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Screening of Geohydrologic Environments in the Basin and Range Province for High-Level Radioactive Waste Disposal

Deeply buried repositories in specially constructed mines offer several properties suitable for disposal of high-level radioactive waste. Principal among these properties are adequate shielding, isolation from the environment, absorption and dispersion of heat generated by the radioactive waste, and protection from intrusion by man. The potential for migration of high-level radioactive wastes from the repository demands that attention be given not only to the selection of a suitable host material for the waste—the rock in which the repository is to be constructed—but also to the selection of the geohydrologic environment of the repository. Ideally, the geohydrologic environment best suited for a high-level radioactive wastedisposal site would have many natural barriers to waste movement. These barriers include the following: • Rock through which water moves

very slowly (low permeability); • Ground-water flow away from the

biosphere;
• Long flow paths to points readily

accessible to people;
Little water movement and great

thickness of the unsaturated zone

above the water table; • Low rainfall;

Strong host rock with few fractures;
Small probability of seismic or

volcanic activity; • Slow rate of erosion; • Ground-water chemistry favoring low

radionuclide solubility; and • High capacity for adsorption

(sorption) or ion exchange of waste

radionuclides. In view of the numerous combinations that are possible among the various natural barriers, the identification of suitable geohydrologic environments dictates that site selection for a highlevel radioactive waste repository be approached using comprehensive analysis. The process used by the U.S. Geological Survey is described below.

The Geological Survey's program for identifying potential environments suitable for locating acceptable repositories in the Basin and Range

province was an outgrowth of a plan developed jointly with the U.S. Department of Energy, which has the responsibility for selecting, building, and operating the repositories. The Geological Survey assists the Department of Energy by providing fundamental geohydrologic information to be used in solving the Nation's problem of selecting high-level radioactive waste disposal sites. Although this is a Geological Survey study, earth scientists from the particular States involved are participating actively.

The screening process consists of several stages. At each stage, it involves geologic and hydrologic description and evaluation of successively smaller land units. According to the plan, the Basin and Range province was divided into smaller land units that were ranked as follows: First, regions (103–105 square miles); second, areas (102-103 square miles); and third, potential sites (about 10 square miles).

Screening of the province and regions is based only on existing data. Field work and collection of new data are not part of the present study. A screening program based on nongeologic factors, such as socioeconomic considerations, is also important in choosing areas for further study to select a site for the disposal of radioactive waste. Such a program would be managed by the U.S. Department of Energy and would be used in conjunction with the geologic screening program described in this article in selecting areas for field investigations.

A Province Working Group, composed of earth scientists from the Geological Survey and several States, was established. The States that are formally participating in the province screening program are Arizona, California, Idaho, Nevada, New Mexico, Texas, and Utah. Oregon is kept informed of progress and has participated informally by reviewing geologic maps.

Basic factors selected for province evaluation were the distribution of potential host rocks, tectonic stability, and ground-water hydrology. Geologic

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information on the following:
• Ground-water flow;
• Potentiometric surface;
• Chemical quality of ground water;
• Natural ground-water discharge

areas;
Depth to water table. This

information is to be used to define
the thickness of the unsaturated
zone, an environment in which vari-
ous rock types might be acceptable

as host media; and • Water use.

The process of the screening of the region by the Province Working Group resulted in the identification of land units considered to be prospective for

containing geohydrologic environments for the isolation of high-level radioactive waste (A part of the figure). Of these land units, regions were selected for further study and are shown in the B part of the figure. The province phase of screening is discussed in detail in a series of three reports, released as U.S. Geological Survey Open-File Reports 83–759, 83-699, and 83–756, which are pending publication in the Circular series as 904-A, 904-B, and 904–C. Currently, the program is screening the regions shown in the figure. The evaluation of the regions and the identification of areas that are prospective for further study will complete the study.

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High Plains Regional Aquifer System Analysis, Phase II Activities

The High Plains is a 174,000-squaremile area of flat to gently rolling terrain east of the Rocky Mountains. It includes parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The High Plains is underlain by unconsolidated alluvial deposits that form a water-table aquifer capable of sustaining well yields of 100 to more than 1,000 gallons per minute. Irrigation from this aquifer has made the High Plains one of the Nation's leading agricultural areas.

Around 1940, a rapid expansion in the use of ground water for irrigation began in the southern High Plains; this expansion in use spread to the central High Plains in the 1950's and to the northern High Plains in the 1960's. By 1980, about 18 million acre-feet of water was pumped annually from about 170,000 wells drilled in the High Plains aquifer to irrigate about 13 million acres.

Although the High Plains is characterized by moderate annual precipitation ranging from 16 to 28 inches, recharge to the ground-water system is generally sparse, and most of the pumpage is from storage within the aquifer materials rather than from water being transmitted through the aquifer. As a result of these withdrawals from storage, widespread declines in water levels have occurred so that the aquifer has been dewatered by more than 50 percent of its saturated thickness in over 3,500 square miles in Kansas, New Mexico, and Texas. These water-level declines have increased the cost of water to irrigators because increased pumping lifts cause higher energy costs and have reduced well yields because of a decrease in the saturated thickness of the aquifer materials.

In 1978, The U.S. Geological Survey began a study of the High Plains aquifer as part of the Regional Aquifer System Analysis Program to understand the flow system and to evaluate the effects of the irrigation development on a regional scale. This study, completed in 1982, indicated that data on pumpage for irrigation and recharge to the

aquifer from irrigation return flow are essential to evaluate water-level declines due to irrigation development; these data, however, were not readily available. To develop a method of estimating these data with an acceptable confidence level, a follow-up project to the program, Phase II, was started in 1983.

Two areas, shown on the accompanying illustration, have been selected for these studies. One area includes Castro and Parmer Counties in the southern High Plains of Texas; the other includes Chase, Perkins, and Dundy Counties in the northern High Plains of Nebraska. The period of pumpage and irrigation return-flow analyses for each area is from 1974 through 1983—10 irrigation seasons. Irrigated acreage is being mapped in both areas for evennumbered years (and including 1983) using Landsat satellite imagery. Dry land and rangeland are to be mapped for the Nebraska area. The Landsat maps will be compared with observed or reported acreage maps, if information is available. The total 1983 pumpage was measured at 52 wells at the Nebraska test area and at about 86 wells in the Texas test area. Irrigated acreages are being mapped from Landsat images for 1974, 1976, 1978, 1980, 1982, and 1983 for the Nebraska and Texas areas. About one-half of the maps showing irrigated land have been completed.

Data on pumpage and irrigated acreage have been collected for about 50 irrigation systems in each area for the 1983 and 1984 irrigation seasons and will be used as the key information upon which pumpage for the entire test area will be estimated. In the Nebraska area, nearly all wells are equipped with inline volumetric flowmeters. These measured flow data also will be used to check the accuracy of flow data measured by portable flowmeters. In the Texas area, pumpage will be measured solely by portable flowmeters and timeof-operation sensors that will be calibrated by about 12 volumetric in-line flowmeters installed at selected sites. Using the data collected at these sites,

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