Kirk‐Othmer Encyclopedia of Chemical Technology

Abstract

replenished.Water lying in deep strata for millions of years is being mined like other minerals, never to be replaced.In Libya, oil drilling found a lake 100 m below the dry sand, hundreds of square kilometers in area, and 750 m deep (3).It has been estimated that this lake will supply irrigation of 800 km 2 for 300 yr, but the pumping of this water is as final an act as the pumping of Libya's petroleum, which probably dates from the same lush geological era.Once pumped, neither resource can be replenished. Transport of FreshwaterFor centuries, containers have been used to carry freshwater, usually for longer distances than would be practical for conduits.Trucks and railways have used tanks.Ships have carried and still do carry water halfway around the world in ballast tanks; tankers that otherwise would be returning empty from oil deliveries may make the return voyage filled with freshwater as precious to the oil-rich, water-poor country as the oil is to its market.Systems of dams, canals, and aqueducts were developed to carry freshwater considerable distances to growing cities, and to irrigate agricultural lands.Records of ancient Rome indicate that 14 aqueducts, averaging over 144 km in length, carried 1.175 × 10 6 m 3 /d of water from the surrounding highlands by gravity.The Romans depended on gravity flow in open channels, apparently with little knowledge of predicting the friction losses in pipes, even though Pliny lists standard lead pipe in circumferences up to ca 2.5 m.The aqueducts of Istanbul are even more dramatic.The engineers' competence developed with increasing needs for water, and conduits became longer.In more modern times, New York City's water system, initiated ca 135 yr ago, stands comparison as a true engineering marvel.Farsighted action in the late nineteenth century also gave the city extensive upstate watershed rights.This system, with a storage capacity of 2.07 × 10 9 m 3 , safely furnishes about 8.5 million people with an average of 5.3 × 10 6 m 3 /d of water.The system is not adequate today, but it has served well except in years of serious drought.Water enters the city via two tunnels, one built in 1917 and the other in 1937.The projected cost of a third tunnel was estimated at $3.5 × 10 9 in 1981 dollars, but could end up being as high as $5 -6 × 10 9 by the time the tunnel is completed, with the first two stages currently scheduled for completion in 2010 (4).Increased population in the vicinity of the water supply reservoirs, a watershed approximately the size of the state of Delaware and as far as 160 km from the city, is increasingly threatening the water quality there.The city has therefore taken steps in the recent years to establish a comprehensive watershed protection program.The 1974 Safe Drinking Water Act put all public water supplies in the United States under Federal supervision.For example, the 94th Congress authorized a six-state water study, the High Plains Study Council, one of several such studies conducted in past years, to develop a set of plans for increasing water supply in a region which supplies about 15% of the nation's wheat, corn, sorghum, and cotton, and 38% of its livestock, and which depends on its rapidly dwindling groundwater (the Ogallala aquifer), mindful of the Dust Bowl drought disaster of the 1930s (5).In 1990 it was estimated that the aquifer would be practically depleted by the year 2020.It was proposed that huge amounts of water from the Missouri and Arkansas Rivers be diverted to the High Plains, with an estimate by the Army Corps of Engineers that it would take as long as 9 yr to design, 25 yr to build, and would cost $6 -25 × 10 9 in 1982 dollars.Rising energy costs, costs of construction, local objections to Federal intervention, environmental and navigation considerations, and various other problems have prevented this and many other large water supply programs from materializing.The problem of bringing water to southern California continues to be one producing controversy.The region is essentially a desert, devoid of any significant water resources, yet its population is growing at a fast pace: in 1990 it stood at 30 million and is expected to reach 49 million in 2020, with half of the increase expected to occur in the arid South Coast region (6).Water is transported from northern California and from the Colorado River, at distances of up to about 1000 km, expected to increase to about 1600 km by the year 2000, often against the objections by those living around these sources.The project to build the so-called Peripheral Canal, a 67-km