Page 297 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water resources 261
An additional problem arises when aquifers are overpumped near the ocean or seacoast. Along a coast- line, fresh groundwater and salty seawater establish a natural interface, or contact surface, with the less-dense freshwater flowing on top. But excessive withdrawal of freshwater can cause this interface to migrate inland. As a result, wells near the shore become contaminated with saltwater, and the aquifer becomes useless as a freshwa- ter source (GIA 9.2, number 13). Pumping freshwater back into the aquifer may halt seawater intrusion, but once contaminated, the aquifer is difficult to reclaim.
Groundwater Mining The utilization of aquifers be- yond their flow and recharge capacities is known as groundwater mining. A major area of groundwater use in Canada is the Regional Municipality of Waterloo in Ontario—the largest urban groundwater municipal- ity in Canada. Here, over a half a million people rely on groundwater for municipal supplies. The depletion of groundwater aquifers has led to extensive conservation efforts and exploration of other water sources.
In the United States, chronic groundwater overdrafts occur in the Midwest, West, lower Mississippi Valley, and Florida and in the intensely farmed Palouse region of eastern Washington. In many places, the water table or artesian water level has declined more than 12 m. Groundwater mining is of special concern for the mas- sive High Plains Aquifer, discussed in Focus Study 9.1.
About half of India’s irrigation water needs and half of its industrial and urban water needs are met by the groundwater reserve. In rural areas, groundwater sup- plies 80% of domestic water from some 3 million hand- pumped bore holes. In approximately 20% of India’s agricultural districts, groundwater mining through more than 17 million wells is beyond recharge rates.
In the Middle East, groundwater overuse is even more severe. The groundwater resources beneath Saudi Arabia accumulated over tens of thousands of years, forming “fossil aquifers,” so named because they re- ceive little or no recharge in the desert climate that exists in the region today. Thus, increasing withdraw- als in Saudi Arabia at present are not being naturally recharged—in essence, groundwater has become a nonrenewable resource. Libya’s water supply comes mainly from fossil aquifers, some of them 75000 years old; an elaborate system of pipes and storage reservoirs in place since the 1980s makes this water available for the country’s population. Some researchers suggest that groundwater in the region will be depleted in a decade, although water-quality problems are already apparent and worsening. In Yemen, the largest fossil aquifer is down to its last years of extractable water.
Desalination In the Middle East and other areas with declining groundwater reserves, desalination of seawater is an increasingly important method for obtaining freshwater. Desalination processes remove organic compounds, debris, and salinity from seawater,
brackish (slightly saline) water found along coastlines, and saline groundwater, yielding potable water for do- mestic uses. More than 14000 desalination plants are now in operation worldwide; the volume of freshwater produced by desalination worldwide is projected to nearly double between 2010 and 2020. Approximately 50% of all desalination plants are in the Middle East; the world’s largest is the Jebel Ali Desalination Plant in the United Arab Emirates, capable of producing 300 million m3 · yr21 of water. In Saudi Arabia, 30 de- salination plants currently supply 70% of the coun- try’s drinking water needs, providing an alternative to further groundwater mining and problems with salt- water intrusion.
In the United States, especially in Florida and along the coast of southern California, desalination is slowly increasing in use. Around Tampa Bay–St. Petersburg, Florida, water levels in the region’s lakes and wetlands are declining and land surfaces subsiding as the ground- water drawdown for export to the cities increases. The Tampa Bay desalination plant was finished in 2008, after being plagued with financial and technical problems during its 10-year construction. The plant met final per- formance criteria for operation in 2013, making it fully operational.
The Carlsbad Desalination Project, including a de- salination plant and a water-delivery pipeline, near San Diego, California, will be the largest in the United States when it is finished in 2016. The plant will use reverse osmosis, a process that forces water through semiper- meable membranes to separate the solutes (salts) from the solvent (water), in effect removing the salt and cre- ating freshwater (see carlsbaddesal.com/pipeline for more information).
Desalination of saline aquifers has also increased in Texas over the past decade. The amount of brackish groundwater beneath the surface is estimated at 3.3 trillion cubic metres, and 44 desalination plants already tap those reserves (none of the plants desalinizes seawater). The largest of the plants is in El Paso, Texas. More projects are in the works as the recent severe drought throughout the state continues. Drawbacks to desalination are that the process is energy-intensive and expensive, and con- centrated salts must be disposed of in a manner that does not contaminate freshwater supplies.
Collapsing Aquifers A possible effect of removing water from an aquifer is that the ground will lose in- ternal support and collapse as a result (remember that aquifers are layers of rock or unconsolidated material). Water in the pore spaces is not compressible, so it adds structural strength to the rock or other material. If the water is removed through overpumping, air infiltrates the pores. Air is readily compressible, and the tremen- dous weight of overlying rock may crush the aquifer. On the surface, the visible result may be land subsid- ence, cracks in foundations, sinkholes, and changes in surface drainage.