Environmental Engineering Reference
In-Depth Information
1
Groundwater recharge
as the rate of recharge increases. Areas of high
recharge are often equated with areas of high
aquifer vulnerability to contamination (ASTM,
2008
; US National Research Council,
1993
).
Locations for subsurface waste-disposal facili-
ties often are selected on the basis of relative
rates of recharge, with ideal locations being
those with low aquifer vulnerability so as to
minimize the amount of moving water com-
ing into contact with waste (e.g. US Nuclear
Regulatory Commission,
1993
). A high profile
example of the importance of susceptibility to
contamination is the study for the proposed
high-level radioactive-waste repository at Yucca
Mountain, Nevada. Tens of millions of dollars
were invested over the course of two decades in
efforts to determine recharge rates at the site
(Flint
et al
.,
2001a
).
Computer models of groundwater-flow are
perhaps the most useful tools available for
groundwater-resource management. Models
are applied in both water-supply and aquifer-
vulnerability studies. We expect that many
readers of this topic will be modelers seeking
recharge estimates for use in groundwater-flow
models or for evaluating model results.
The primary objective of this text is to pro-
vide a critical evaluation of the theory and
assumptions that underlie methods for estimat-
ing rates of groundwater recharge. A complete
understanding of theory and assumptions is fun-
damental to proper application of any method.
Good practice dictates that recharge estimation
techniques be matched to conceptual models of
1.1 Introduction
Groundwater is a critical source of fresh water
throughout the world. Comprehensive statistics
on groundwater abstraction and use are not
available, but it is estimated that more than
1.5 billion people worldwide rely on ground-
water for potable water (Clarke
et al
.,
1996
).
Other than water stored in icecaps and glaciers,
groundwater accounts for approximately 97% of
fresh water on Earth (Nace,
1967
; Shiklomanov
and Rodda,
2003
). As the world population con-
tinues to grow, more people will come to rely
on groundwater sources, particularly in arid
and semiarid areas (Simmers,
1990
). Long-
term availability of groundwater supplies for
burgeoning populations can be ensured only
if effective management schemes are devel-
oped and put into practice. Quantification of
natural rates of groundwater recharge (i.e. the
rates at which aquifer waters are replenished)
is imperative for efficient groundwater man-
agement (Simmers,
1990
). Although it is one
of the most important components in ground-
water studies, recharge is also one of the least
understood, largely because recharge rates vary
widely in space and time, and rates are difficult
to directly measure.
The rate, timing, and location of recharge
are important issues in areas of groundwater
contamination as well as groundwater sup-
ply. In general, the likelihood for contami-
nant movement to the water table increases
