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Upper Agua Fria Basin
The Agua Fria basin occupies about 1,200 square miles in
central Arizona. The northern half
of the basin is in the Central highlands physiographic province, and the
southern half is in the Basin and Range province. The basin’s main drainage is the Agua Fria River, which flows north to
south through the basin and empties into Lake Pleasant. Major tributaries to the Agua Fria are Big Bug, Silver, Sycamore, and
Yellow Jacket Creeks. The Agua Fria and its tributaries are generally intermittent
streams except for some perennial stretches where impermeable bedrock forces
groundwater into the streambed. The
basin is bounded on the north by Hickey Mountain, on the west by the Bradshaw
and Buckhorn Mountains, on the south by Lake Pleasant, and on the east by the
Black Hills and New River Mountains. Land-surface
elevations in the Agua Fria basin vary from 1,570 feet above mean sea level at
Lake Pleasant to 7,800 feet above mean sea level in the Bradshaw Mountains.
Rock units in the Agua Fria basin can be divided into
four broad groups based on general geologic character and their ability to yield
water. The units are from youngest
to oldest: basin-fill sands and gravels, volcanic rocks, conglomerates, and
igneous and metamorphic rocks.
Groundwater occurs in all four rock units in the Agua
Fria basin. The main water-bearing
units are the basin-fill sands and the conglomerates. The volcanics and crystalline rocks yield only small amounts of water.
The basin-fill consists of sands and gravels and readily
transmits recharge into the underlying conglomerate (Wilson, 1988). Because the basin-fill unit is thin it does not contain large quantities
of groundwater in storage.
The volcanic rocks provide small amounts of water to
low-yield stock wells in the northeastern sections of the basin. Well yields are best from cinder beds and fractured sections of the
volcanics. A number of seasonal
springs flow from the volcanics in response to precipitation or snowmelt (Littin,
1981).
Conglomerates occur widely throughout the basin and
contain the largest volume of groundwater. Faulting formed the present-day drainage basins and separated the unit
into several smaller, discrete groundwater basins that are separated by
impermeable crystalline rocks. As a
result, there is little direct subsurface hydrologic connection between the,
sedimentary units in the smaller groundwater basins (Wilson, 1988).
The water-bearing ability of the igneous and metamorphic
rocks depends on their degree of fracturing. Most wells have low yields, however, near Black Canyon City wells drilled
into the Precambrian schist can produce up to 20 gallons per minute (Littin,
1981). A number of perennial
springs flow from the crystalline rocks. Normal
discharges are 1 to 5 gallons per
minute. Castle Hot Springs, located
in the southwest part of the basin, discharges 200 gallons per minute from the
Precambrian rocks (Littin, 1981).
Development of groundwater resources is increasing in the
Agua Fria basin. Population growth
in recent years has resulted in increased pumpage. The U.S. Geological Survey (1986) has estimated that
groundwater pumpage increased from 3,000 acre-feet per year in 1979 to 10,000
acre-feet per year in 1987 (Alice Konieczki, U.S. Geological Survey, written
commun., 1991). Although the U.S.
Geological Survey groundwater basin boundary for the Agua Fria basin differs
from the Arizona Department of Water Resources’ boundary and includes the
southeastern part of the Prescott AMA, this pumpage estimate is the best
available at this time. Despite
increased groundwater pumpage, water levels generally have not declined in the
basin. The only area of declining
water levels is around Cordes Junction where declines of several feet have been
reported (Wilson, 1988). This
suggests that overall the basin is still in a steady-state situation. Total groundwater reserves in the Agua Fria basin are estimated to be 3.5
million acre-feet (Arizona Department of Water Resources, 1988).
Water quality in the basin generally is good. Near Black Canyon City, arsenic has been detected in the groundwater and
is associated with the volcanic formations in the area (Arizona Department of
Environmental Quality, 1990).
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