Environmental Engineering Reference
In-Depth Information
7
Chemical tracer methods
environmental tracer is chloride (Cl) (Allison
and Hughes,
1978
). Ocean water, through the
process of evaporation, is the primary source
of atmospheric Cl. Other tracers in this cat-
egory include chlorine-36 (
36
Cl) and tritium (
3
H);
these two isotopes are produced naturally in
the Earth's atmosphere; however, there are add-
itional anthropogenic sources of them.
Historical tracers refer to tracers that have
been widely introduced to the atmosphere by
human activity. These are sometimes referred
to as anthropogenic tracers (Cook and Böhlke,
2000
). Included in this category are
3
H and
36
Cl
(
F ig u r e 7.1
), which were produced by nuclear
weapons testing in the 1950s and early 1960s
(Phillips
et al
.,
1988
; Scanlon,
1992
; Cook
et al
.,
1994
); iodine-129 (
129
I) from nuclear reprocess-
ing and the Chernobyl accident (Englund
et al
.,
2008
); and gases such as chlorofluorocarbons
(CFCs) and sulfur hexafluoride (SF
6
), which are
generated by industrial activities (Plummer
and Busenberg,
2000
; Plummer,
2005
). As with
environmental tracers, non-gaseous historical
tracers arrive at the land surface as wet or dry
atmospheric deposition and enter the subsur-
face in a dissolved phase. Gaseous tracers, on
the other hand, are assumed to diffuse from the
atmosphere through the unsaturated zone and
to dissolve in groundwater at the water table.
Applied tracers are those that are applied
directly to land surface (or in some instances
in the shallow subsurface) as a result of human
activity. The application can be specifically for
purposes of conducting a tracer experiment
7.1 Introduction
Tracers have a wide variety of uses in hydrologic
studies: providing quantitative or qualitative
estimates of recharge, identifying sources of
recharge, providing information on velocities
and travel times of water movement, assessing
the importance of preferential flow paths, pro-
viding information on hydrodynamic disper-
sion, and providing data for calibration of water
flow and solute-transport models (Walker,
1998
;
Cook and Herczeg,
2000
; Scanlon
et al
.,
2002b
).
Tracers generally are ions, isotopes, or gases
that move with water and that can be detected
in the atmosphere, in surface waters, and in the
subsurface. Heat also is transported by water;
therefore, temperatures can be used to trace
water movement. This chapter focuses on the
use of chemical and isotopic tracers in the sub-
surface to estimate recharge. Tracer use in sur-
face-water studies to determine groundwater
discharge to streams is addressed in
Chapter 4
;
the use of temperature as a tracer is described
in
Chapter 8
.
Following the nomenclature of Scanlon
et al
.
(
2002b
), tracers are grouped into three categor-
ies: natural environmental tracers, historical
tracers, and applied tracers. Natural environ-
mental tracers are those that are transported
to or created within the atmosphere under
natural processes; these tracers are carried to
the Earth's surface as wet or dry atmospheric
deposition. The most commonly used natural
