Environmental Engineering Reference
In-Depth Information
(7% of precipitation) over the Basin and Range
carbonate-rock aquifer system in Nevada and
parts of Utah. An empirical model based on
annual precipitation and elevation developed
by Maxey and Eakin (
1949
) is still extensively
used in Nevada (
Section 3.7
). Unsaturated-
zone tracer profiles of bomb-pulse tritium and
chlorine-36 have been used to estimate rates
of drainage through the unsaturated zone that
are averaged over the time period between the
1950s and 1960s, when peak atmospheric con-
centrations occurred, and the time at which
samples were collected (Phillips
et al
.,
1988
;
Scanlon,
1992
). In particularly arid areas, the
chloride mass-balance method has been used
to estimate drainage rates through the unsat-
urated zone that are averaged over periods of
hundreds to thousands of years (Phillips,
1994
;
Tyler
et al
.,
1996
). Numerous additional studies
of recharge to mountains and basin aquifers are
described in Wilson (
1980
), Hogan
et al
. (
2004
),
and Stonestrom
et al
. (
2007
).
In open basins that are traversed by peren-
nial streams, focused recharge to underlying
aquifers may support evapotranspiration by
riparian phreatophytes, and thus not be avail-
able for human consumption. Culler
et al
. (
1982
)
used a stream water-budget approach to deter-
mine evapotranspiration rates before and after
phreatophytes were cleared from the Gila River
floodplain in Arizona. In closed basins with
no surface water outflow, groundwater is dis-
charged to the atmosphere through evapotran-
spiration or to adjoining basins as groundwater
flow. Nichols (
2000
) used evapotranspiration
measurements and estimates of interbasin
flow to generate water budgets and recharge
estimates for basins in the Great Basin area of
Nevada.
Alluvial valleys actually exist in many
regions. In his description of groundwater
regions of the United States, Heath (
1984
) listed
Alluvial Valleys as a separate noncontiguous
region. The hydrology of these valleys is simi-
lar, in some ways, to the hydrology described
above; water flows to the valleys from adjacent
uplands. Climate, however, plays an important
role in the hydrology of these systems. In humid
areas, recharge is predominantly diffuse, and
streams in alluvial valleys are usually sinks for
discharging groundwater, as opposed to being
sources of focused recharge in arid and semi-
arid regions. A particular estimation method
may be appropriate in one alluvial valley but
inappropriate in another.
Example: Yucca Mountain, Nevada
Throughout the 1980s and 1990s, studies were
conducted to assess the suitability of Yucca
Mountain as a repository for high-level radioac-
tive waste. Of major concern and interest was the
amount of water that waste would be exposed
to, i.e. the rate of water drainage through the
unsaturated zone. Flint
et al
. (
2001a
) provided an
excellent description of the conceptual model
of water flow through the unsaturated zone
and discussion of how the conceptual model
evolved over time as more data and results of
computer simulations became available. The
Yucca Mountain study may be the most expen-
sive and most detailed study ever conducted on
water movement through an unsaturated zone.
The unsaturated zone at Yucca Mountain
is approximately 500 m thick at the planned
repository location and consists of layers of vol-
canic tuff, some of which are highly fractured,
others of which have few fractures but are pen-
etrated by faults. Average annual precipitation
is about 170 mm. Initial estimates of drain-
age rates of less than 1 mm/yr were based on
few data and on what turned out to be flawed
hypotheses on the manner in which water
moves through variably saturated fractured
rock. Data collected during the study included
concentrations of natural chemical and isotopic
tracers in the unsaturated and saturated zones
(in rock matrix and on fracture walls), soil-wa-
ter content and chemistry, soil-gas chemistry,
soil matric potential, subsurface temperatures,
groundwater levels and groundwater chemistry,
precipitation, evapotranspiration, and runoff.
Results from simulation models were crucial
for improving the understanding of water
movement through the mountain. Applied
models included groundwater-low models, var-
iably saturated flow models, water-budget and
energy-budget models, watershed models, and
various geochemical models. Many different
