WATER RESOURCE

any of the entire range of natural waters that occur on the Earth, regardless of their state (i.e., vapour, liquid, or solid) and that are of potential use to humans. Of these, the resources most available for use are the waters of the oceans, rivers, and lakes; other available water resources include groundwater and deep subsurface waters and glaciers and permanent snowfields. Human use of natural waters, particularly of freshwater resources, has increased steadily over the centuries. It is unlikely that this trend will change given the continued growth of population and the ever-widening utilization of water for agricultural, industrial, and recreational purposes. This situation has given rise to growing concern over the availability of adequate water supplies to accommodate the future needs of society. Surface-water resources are already being used to their maximum capacity in various regions of the world, as, for example, in the southwestern United States. Quantity of water is not the only concern. Overuse has resulted in the progressive deterioration of water quality. Seepage of mineral fertilizers (phosphates and nitrates), pesticides, and herbicides into surface and subsurface waters has not only rendered them unfit for human consumption but also disrupted aquatic ecosystems. Lakes and rivers also have been contaminated by the improper disposal of sewage, the discharge of untreated industrial wastes (including such toxicants as polychlorinated biphenyls, or PCBs), and the release of heated wastewater from nuclear-power plants and other industrial facilities, which results in thermal pollution and its attendant problems. Efforts are being made to curb the contamination of water resources. For example, regulatory action by the U.S. government to reduce phosphorous input into the Great Lakes has had measurable results, as has the implementation of improved waste-purification technology by certain municipalities in the region. The latter not only helps to restore water resources but also conserves the water supply by effective recycling. Advanced sewage-treatment facilities have made it possible to obtain potable water purer than most stream water. Projects to remove salt and other dissolved solids from brackish surface water as well as from seawater have been undertaken in such countries as Australia, Kuwait, and the United States. Water from desalination plants is generally suitable for household use and for irrigation. Other procedures employed for relieving water shortages include control of runoff and the reduction of evaporation by means of agricultural-engineering measures.