KID’S PAGE

By Trish Penny

 

 

 

 

GROUND WATER

 

Some water underlies the Earth’s surface almost everywhere, beneath hills, mountains, plains, and deserts.  It is not always accessible, or fresh enough for use without treatment, and it’s sometimes difficult to locate or measure.  This water can lie close to the land surface, as in a marsh, or it may lie many hundreds of feet below the surface.  Water at a very shallow depth may be just a few hours old; at moderate depths, it may be 100 years old; and at great depth or after having flowed long distances from place of entry, water may be several thousands of years old.

 

Ground water is stored in, and moves slowly through permeable rocks called aquifers.  Aquifers literally carry water underground.  An aquifer may be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubbly top or base of lava flows, or even a large massive rock, such as fractured granite, that has sizeable openings.  Ground water is the largest single supply of fresh water available for use by humans.

 

An estimated one million cubic miles of the world’s ground water is stored within one-half mile of the land surface.  Only a fraction of this reservoir of ground water can be practicably tapped and made available on a perennial basis through wells and springs.

 

Some people believe that ground water collects in underground lakes or flows in underground rivers.  Ground water is simply the subsurface water that fully saturates pores or cracks in soils and rocks.  Ground water is replenished by precipitation and depending on the local climate and geology is unevenly distributed in both quantity and quality.  When rain falls or snow melts, some of the water evaporates, some is transpired by plants, some flows overland and collects in streams, and some filtrates into the pores or cracks of the soil and rocks.  The first water that enters the soil replaces water that has been evaporated or used by plants during a preceding dry period. Between the land surface and the aquifer water is a zone that hydrologists call the unsaturated zone.  In this zone, there usually is

at least a little water, mostly in smaller openings of the soil and rock; the larger openings usually contain air instead of water.  After a significant rain, the zone may be almost saturated ; after a long dry spell it may be almost dry.  Some water is held in the unsaturated zone by molecular attraction, and it will not flow toward or enter a well.

 

After the water requirements for plants and soil are satisfied, any excess water will infiltrate to the water table (the top of the zone below which the openings in rocks are saturated).

 

Below the water table, all the openings in the rocks are full of water that moves through the aquifer to streams, springs, or wells from which water is being withdrawn.  Natural refilling of aquifers at depth is a slow process because ground water moves slowly through the unsaturated zone and the aquifer.  The rate of recharge is an important consideration because if an aquifer was emptied that underlies an area of minimal precipitation it could take centuries to refill.  In contrast, a shallow aquifer in an area of substantial precipitation may be replenished almost immediately.

 

Aquifers can be replenished artificially in two main ways:  One way is to  spread water over the land in pits, furrows, or ditches, or to erect small dams in stream channels to detain and deflect surface runoff, allowing it to infiltrate to the aquifer; the other way is to construct recharge wells and inject water directly into an aquifer. The second way is very expensive but may be justified where spreading method is not feasible.  Unfortunately, artificial recharge projects have not always been successful.

 

The distribution of water in both space and time is irregular, and some areas already face serious regional water shortages because of using some water faster than is naturally replenished.  Further development of energy, mineral, and agricultural resources is dependent largely upon adequate water supplies.  Therefore, the ground-water resources will become even more valuable in the years ahead as the Nation copes with growing natural-resource and environmental problems and increased water demands.

 

A major responsibility of the U.S. Geological Survey is to assess the quantity and quality of the Nation’s Water Supplies.  The Geological Survey in cooperation with other Federal, State, and local agencies, maintains a nationwide hydrologic-data network, carries out a wide variety of water-resources investigations, and develops new methodologies for studying water.  The results of these investigations are indispensable tools for those involved in water-resources planning and management. 

 

Source:  USGS