Environmental Engineering Reference
In-Depth Information
Profile, peak-displacement, and mass-
balance methods can be applied with ground-
water tracer data as described in
S e c t ion 7. 2
.
Groundwater age-dating methods can also be
used to estimate recharge. The profile and peak-
displacement methods, described in
Section
7. 3. 3
, are based on assumptions of a pulse
tracer input (such as bomb-pulse
3
H or
36
Cl) and
vertical groundwater flow. Groundwater sam-
ples need to be collected at several depths to
define the concentration-depth profile. These
requirements preclude application of profile
and peak-displacement methods at many sites.
The mass-balance method is often applied
with groundwater data, most often on the
basis of chloride or tritium concentrations.
The approach is similar to that described for
unsaturated-zone application in
S e c t ion 7. 2 . 3
and is discussed in
Section 7.3.2
. Groundwater
age-dating techniques are described in the
following section. Applied tracers are used
in many groundwater studies to determine
groundwater velocities and travel paths.
However, applied tracers are not commonly
used to quantify recharge rates, and, therefore,
they are not addressed herein.
Age-dating methods rely on a simple
equation to estimate recharge (Böhlke,
2002
):
Rv
(7.16)
=
ϕ
v
where
ϕ
is porosity and
v
v
is vertical water vel-
ocity at the water table. Apparent age dates of
groundwater, determined at one or more sam-
pling points below the water table, and an
assumption on the travel path of water from
the water table to the sampling points are used
to estimate
v
v
. The following paragraphs pro-
vide an analysis of assumptions related to the
use of apparent groundwater ages to estimate
recharge. Subsequent sections provide spe-
cific information for some of the more widely
used tracers. Additional details can be found
in Kendall and McDonnell (
1998
), Cook and
Herczeg (
2000
), and the US Geological Survey
CFC Laboratory website (http://water.usgs.gov/
lab; accessed April 10, 2009).
A number of compounds that have been
introduced to the atmosphere by human activ-
ity since the 1930s (e.g.
3
H, CFCs, SF
6
, and
85
Kr)
are used to date groundwater with ages less
than 50 to 60 years. Tritium was discussed in
Section 7.2.3
. The other compounds are gases
whose atmospheric concentrations change over
time (
F ig u r e 7. 2
). The apparent groundwater age
is determined by analyzing the concentrations
of these gases in groundwater and then find-
ing (after possible corrections) the year, from
F ig u r e 7. 2
, whose concentration matches that
of the sample. That year is referred to as the
recharge date; apparent age is the difference
between sampling date and recharge date.
This age-dating approach relies on the
assumption that tracer gases become dissolved
in recharging water right at the water table and
that gas concentrations in the unsaturated zone
above the water table at the time of recharge
are equal to atmospheric levels. Thus, time lags
for the movement of gases from land surface to
the water table are ignored. This assumption
may be appropriate for relatively thin unsatu-
rated zones where depth to the water table is a
few meters or less. But for thicker unsaturated
zones, in which diffusion is the dominant gas-
transport mechanism (Weeks
et al
.,
1982
), this
assumption can introduce important errors
7.3.1 Age-dating methods
Age refers to the elapsed time since a tracer
moved across the water table and became
isolated from the atmosphere, its presumed
source. We commonly refer to the age of a
groundwater sample; however, it is the age of
the dissolved tracer, not the water itself, that
is calculated from tracer concentrations. It is
more appropriate to use the term
apparent age
when referring to water. Differences may exist
between the actual and apparent ages of water
because various physical and chemical proc-
esses can affect tracer concentrations. These
processes include mixing of water, adsorption
of tracers onto solids, biodegradation of trac-
ers, entrapment of air, and the presence of
alternate tracer sources (especially for CFCs).
Plummer (
2005
) discussed these complications
in some detail and described techniques for
assessing their importance (e.g. measuring
concentrations of a suite of noble gases in the
water sample).
