6
, and carbon-14,
14
C). Contaminants in groundwater can some-
times be used as tracers. Groundwater samples
are usually more easily obtained than unsatu-
rated-zone pore-water samples, and there is a
wealth of groundwater chemistry data avail-
able in national databases. Tracer data collected
in both the unsaturated and saturated zones
can be used to calibrate water-flow and solute-
transport models for estimating recharge.
∞
z dt
(7.14)
=
∫
∫
t z
()
tC z
()() /
θ
z dt
C z
()()
θ
T
T
T
T
T
T
0
0
