Freshwater Resources Chapter 7

Water: A Vital Resource Water is fundamental to life as we know it. A total volume of 325 million cubic miles covers 71% of Earth’s surface. About 97.5% of this volume is salt water of the ocean and seas. The remaining 2.5% is fresh water – water with a salt content of less .1% Evaporation from the oceans combines with precipitation to re-supply the small percentage continually through the solar powered water cycle. Thus, fresh water is a continually renewable resource.

Streams, rivers, ponds, lakes, swamps, estuaries, groundwater, bays, oceans, and the atmosphere all contain water, and they represent ecosystem capital – goods and services vital to human interests.

Groundwater Most percolates into the soil layers, moving downward to become groundwater, which makes up 1/5 of Earth’s freshwater supply and plays a key role in meeting human water needs. Groundwater is contained within aquifers, porous, spongelike layers of rock, sand or gravel that holds water. It is easy for us to understand the movement of surface water because we witness it all the time, but the role of groundwater in the water cycle is more difficult to visualize. Any precipitation reaching Earth’s land surface that does not evaporate, flows into waterways, or get taken up by organisms or infiltrates the surface.

An aquifer’s upper layer, or zone of aeration, is generally riddled with open spaces that contain both water and air. In the lower layer, or the zone of saturation, the spaces contain nearly all water. The boundary between them is the water table. Picture a sponge resting in a tray of water – the lower part of the sponge is completely saturated, while the upper portion may be moist but contains plenty of air in its pores. Any geographic area where water infiltrates Earth’s surface and reaches an aquifer below is known as an aquifer recharge zone.

Confined Aquifer (Artesian Aquifer) Exists when a water-bearing porous layer of rock, sand, or gravel is trapped between an upper and lower layer of less permeable substrate (often clay). In such a situation, water may be under great pressure because it is trapped between these layers.

Unconfined Aquifer A layer of less permeable substrate underlies unconfined aquifers. There is no upper layer to confine them. The water they contain us under considerably less pressure than that of a confined aquifer.

We use aquifers as a source of drinking water and of water to irrigate crops or to use in industry, pumping water from the aquifer using a well. As with any container of water, pumping from the aquifer empties it--or at least decreases the amount of water it holds. Aquifers are refilled, or recharged, in areas where they are exposed on the surface of the earth. Water can re-enter the aquifer in these recharge areas. In an artesian well, water rises from an underground water-containing rock layer under its own pressure. Rain falls at one end of the water-bearing layer, or aquifer, and percolates through the layer. The layer fills with water up to the level of the water table. Water will flow from a well under its own pressure if the well head is below the level of the water table.

Groundwater Aquifers in the United States hold 30 times more water than all U.S. lakes and rivers. Groundwater supplies almost 40% of freshwater in the U.S. Groundwater is not stationary. A typical rate of groundwater flow might be about 3 feet per day. Because of the slow movement, groundwater may remain in the aquifer for a long time. In fact, it could be ancient. The average age of groundwater has been estimated about 1400 years old.

Groundwater The Ogallala aquifer lies between Texas and N. Dakota. Pumping up water from this aquifer has made this arid region of the U.S. into some of the most productive farmland in the country. Removal of water from Ogallala has occurred at a faster rate than recharge rate resulting in a drop in water table and subsidence (sinking of the land) in some areas. Groundwater depletion will bring an end to this type of farming. Other areas of major subsidence include the San Joaquin Valley of California and Mexico City.

If water withdrawal is faster than recharge: Cone of depression.

Depletion of water in aquifers also leads to sink holes.

Consequence of Overdrawing water from aquifers.

Saltwater Intrusion Where aquifers open into the ocean, freshwater is maintained in the aquifer by the head of freshwater inland. Excessive removal of water may reduce the pressure, so that salt water moves into the aquifer.

NJ has had to deal with saltwater intrusions that were putting salt water into farm irrigation wells. Solution: Build surface water reservoirs for irrigating farmland.