Environmental Engineering Reference
In-Depth Information
water needs for rapidly growing urban areas, and growing demands for sustainable water
supplies for riparian ecosystems.
4.6 Groundwater
Groundwater consists of the two fundamental states of unsaturated and saturated zones
delineated vertically by the water table. The unsaturated zone, as the name implies, is a
three-phase system of gases, water, and sediment, with both gases and water in motion.
In the unsaturated zone, water generally moves downward, but water may move upward
either by capillary action, vapor transport, or hydraulic lift by plants. A contiguous satu-
rated zone in permeable rocks, regardless of extent, is called an aquifer. Like surface water,
groundwater moves according to elevation and pressure gradients, only the rates of move-
ment are orders of magnitude lower than surface water.*
Major aquifers in the Basin and Range part of the Southwest deserts are characterized by
having large volumes of water stored in pore spaces in the gravel, sand, silt, and clay that fill
the basin. Most natural recharge occurs along edges of the basin fill where runoff crosses
the interface between low-permeability rocks of the mountains and runs down coarse-
grained alluvial channels. In areas with permeable rocks in the mountains, water can enter
the aquifers as mountain-block recharge. Influent stream reaches can also be considered a
source of recharge, although a mass balance would likely indicate that the net change water
available to an alluvial aquifer traversed by a stream is negative when both influent and
effluent reaches and use by riparian vegetation is considered. Regardless of the mechanism,
annual recharge to aquifers is usually small in relation to the volume of fresh water stored
in the aquifers. Natural discharge from these aquifers occurs by flow to streams, springs,
and wetlands; uptake by plants; and groundwater underflow to adjacent basin aquifers.
Major aquifers in the Colorado Plateau are characterized as having large volumes of
water in areally extensive sandstone and other consolidated rock units. Recharge occurs
through direct infiltration of rainfall and snowmelt, as well as through infiltration of run-
off into narrow alluvial channels incised into the consolidated rocks. Like the basin aqui-
fers, annual recharge to aquifers beneath the Colorado Plateau is small in relation to the
volume of water in storage. From recharge areas, groundwater moves toward regional
drains that include streams and springs below the Mogollon Rim (along the south edge of
the Colorado Plateau), the Verde River, the Little Colorado River, and springs and streams
in the Grand Canyon and tributary canyons. In addition to discharging to these features,
some water also is used by phreatophytic vegetation where the water table is near land
surface. The water table is relatively close to the land surface adjacent to much of the Verde
and Little Colorado Rivers, but it can be at great depths exceeding several thousand feet
below land surface under parts of the plateau adjacent to deep canyons and in structural
basins. For individual aquifers underlying the Colorado Plateau, natural discharge also
can occur as vertical movement to underlying or overlying aquifers.
Base flow in most perennial streams and rivers in the region ultimately is generated
from groundwater discharge, either locally or, in the case of the Colorado River, from a
long distance away. For rivers solely benefiting from groundwater discharge, base flow has
little interannual variation but may have long-term trends owing to a variety of conditions,
* For a recent review of groundwater, see Alley et al. 25
An excellent source of information on groundwater recharge is contained in Stonestrom et al. 26
 
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