DEFINITION OF FRACTURE ZONE AQUIFERS

Faults and fractures develop at various

scales from faults that cross continents to fractures that

are only visible microscopically. These discontinuities in the

rock fabric are located and oriented according to the internal

properties of the rock and the external stresses imposed on it.

Fractures at various scales represent zones of increased porosity

and permeability. They may form networks and, therefore,

are able to store and carry vast amounts of water.

The concept of fracture zone aquifers explains the

behavior of groundwater in large fault-controlled watersheds.

Fault zones in this case serve as collectors and transmitters of

water from one or more recharge zones with surface and subsurface

flow strongly controlled by regional tectonism.

Both the yield and quality of water in these zones are

usually higher than average wells in any type of rock. Highgrade

water for such a region would be 250 gallons per

minute or greater. In addition, the total dissolved solids measured

in the water from such high-yielding wells will be lower

than the average for the region.

The fracture zone aquifer concept looks at the variations

in groundwater flow as influenced by secondary porosity

over an entire watershed. It attempts to integrate data on a

basin in an effort to describe the unique effects of secondary

porosity on the processes of groundwater flow, infiltration,

transmissivity, and storage.

The concept includes variations in precipitation over the

catchment area. One example is orographic effects wherein

the mountainous terrain precipitation is substantially greater

than at lower elevations. The rainfall is collected over a large

catchment area, which contains zones with high permeability

because of intense bedrock fracturing associated with major

fault zones. The multitude of fractures within these highly

permeable zones “funnel” the water into other fracture zones

down gradient. These funnels may be in a network hundreds

of square kilometers in area.

The fault and fracture zones serve as conduits for groundwater

and often act as channelways for surface flow. Intersections

form rectilinear drainage patterns that are sometimes

exposed on the surface but are also represented below the surface

and converge down gradient. In some regions, these rectilinear

patterns are not always visible on the surface due to

vegetation and sediment cover. The convergence of these

groundwater conduits increases the amount of water available

as recharge. The increased permeability, water volume, and

ratio of water to minerals within these fault/fracture zones help

to maintain the quality of water supply. These channels occur

in fractured, nonporous media (crystalline rocks) as well as in

fractured, porous media (sandstone, limestone).

At some point in the groundwater course, after convergence,

the gradient decreases. The sediment cover over the

major fracture zone becomes thicker and acts as a water storage

unit with primary porosity. The major fracture zone acts

as both a transmitter of water along conduits and a water

storage basin along connected zones with secondary (and/or

primary) porosity. Groundwater within this layer or lens

often flows at accelerated rates. The result can be a pressurization

of groundwater both in the fracture zone and in the

surrounding material. Rapid flow in the conduit may be

replenished almost instantaneously from precipitation. The

surrounding materials are replenished more slowly but also

release the water more slowly and serve as a storage unit to

replenish the conduit between precipitation events.

Once the zones are saturated, any extra water that

flows into them will overflow, if an exit is available. In a

large area watershed, it is likely that this water flows along

subsurface channelways under pressure until some form of

exit is found in the confining environment. Substantial

amounts of groundwater may flow along an extension of

the main fault zone controlling the watershed and may vent

at submarine extensions of the fault zone forming coastal or

offshore freshwater springs.

The concept of fracture zone aquifers is particularly applicable

to areas underlain by crystalline rocks and where these

rocks have undergone a multiple deformational history that

includes extensional tectonics. It is especially applicable in

areas where recharge is possible from seasonal and/or sporadic

rainfall on mountainous regions adjacent to flat desert areas.

Fracture zone aquifers are distinguished from horizontal

aquifers in that: (a) they drain numerous streams in extensive

areas and many extend for tens of kilometers in length; (b) they

constitute conduits to mountainous regions where the recharge

potential from rainfall is high; (c) some may connect several

horizontal aquifers and thereby increase the volume of accumulated

water; (d) because the source of the water is at higher elevations,

the artesian pressure at the groundwater level may be

high; and (e) they are usually missed by conventional drilling

because the water is often at the depth of hundreds of meters.

The characteristics of fracture zone aquifers make them

an excellent source of groundwater in arid and semiarid environments.

Fracture zone aquifers are located by seeking

major faults. The latter are usually clearly displayed in

images obtained from Earth orbit, because they are emphasized

by drainage. Thus, the first step in evaluating the

groundwater potential of any region is to study the structures

displayed in satellite images to map the faults, fractures, and

lineaments. Such a map is then compared to a drainage map

showing wadi locations. The combination of many wadis and

major fractures indicates a larger potential for groundwater

storage. Furthermore, the intersection between major faults

would increase both porosity and permeability, and hence,

the water collection capacity.

It must be recognized that groundwater resources in arid

and semiarid lands are scarce and must be properly used and

thoughtfully managed. Most of these resources are “fossil,”

having accumulated under wet climates during the geological

past. The present rates of recharge from the occasional rainfall

are not enough to replenish the aquifers. Therefore, the

resources must be used sparingly without exceeding the optimum

pumping rates for each water well field.

See also GROUNDWATER; STRUCTURAL GEOLOGY.

freezing rain See PRECIPITATION.

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