Page PLATE 1. Maps showing irrigated areas, well locations, and computed effects of irrigation and pumping, Yuma area, Arizona ___________________________ _____________________________ In pocket FIGURE 1. East-west cross section of Yuma area showing hydrologic model and analog representation ____ 12 2. Graphs showing computed change in ground-water flow across international boundaries, 1925-2000 6 3. Graph showing computed decrease in ground-water flow across international boundaries resulting from pumping in United States, 1965-2000 ________________________________________ 8 4. Profiles of change in water levels resulting from pumping, 1925-2000: cross section eastward from about Tecolote, Mexico __________________________________________ ___ 9
INTRODUCTION Ground water is among the Nation's most important natural resources. It is the source of drinking water for more than half of the total population and irrigation water for a multibilliondollar agricultural industry. Development of groundwater resources has steadily progressed during the last century, accompanied by a gradually shifting emphasis from exploration for new supplies to management of existing supplies. That shift has occurred partly because much is now known about die distribution, physical characteristics, and potential yield of aquifer systems (water-bearing rocks), and partly because the groundwater resource has become highly developed. The increased development in places has resulted in significant waterlevel declines, high pumping costs, and, in some cases, irreversible depletion of ground water. During the past decade, there has been a growing public awareness that ground water can be contaminated by toxic or hazardous industrial, domestic, and agricultural chemicals and byproducts. The need to define the extent, nature, and degree of contamination, as well as the effects of the contaminants, now dominates the national concern. The United States Geological Survey (USGS) is the principal source of scientific and technical expertise in the earth sciences within the Department of the Interior and the Federal government. Its activities span a wide range of earth-science research investigations and services in the fields of geology, hydrology, and cartography, and represent the continuing pursuit of a long-standing scientific mission. The USGS does not manage, regulate, or formulate policy related to ground water. As a result, it has been able, for more than 100 years, to provide water managers and policymakers technically sound and unbiased data and reports that describe the occurrence and quality of ground water. More than one-third of the Geological Survey's total resources are directed toward water-related programs. The hydrologic professional staff of some 1,700 hydrologists, geologists, engineers, chemists, and biologists (the largest such group in the United States) is currently conducting field investigations in the SO States, Puerto Rico, and the Trust Territories. These investigations are conducted at the "grass-roots" level. The workforce is located at four regional offices, three research centers, 43 District Offices that largely coincide with State boundaries, and more than 150 field offices. Total funding for USGS water-related programs in fiscal year 1987 was about $267 million. A little more than half of this amount came from directly appropriated funds, while the
Instrumentation and theory__________________________________ Calibration and zones of investigation______________________ Borehole effect__________________________________________._ Mechanism of electrical conduction in earth materials._____________ General lithologic interpretation...........______________________ Interpretation based upon the relative magnitudes of the normal curves.. _ _______________________________________ 11 Interpretative practices in the oil industry.___________________
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