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Water-bearing formations and their geographic distribution. -The United States an be divided into four regions with respect to ground water, that is, the water hat supplies the springs and wells (1) the East-Central old-rock region, (2) the .tlantic and Gulf Coastal Plain region, (3) the Great Plains region, and (4) the Western Mountain region. The East-Central region includes about one-third of the area and two-thirds f the population of the country. Most of the region is underlain by Paleozoic )cks, including water-bearing sandstones and limestones and unproductive shales; le deep water from these rocks is highly mineralized. Parts of this region are nderlain by Pre-Cambrian or Triassic rocks that yield small supplies. The glacial rift, in the northern part, and the glacial outwash sands and gravels beyond the rift border yield numerous large water supplies. The wide and well populated coastal plain bordering the Atlantic Ocean and ulf of Mexico is underlain by Cretaceous, Tertiary, and Pleistocene formations, eluding many beds of sand and limestone that yield numerous supplies of water, .any of which are large. The semiarid or subhumid plains region lying east of the Rocky Mountains conins extensive Tertiary and Pleistocene deposits of water-bearing sand and gravel id Cretaceous sands that yield highly mineralized artesian water, especially in orth Dakota and South Dakota; also water-bearing glacial drift in the north and oductive Permian limestone in the Roswell artesian basin in New Mexico. In .e areas of Cretaceous shale the water supplies are scarce. The Western Mountain region, which occupies about one-third of the area the country, is chiefly arid and contains extensive tracts in which water supies are scarce. Its principal water-bearing beds are the sand and gravel in e Pleistocene or Tertiary valley fill, which in some parts, especially in Calirnia, yield very large supplies. In some places in the Northwest the Tertiary Icanic rocks yield much water to springs and wells. Water-table conditions. The water supplies of the springs and wells in the lited States are largely derived from formations that have essentially waterble conditions. These are the surficial deposits of sand and gravel; the sand)nes and limestones in their outcrop areas; the extrusive volcanic rocks of the arthwest; and the dense igneous and metamorphic rocks and clays and shales, lich are commonly rendered somewhat permeable by weathering near the rface. As these formations are extensively exposed at the surface, their supes are readily replenished by the water from rain and snow. The largest pplies of water come from the deposits of sand and gravel, and these may grouped as follows: (1) Glacial outwash from the continental ice sheets occur.g from the Atlantic to the Pacific, (2) Valley fill in the Western Mountain 'ion (in part artesian), (3) Tertiary and Quaternary deposits in the Great lins region, and (4) Tertiary and Quaternary terrace and lowland deposits the Atlantic and Gulf Coastal Plain. 157 158 CONTRIBUTIONS TO HYDROLOGY, 1938-39 Artesian conditions. The principal artesian systems in the United States ar< as follows: (1) the extensive Paleozoic artesian system occupying a large part o the East-Central region, in which the shales confine water under artesian pressur in the sandstones and limestones; (2) the small but productive Roswell artesia] basin, in New Mexico, in which cavernous Permian limestone is the artesian for mation; (3) the artesian system formed by the entire Atlantic and Gulf Coasts Plain, in which Cretaceous and Tertiary strata dip toward the sea, and th water in the sands and limestones is held under artesian pressure by the interbedde' shales; (4) the Cretaceous artesian system of the northern and central parts c the Great Plains region, in wljich water is confined in the sandstones, under grea artesian pressure, by the thick, dense, overlying shales; (5) numerous small an> imperfect artesian systems in the glacial drift; and (6) numerous artesian system in the valley fill of the Western Mountain region, the largest of which are in th San Joaquin Valley, in California, and the San Luis Valley, in Colorado. In most areas of artesian flow the pressure has greatly diminished, and larg supplies are generally obtained by pumping. Discharge of ground water through springs and by evaporation and transpiration.In the relatively humid eastern part of the United States, the streams are generall fed by numerous branches that receive effluent seepage at many points, excef in the limestone terranes, in which the springs are larger but less numerous. I the arid and semiarid western parts of the country, the recharge and hence also tl discharge of ground water are reduced in even greater proportion than the precip tation, as compared with the humid East, and the discharge is largely by transpirs tion of the plants known as phreatophytes. According to available data, there are in the United States 65 springs of tl first magnitude, that is, springs yielding more than 100 second-feet. Of these, J rise in volcanic rock or associated gravel in the Northwest; 24 rise in limeston chiefly in Florida, Missouri, and Texas; and 3 rise in sandstone in Montan Silver Spring, Florida, which is the largest limestone spring in the country, has s average discharge of 808 second-feet. The springs that issue from volcanic roc and associated gravel along a 50-mile stretch of Snake River, Idaho, have an aggr gate discharge that averages 5,085 second-feet. In a recently published report (Water-Supply Paper 679-B) are listed 1,0. thermal springs or spring localities, all of which are in the Western Mounta region except 52 in the East-Central region, and 3 in the Great Plains regie (Black Hills). Yellowstone National Park exceeds all other areas in the abu dance of springs of high temperature. Nearly two-thirds of the thermal sprin issue from igneous rock, chiefly intrusive magmas, deriving their heat large from magmatic sources and their water largely from surface sources. Those the eastern part of the country are largely due to artesian structure. There are in the United States 23 known periodic or ebbing and flowing spring of which 9 are in Virginia and 4 in Missouri. All or nearly all are in limestone. Utilization of ground water. About 10,000 communities, with about 75,000,0 inhabitants (1930 census), have public waterworks, of which about 6,500 coi munities, with about 20,000,000 inhabitants, are supplied with ground wai from wells. The total yielded by wells for public supplies is estimated at 2,001 000,000 gallons a day. One of the greatest achievements in American history has been the develc ment of water supplies of good quality for domestic use, that is, for drinkii cooking, and laundry and toilet purposes. A very large proportion of the 75,00' 000 people in communities served by public waterworks have ample and perenn supplies of safe water delivered under pressure. About 47,000,000 people livi in several thousand small communities and on about 6,500,000 farms have priv£ water supplies, chiefly from wells, supplemented in hard-water areas with ra: GROUND WATER IN THE UNITED STATES 159 pater cisterns for toilet and laundry uses. Much progress has been made in mprovement of supplies from private wells but much remains to be done, largely hrough advances in well drilling practices. Water supplies for livestock are partly the same as the domestic supplies but nany additional supplies have been developed to make it possible for the live,tock to utilize all the available forage. This has been done chiefly by sinking veils but also by improving springs and building small reservoirs. Numerous large and small private water supplies have been developed from veils for industries and institutions in areas not reached by public waterworks ind also to a considerable extent in the large cities. Over 2,100,000 acres was irrigated in 1929 with water from wells, about two-hirds of which was in California. Only 2 or 3 percent of this acreage was irrigated with water from flowing wells, the rest from pumped wells. The area rrigated with water from wells is about 11 percent of the total irrigated area, iut the value of the product is proportionately greater because it consists largely )f fruits and vegetables. About twice as much water from wells is used annually 'or irrigation as for public supplies. In the United States there are some well-equipped health resorts at watering places and many that are only modestly or poorly equipped. A total of 184 ;hermal springs are reported to be used for resorts, including sanitariums. In L923 about 45,000,000 gallons of natural waters were sold, from 433 commercial springs, for table and medicinal uses. The American people rank high in the personal use of water, both external and internal. From a broad viewpoint, they ioubtless rank higher than is generally assumed in its therapeutic use. Methods and results of ground-water investigations. The ground-water investigations have been of two principal kinds (1) systematic areal surveys and (2) special investigations, which are generally more intensive and contribute to a Tiore detailed knowledge of hydrologic principles. It has not been possible to ;arry out a uniform ground-water survey of the entire country, but more or less standardized methods are applied as opportunity is afforded. The intensive studies have related largely to the quantities of ground water ;hat can be obtained from underground storage and from recharge. Quantitative nethods are grouped as intake, discharge, storage, and transmission methods. Water-bearing formations are studied with respect to their functions as reservoirs ind as conduits. Studies have been made of the infiltration of rain and stream water, the retention of the water in the root zone, and its transmission to the water table; the rate of discharge of ground water by springs, effluent seepage, evaporation, and transpiration fro

Ground water in the United States, a summary of ground-water conditions and resources, utilization of water from wells and springs, methods of scientific investigation, and literature relating to the subject