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Ground-water resources in the semiarid
Verde River headwaters of Arizona are under increasing pressure from
population growth and residential development. To address this issue, U.S.
Geological Survey (USGS) scientists are using multi-disciplinary approaches
to identify the most important ground-water flowpaths through aquifers in
Big and Little Chino Valleys—tributary aquifers which provide
life-sustaining flow to the upper Verde River north of Prescott, Ariz.
A new USGS report uses 3-dimensional
mapping of rock and aquifer units, airborne geophysical surveys, tracer and
synoptic water-chemistry studies, environmental isotopes, and geochemical
modeling to describe the geologic framework of the major aquifers and also
to trace key pathways of ground-water movement.
“Interpretations from several
independent investigations were combined to create a detailed conceptual
model of the ground-water system,” said USGS scientist Laurie Wirt, the lead
author of the 209-page report.
Results of the USGS study are expected
to promote science-based decision making by providing technical information
to water-resource managers in charge of allocating limited water resources.
Findings may also be used by other scientists who are developing numerical
ground-water flow models to predict the results of future pumping likely to
occur in Big Chino Valley.
Key findings of the research include:
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Gravity data indicate an asymmetric
basin at least 1–2 km deep and beneath Big Chino Valley. The lack of a
distinct gravity low suggests a much thinner basin (less than 1 km)
beneath Little Chino Valley. The size and geometry of the aquifers has
important implications for the amount of water held in storage.
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A large part of the regional
carbonate aquifer beneath the Colorado Plateau lies north of a
ground-water divide that follows the crest of Big Black Mesa and probably
contributes very little, if any, recharge to Big Chino Valley or the upper
Verde River.
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Geochemical modeling indicates that
the Big Chino aquifer, including its underlying carbonate units, provides
80 to 86 percent of the base flow in the upper Verde River at the USGS
gauge near Paulden. About 14 percent is supplied by the Little Chino
aquifer, with the part of the carbonate aquifer north of the upper Verde
River contributing less than 6 percent.
“These new findings have been compared
to earlier studies, and for the most part, helped to confirm and expand on
what we already knew,” said Wirt. “By relying on multiple lines of
geological evidence, we greatly improve our confidence in our understanding
of the aquifer system. We can better evaluate how much water is available
and how the plumbing system operates.”
The USGS report was supported by a
research grant from the Arizona Water Protection Fund Commission, which is
administered through the Arizona Department of Water Resources. The report
will provide technical background for water-resource policy issues, as well
as for future scientific studies involving aquifer studies and ground-water
modeling.
Copies of Geologic Framework of
Aquifer Units and Ground-Water Flowpaths, Verde River Headwaters,
North-Central Arizona, edited by Laurie Wirt, Ed DeWitt, and Victoria E.
Langenheim and published as USGS Open-File Report 2004–1411, Chapters A–G
with Appendices, are available at:
http://pubs.usgs.gov/of/2004/1411/ |
