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Use of the Geohydrologic Environment for High-Level Radioactive Waste Disposal
By George Dinwiddie
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 accessible environment, absorption and dispersion of heat generated by the radioactive waste, and protection from intrusion by man. The potential for migration of highlevel radioactive wastes from the repository demands that attention be given not only to selection of a suitable host material for the waste-the rock in which the respository is to be constructed-but also to selection of the geohydrologic environment of the repository. Ideally, the geohydrologic environment best suited for a high-level radioactive waste disposal site would have all of the 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; . Slow rates of ground-water movement; Long flow paths to points readily accessible to humans; Deep 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 high-level radioactive waste repository be approached using systems analysis. The process used by the U.S. Geological Survey is described below.
The Geological Survey’s program for identifying environments potentially suitable for locating acceptable repository sites 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’s effort, one of its most important applied studies, is aimed at assisting in solving the Nation's dilemma in selecting high-level radioactive waste disposal sites. Although this is a Geological Survey study, there is active participation by earth scientists from the particular States involved.
The screening process consisted initially of identifying 11 provinces in the centerminous United States (see fig. 1) for evaluation. According to the plan, a province is successively divided into smaller land units which are ranked: first, regions (103-105 square miles); second, areas (102-103 square miles); and, ultimately, potential sites (about 10 square miles). At each stage, the screening process involves geologic and hydrologic description and evaluation of the land units with respect to pre established guidelines for radioactive waste isolation.
Initial screening, province evaluation, is based only on existing data. Field work and collection of new data are not part of the first phase of study. One of the most important criteria in the screening process is the occurrence of suitable host rocks. The primary factors in selecting a host rock are mineability, thermal conductivity, frequency and extent of permeable fractures, permeability, thickness, areal extent, depth of burial, homogeneity, sorption capacity, and other geochemical properties.
The principal candidates presently being considered are crystalline rocks, such as granite and gneiss; salt formations, either bedded salt or salt domes; basalt; tuff; argillaceous (clayey) formation; and unsaturated Valley-fill sediments. These candidate host rocks are distributed widely, though unevenly, throughout the centerminous United States.
The other most significant factor considered in characterizing a province is ground-water flow systems. Assuming that a repository will be located in an area unlikely to be affected by earthquakes, volcanoes, and so forth, and that the risk of exhuma
tion of wastes by erosion, catastrophic event (for example, meteorite impact), or intrusion by man is minimal, it is generally acknowledged that the single most likely means by which radionuclides could reenter the biosphere is by way of ground water. Definition of these flow systems is one of the most basic steps in hydrologic characterization of a province, and boundaries between the various flow systems are the basis for the division of a province into regions.
The components most basic to delineating ground-water flow systems are potentiometric head (the level to which water will rise in a tightly cased well that taps a particular rock unit), geology, geochemistry and temperature of ground-water recharge and discharge zones, and water quantities of water inflow, storage, and outflow. Potentiometric head is usually the most significant of these factors in defining ground-water flow within a system and in delineating system boundaries. Geology is necessary to defining ground-water flow systems because the rocks are the framework within which the system operates. Geochemistry and temperature of the ground water are indicators of the history of the ground water before it reaches the observation point; they may also control leaching and chemical reactions of waste nuclides.
The Basin and Range province (province 9 in fig. 1) was selected for a prototype study to determine the feasibility of the screening procedure. This province appeared to be suitable for such a study because it offered a large variety of geologic conditions and because the Geological Survey has been studying the geology of the province for many years.
A Province Working Group, composed of earth scientists from the Geological Survey and the States, was established. The States that are formally participating in the province screening program are Arizona, Idaho, Nevada, New Mexico, Texas, and Utah. Oregon is kept informed of progress, and California is participating informally.
Province screening was based initially on information available from published reports and files. It was recognized that availability of data would be inconsistent throughout the province and that lack of information might preclude evaluation of some parts of the province. Coincidentally, it was recognized that all available geologic and hydrologic data could not be compiled and assimilated during the first phase of province evaluation.
Basic factors selected for province evaluation were the distribution of potential host rocks, tectonic stability, and ground-water hydrology. Geologic data to be evaluated include information on the following:
Distribution of rock types considered to be potential host rocks. Surface outcrops of basalt, granitic rocks, argillaceous rocks, tuff, salt, and anhydrite were mapped. In addition, data on subsurface occurrences of salt and anhydrite were compiled;
' Quaternary volcanism;
' Quaternary faulting;
' Heat flow; and
' Mineral resources.
Hydrologic data to be evaluated include in
formation 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 various rock types might be acceptable as host media; and
' Water use.
The Basin and Range province has been subdivided into regions, and those considered most suitable will be described and evaluated. Initial evaluation of characteristics of the Basin and Range indicates that about 40 percent of the province appears to be worthy of further attention. The second phase of evaluation will focus more on the identification and evaluation of specific geohydrologic barriers in the subsurface. This effort will result in identification of areas where further more intensive study appears warranted.
As mentioned earlier, depth to water (or thickness of the unsaturated zone) is one factor considered in screening the Basin and Range province. Disposal of high-level radioactive waste in thick and relatively dry unsaturated rock units, principally valley fill and volcanic tuff, appears to offer an economical and workable alternative to burial in deep or saturated geologic formations. The Basin and Range province has some of the thickest accumulations of unsaturated materials in the United States and
a favorable arid to semiarid climate. Several advantages of burying waste at somewhat shallower depths in unsaturated material rather than in deep or saturated environments include the following:
' Low rate and volume of water movement through unsaturated materials under present climatic conditions;
' High absorptive capacity of most Valley-fill sediments and some tuffs;
' Inhibition of vertical movement of water by properly enginering the emplacement of materials used for overpack and backfill;
Minimal water-related problems during construction of the repository after sealing it; and
' Ease of monitoring or removing of wastes from a shallow repository.
The national screening program for identifying potentially suitable disposal sites allows for a systematic consideration of the effects of waste disposal by focusing geologic and hydrologic expertise on the problem at the primary stage. This type of systematic approach assures not only that the most suitable areas will be considered but also that potentially suitable areas are not likely to be overlooked. Although the systematic nature of the screening program probably precludes its contributing significantly to selection of the site for the first repository, the ongoing program and the program methodology should be useful in the selection of subsequent sites. A screening program based on non geologic factors, such as socioeconomic considerations, is also important in choosing areas for further study to select a site for disposal of radioactive waste. Such a program will be managed by the U.S. Department of Energy and its contractors for the Basin and Range province.
The National Digital Cartographic Data Base
By Sheila E. Martin
Digital cartography provides a new computer-based means to support the management of the Nation's natural resources. To do this effectively requires collecting data from a wide variety of sources. With increasing frequency, this information is analyzed using computer models.
Information from U.S. Geological Survey topographic maps is fundamental in resource studies that require geographic control, terrain elevations, boundary portrayal, and stream and highway patterns. Responding to this need, the Geological Survey has developed a considerable program involving cartographic and geographic information in digitial form.
Computer technology and recent advances in computer graphics have dramatically changed our concepts of collecting and communicating map information. Digital cartography will soon surpass the major advances in mapping that resulted from developments in printing and photography during the late 19th century and in aerial photographic surveying and photogrammetry in the early 20th century. Maps produced by automatic plotters using digitial data are equal or superior in accuracy and graphic quality to those produced by manual techniques. Automated techniques can use the same data at varying graphic scales and formats and can provide for a wide variety of applications. Automation is less expensive.
Although the method of portraying terrain information in the form of a topographic map will still be used for many decades, the format and portrayal of map information are changing. Images presented on the cathode ray tube, where scale and viewing direction can be rapidly changed, have already replaced traditional printed maps for many applications. In applications such as weather reporting, where up-to-the-minute data are necessary, high-resolution images can be obtained instantly through commands to an orbiting satellite.
Recognizing that traditional cartographic methods would someday be replaced by automated techniques, the Survey began in the early 1970's to conduct research in digital cartography and automated mapmaking techniques. By 1977, the Survey had started to focus on the development of a national cartographic and geographic digital data base, a major component of the present Digital Cartography Program.
The Survey has digitized considerable data from 1:24,000-scale maps, but, because there are over 54,000 different 1:24,000-scale maps of the conterminous United States, it was realized that digitizing all these maps would exceed the resources available. (In addition, many requirements appeared for data covering large areas that could not be satisfied by existing small-scale data bases.) Therefore, the decision was made in 1978 to proceed with a national small-scale digital cartographic data base.
The first phases of the project have recently been completed. The 21 sectional maps of the National Atlas of the United States of America, at a scale of 1:2,000,000 (1 inch equals about 32 miles) portray a relatively high level of detail for this small scale (fig. 1). These maps are now available in digital form through the National Cartographic Information Center. Applications of the data for automated map production and computer analysis have begun.
Earlier Data Bases
Four earlier small-scale digital cartographic data bases have been used extensively. The Dahlgren data base was developed in the early 1960's. It is a global data base of coastlines and international boundaries digitized mostly from 1:12,000,000-scale maps using 8,300 coordinate points. The coastlines and the State boundaries of the United States were obtained from
Figure 1. —lndex map show
ing location of 21 areas of 1:2, 000, 000-scale digital cartographic data available on computer tapes for the United States.
1:1,000,000-scale maps. The data provide a general portrayal of the coastlines, countries, and States.
Another data base was developed in the mid-1960's by the Department of Transportation as a county boundary file of the United States. County and State boundaries, together with the coastline, were digitized from 1:5,000,000-scale maps using 115,000 coordinate points.
The World Data Bank 1 was developed by the Central intelligence Agency in 1966. It contains world coastlines and international boundaries digitized from 1:12,000,000-scale maps using 100,000 coordinate points; the United States coastline is described by 20,000 points.
A second World Data Bank was completed in 1977 using source maps which ranged from 1:4,000,000 to 1:1,000,000 in scale. This data base contains country and State boundaries, coastlines, islands, lakes, rivers, and selected roads and railroads. The number of digitized points increased dramatically to 6 million. Source materials for the United States were digitized from 1:3,000,000-scale maps using 1.5 million points.
Some of the limitations associated with these earlier data bases are:
' The very small scale of the source maps used for digitization;
' The limited range of features digitized;
' The lack of current information;
' Data formats designed primarily for graphic applications;
' Limited flexibility for combining these data with other thematic data available in digital form.
Experience with these earlier data bases has shown that, while they meet their objectives, their content is not sufficient for all user applications (fig. 2).
The U.S. Geological Survey Small-Scale Data Base
The Survey’s small-scale digital cartographic data base was prepared from an updated National Atlas of the United States of America. For comparison, World Data Bank l contained about 100,000 points and World Data Bank II had about 6 million points to describe the entire globe, while the Survey’s small-scale data base has some 7 million points to describe the United States alone.
The content of the data base includes political boundaries (State and county level), Federal lands, transportation networks (roads and railroads), hydrographic features (streams and water bodies), and populated places. These data were entered into the computer in a format that records both the location of a feature and its relations with similar features on the source map. This format allows graphic applications, such as drawing streams and roads for automatic map plotting, as well as analytical applications, such as area calculations and verifying the data for consistency and accuracy. The format also permits automatic smoothing to