Environmental Engineering Reference
In-Depth Information
over predevelopment totals as a result of return
flow (Vaccaro and Olsen,
2007
).
Nimmo
et al
. (
2002b
) described a large-scale
tracer test above the Snake River aquifer in
Idaho. Focused recharge from floodwaters of
the Big Lost River were followed by means of
a chemical tracer. Movement of the water in
the subsurface was tracked by analyzing water
samples obtained from numerous wells and
piezometers. Estimates of recharge were not
obtained, but the experiment provided insight
into the rate and direction of water movement
through the alternating layers of basalts and
sediment that make up the 200 m thick unsat-
urated zone.
Estimates of natural diffuse recharge in
more arid, nonirrigated areas of the plateau
have been made for purposes of assessing
potential mobility of buried waste material. A
variety of approaches have been used to esti-
mate drainage rates through the unsaturated
zone near Hanford, Washington: Gee
et al
.
(
1992
) used lysimeters; Prych (
1998
) applied
the unsaturated-zone chloride mass-balance
and chlorine-36 tracer profile methods; and
Fayer
et al
. (
1996
) used lysimeters, the zero-
flux plane method, and a numerical model.
Estimated drainage rates were generally less
than 2 mm/yr. Nimmo and Perkins (
2008
)
also estimated rates of less than 2 mm/yr for
a site in eastern Idaho by applying the Darcy
unsaturated-zone unit-gradient method.
1991
). Focused recharge from streams is also
important in some areas.
A variety of methods have been used to
estimate recharge in this region. Watson
et al
.
(
1976
) and Freethey and Cordy (
1991
) used
modified versions of the Maxey-Eakin method
(Maxey and Eakin,
1949
;
Section 3.7
). Freethey
and Cordy (
1991
) estimated average recharge for
the upper Colorado River Basin to be 15 mm/yr.
Heilweil and Freethey (
1992
) used an aquifer
water-budget method for a part of the Navajo
Sandstone, with measured or estimated ground-
water discharges, to estimate recharge at 3 to
4 mm/yr. Recharge rates in the Black Mesa area
of northeastern Arizona were estimated to be in
the range of 13 to 19 mm/yr (4 to 6% of annual
precipitation) on the basis of carbon-14 age dat-
ing and groundwater-flow modeling (Zhu,
2000
)
and at 10 mm/yr by using the groundwater
chloride mass-balance method (Zhu
et al
.,
2003
).
Danielson and Hood (
1984
) applied the stream
water-budget method and estimated focused
recharge to the Navajo Sandstone from the
Fremont River and other smaller streams to be
about 2 500 000 m
3
/yr in the lower Dirty Devil
River Basin in south-central Utah. Heilweil and
Freethey (
1992
) reported similar rates of focused
recharge from the North and East Forks of the
Virgin River.
Heilweil
e t a l
. (
2007
) applied a variety of tracer
techniques to estimate recharge to the Navajo
Sandstone at Sand Hollow in southwestern
Utah. The groundwater chloride mass-balance
method was used to estimate a basin-wide aver-
age recharge of 8 mm/yr (4% of precipitation).
Point estimates of unsaturated-zone drain-
age rates, determined with the unsaturated-
zone tritium profile and tritium mass-balance
methods at 18 borehole sites, ranged from 1 to
57 mm/yr; the wide range in estimates supports
the hypothesis that recharge occurs primarily
as focused recharge of runoff from bedrock out-
crops and as direct infiltration in areas where
surficial soils are coarse.
9.4.4 Colorado Plateau
This region consists of thick sedimentary
strata (mostly interbedded shales, sandstones,
and coal). The beds are roughly horizontal but
may be folded, tilted, or broken by faults. The
plateau has deeply incised streams. The climate
in this region is semiarid, but mountainous
areas may receive substantial precipitation.
There are five regionally extensive bedrock
aquifers of Mesozoic age, the most impor-
tant of which may be the Navajo Sandstone.
Recharge occurs mostly at higher elevations
along margins of uplifts where aquifer rocks
outcrop or are covered by a thin layer of per-
meable material and precipitation is greater
than about 300 mm/yr (Freethey and Cordy,
9.4.5 High Plains
The High Plains region, comprising an area of
about 450 000 km
2
in the central Great Plains, is
a remnant of a vast plain formed by sediments
