Environmental Engineering Reference
In-Depth Information
source. Runoff diversions may be important in
terms of aquifer vulnerability to contamination
because they can quickly funnel contaminants
to the subsurface.
Delivery systems for water supply and treat-
ment are additional artifacts of urbanization
that can affect recharge processes, both in
terms of water supply and potential for contam-
ination. These systems consist of open channels
or water pipes and sewers. Invariably, there is
leakage associated with any delivery system.
This leakage, a form of potential recharge, may
become actual recharge. Norin
et al
. (
1999
) esti-
mated that 26% of water transmitted through
water mains in Göteborg, Sweden, was lost to
leakage.
and timing for data collection. Application of
numerical or analytical models provides ben-
efits at all stages of a recharge study. The con-
ceptual and numerical models are both part of
an iterative process, whereby the models are
continually refined and revised as new data,
interpretations, and simulation results become
available.
1.4.8 Use of existing data
Construction of a conceptual model should
make use of all available data for the study area
and surrounding areas. Many of the recharge
estimation methods described in the following
chapters, including watershed and groundwa-
ter-flow models, can be applied without collect-
ing any new data. Careful analysis of all existing
data precedes any decisions on the collection of
new data. Existing databases contain climato-
logical data, surface-water flow data, land-use
data, groundwater levels, chemistry of surface
water and groundwater, physical and hydraulic
properties of soils, and land-use characteristics.
Pertinent data sources are described more thor-
oughly in
Chapters 2
and
3
.
1.4.7 Integration of multiple factors
A conceptual model of recharge processes is
formed by integrating the above factors, and
perhaps other factors as well, into hypotheses
on where, when, and why recharge occurs. For
example, the timing and location of recharge
in high mountainous valleys is often controlled
by geology, climate, and hydrology. Snowfall
in the mountains from late fall through early
spring is the source of recharge water. Water is
stored in the snowpack until late spring, when
it is released to streams as rising air tempera-
tures melt snowpacks. As swollen streams flow
to valleys, water seeps downward, recharging
underlying aquifers. Variations of this predict-
able pattern of seasonal recharge occur in many
mountainous regions.
Numerical or analytical models of climatic
conditions, watershed processes, surface-water
flow, groundwater flow, or unsaturated-zone
flow are useful tools for integrating the factors
affecting the conceptual model of recharge.
The suggestion of using a numerical model in
the initial stages of a recharge study may seem
unusual because oftentimes the specific goal
of a recharge study is to develop estimates for
use in groundwater-flow models or for com-
parison with model results. Nonetheless, a
simple numerical model can be a useful tool
for identifying important mechanisms, evalu-
ating hypotheses included in a conceptual
model, and determining optimum locations
1.4.9 Intersite comparison
As a first estimate of recharge for a particular
study area, one might use an estimate derived
from a site with similar climate, land use, and
other features. A review of literature for simi-
lar sites is always a worthwhile endeavor. Such
a review would benefit from a common clas-
sification scheme for climatic/hydrologic/geo-
logic provinces. Such a scheme would facilitate
intersite comparisons and would also be useful
in the construction of conceptual models and
selection of appropriate techniques. Currently
(2010), there is no such scheme in widespread
use, although classification schemes suggested
by Salama
et al
. (
1994b
) for hydrogeomorphic
units and by Winter (
2001
) for hydrologic land-
scapes hold promise. For discussion of generic
recharge processes, we resort to the ground-
water regions of the United States defined by
Thomas (
1952
); this discussion is provided
in
Chapter 9
so that concepts and specific
methods can be discussed in complementary
fashion.
