Environmental Engineering Reference
In-Depth Information
8
Heat tracer methods
to precipitation, infiltration, and drainage
through the unsaturated zone. Estimates of dif-
fuse recharge determined using measured tem-
peratures in the unsaturated zone are referred
to as
potential recharge
because it is possible
that not all of the water moving through the
unsaturated zone will recharge the aquifer;
some may be lost to the atmosphere by evap-
oration or plant transpiration. Estimated fluxes
across confining units in the saturated zone
are referred to as
interaquifer flow
(
Chapter 1
).
Focused recharge
is that which occurs directly
from a point or line source, such as a stream, on
land surface. Focused recharge may vary widely
in space and time. If the water table intersects
a stream channel, estimates of stream loss are
called actual recharge, or just recharge. If the
water table lies below the stream channel, esti-
mates are referred to as potential recharge. For
simplicity, all vertical water fluxes are referred
to as
drainage
throughout this chapter. Whether
the estimated quantity represents actual or
potential recharge or drainage depends on the
circumstances of each individual study.
8.1 Introduction
The flow of heat in the subsurface is closely
linked to the movement of water (Ingebritsen
et al
.,
2006
). As such, heat has been used as a
tracer in groundwater studies for more than 100
years (Anderson,
2005
). As with chemical and
isotopic tracers (
Chapter 7
), spatial or temporal
trends in surface and subsurface temperatures
can be used to infer rates of water movement.
Temperature can be measured accurately, eco-
nomically, at high frequencies, and without the
need to obtain water samples, facts that make
heat an attractive tracer. Temperature measure-
ments made over space and time can be used to
infer rates of recharge from a stream or other
surface water body (Lapham,
1989
; Stonestrom
and Constantz,
2003
); measurements can also
be used to estimate rates of steady drainage
through depth intervals within thick unsat-
urated zones (Constantz
et al
.,
2003
; Shan and
Bodvarsson,
2004
). Several thorough reviews
of heat as a tracer in hydrologic studies have
recently been published (Constantz
et al
.,
2003
;
Stonestrom and Constantz,
2003
; Anderson,
2005
; Blasch
et al
.,
2007
; Constantz
et al
.,
2008
).
This chapter summarizes heat-tracer approaches
that have been used to estimate recharge.
Some clarification in terminology is pre-
sented here to avoid confusion in descrip-
tions of the various approaches that follow.
Diffuse recharge
is that which occurs more or
less uniformly across large areas in response
8.2 Subsurface heat flow
Heat flow within the subsurface is driven by
two phenomena. High temperatures in the
Earth's interior produce a flow of heat outward
from the core toward land surface. Near land
surface, diurnal, seasonal, and climatic trends
in solar radiation produce temporal trends in
