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These properties and many others distinguish one kind of water from another. To speak of the quality of water is to describe its individuality. Today, much is heard about water quality. In a practical sense this term means those traits that are distinctive to a body or supply of water in relation to some use for example, drinking, irrigation of croplands, manufacturing, food processing, or recreation. Water quality is relative. Rapid-forming deposits in hot-water tanks and in tea kettles plague people who must use unsoftened water. But if the water is too soft, the coffee doesn't taste right. On vacation, you may have trouble adjusting to a new water supply if its quality is different from that back home. Even though a domestic water supply is satisfactory in all respects for drinking purposes, distilled water is preferred for use in automobile storage batteries. Why is this so? Because living cells accept, in fact require, certain minerals in the water, but for battery cells the opposite is true. "Quality" then must be associated with the intended use of the water and has many meanings. No one definition means the same thing to all people. Water quality is influenced by natural factors and by the activities of man. Before considering these effects upon water, you should know about the structure and unusual properties of water as a chemical compound. In 1934 the concept of the water molecule had to be changed because another kind of hydrogen was discovered in water's structure. This new substance, called deuterium, is twice the weight of the normal hydrogen atom. The symbol "D" is assigned to deuterium, which also combines with oxygen in the ratio of two to one to form DzO. A common name for D2O is "heavy water." Later, still another kind of hydrogen, called tritium, was discovered. Tritium ("T") is three times the weight of ordinary hydrogen and combines with oxygen to form T2O. Pure water contains very small quantities of D2O and T2O in proportion to H2O. Deuterium and tritium are isotopes of hydrogen, that is, they are varieties of the same element, similar in chemical behavior, but differing in weight. Hydrogen, deuterium, and tritium are like brothers who are triplets. The three brothers have about the same behavior patterns and personality traits, but one boy weighs only 100 pounds, the second boy weighs 200 pounds, and their brother, the heaviest of all, weighs 300 pounds. The structure of water is further complicated by the fact that oxygen, like hydrogen, has three isotopes. The combining of the hydrogen and oxygen isotopes in the ratio of two to one results in a mixture of 33 different substances, some made up of distinct molecules and some of which exist only as electrically charged parts of molecules, called ions. More will be said about ions in the discussion of water as a solvent. The common formula for water, H2O, need not be rejected, because pure water is virtually what this formula says it is. It is important to remember, however, that water is not a single, simple compound. Actually, less is kn...

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T.he occurrence of abundant groundwater supplies in the lower Platte Biver Valley has made possible the present agricultural and industrial economy of the area. Likewise, the future development of the area is dependent on the wise use of this important resource. The current investigation, on which this report is based, is a necessary step in the planning for the greatest ultimate utilization of the water resources in the lower Platte River Valley. The area covered by this study is the floor of the lower Platte River Valley between North Platte and Fremont and embraces about 2,500 square miles. The entire valley floor is underlain by unconsolidated Pleistocene sediments which consist of clay, silt, sand and gravel and range in thickness from less than 20 feet to nearly 200 feet. Westward from Cozad these sediments were deposited in a valley entrenched in bedrock, but east of Cozad they are continuous with similar deposits which underlie the adjacent uplands. Bedrock formations of Tertiary age are in contact with the basal Pleistocene sediments from the west end of the area to about Central City. Fram Central City eastward, formations of. Cretaceous age immediately underlie the Pleistocene deposits. INTRODUCTION 3 in this area will be required to evaluate the effects of the re-use of drainage waters and to provide information to~ salinity control. Groundwater supplies are drawn upon extensively tor irrigation purposes, tor municipal supplies, and fo~ rural, domestic and stock supplies. Nearly 4,000 irrigation wells are known to exist in the area, the greatest concentrations of these being in Dawson, Buffalo, and Hall Counties.

A study of the geology and the groundwater conditions of the Helena Valley, in the west-central part of Montana, was made during the summer of 19*^80 The purpose of the study was to gather data on the occurrence of ground water and its relation to present and proposed irrigation in the area. With such data one may better predict the effects of the application of irrigation water to additional lands in the area. 2 GEOLOGY AND GROUNDWATER RESOURCES OF THE HELENA VALLEY, MONTANA Ground vaters in the valley contain moderate amounts of dissolved solids, the predominant ions being calcium, bicarbonate, and sulfate, Little difference is discernible in the composition of waters in the Quaternary and Tertiary deposits; the dilute nature of these waters indicates that the valley fill is derived from formations containing only small amounts of soluble minerals. The quality of the ground water appears to be little affected by recharge of the groundwater reservoir from return irrigation flows. Waterlogged areas may in time show an increase in mineralization as a result of evaporation. Both ground and surface waters are classified as "excellent to good" on the basis of Wilcox's criteria for irrigation waters. Boron, percent sodium, and dissolved solids are relatively low. Waters used for drinking or domestic purposes are generally considered satisfactory except, in some instances, for hardness. INTRODUCTION Purpose and Scope of Investigation The purpose of this investigation was to gather data on the occurrence, conditions, and potentialities of the ground water in the Helena Valley. Special consideration was given to the source of the ground water, to its direction of movement, to the fluctuations of water levels in wells, to the depth to the water table below the land surface, to the available supply, and to the present and potential extent of waterlogging and means for alleviating it. This study was a part of the program of the Interior Department for development of the Missouri River basin, and it is directly related to the proposed Canyon Ferry project which calls for diversion of water from the Missouri River into the Helena Valley to irrigate about 10,000 acres of land not now irrigated and to furnish additional water to about 5*000 acres of land already irrigated. This report is based principally on field work done by Howard W. Lorenz between July and early November 19^8. However, water-level measurements made during April, May, and June, 19^8, by the Bureau of Reclamation are incorporated in this report. The geology of the area was mapped on aerial photographs, and the field data were later transferred to a base map by use of a sketchmaster. An inventory was made of all wells in the area; water levels in observation wells were measured periodically; and a watertable contour map was constructed from instrumentally determined altitudes of water levels in wells. The field investigation was under the general supervision of A. N. Sayre, chief of the Ground Water Branch of the Federal Geological Survey, and G. H. Taylor, re...