WYOMING SOUTH DAKOTA IOWA NEBRASKA COLORADO KANSAS NEW MEXICO ALASKA 3 WISCONSIN MISSOURI TENNESSEE 12 YORK PENNSYLVANIA WEST VIRGINIA 11 VIRGINIA NORTH CAROLINA. SOUTH CAROLINA GEORGIA MISSISSIPPI ALABAMA 6 LOUISIANA Figure 1. The 50 States, Puerto Rico, and the Virgin Islands have been separated into 13 regional areas. Each area is described in a chapter of the Ground Water Atlas of the United States (Puerto Rico and the Virgin Islands not shown). The Atlas is written so that it can be readily understood by readers who are not hydrologists. Therefore, it will be useful as a teaching tool in colleges and universities and as an introduction to ground-water hydrology for those in Federal and State agencies, the Congress, and State Legislatures who need to understand ground-water occurrence, movement, and quality. Atlas Design.-The complete color Atlas will have 14 chapters, 13 of which describe regional areas that collectively cover the 50 States, Puerto Rico, and the Virgin Islands (fig. 1). The introductory chapter to the overall publication will be the last chapter written. Two objectives used in delineating the regional areas are (1) to keep each area at a size within which all the principal aquifers can be shown at a reasonable map scale and (2) to contain the most important regional aquifer system or systems entirely within an area. Minor aquifers and minor features of the geology and hydrology of the principal aquifers are not shown because of the scale of the maps. Maps in the Atlas are supplemented by charts, cross sections, hydrographs, block and pie diagrams, and photographs that describe the location, geology, and hydrology of the principal aquifers of the United States. The scales of the maps vary from nationwide-sized maps that illustrate problems, such as saltwater encroachment or land subsidence caused by ground-water pumping, to maps of specific aquifers, such as the Biscayne aquifer that extends over only a three-county area in southeastern Florida. Figure captions describe in simple language the principal features of each illustration. Also, pertinent references are listed for each described aquifer for readers who need detailed information. Each of the 13 descriptive chapters will be published as a separate Hydrologic Investigations Atlas (HA). The first chapter of the Atlas to be published, "Hydrologic Investigations Atlas 730-G," was issued in 1991 and includes aquifers in Alabama, Florida, Georgia, and South Carolina. After all 13 reports are published, the introductory chapter will be written and the 14 chapters will be published as a single book. The introductory chapter will present an overview of groundwater conditions nationwide and discuss the effects of human activities on ground water. Each descriptive chapter of the Atlas begins with an overview of geologic and hydrologic conditions throughout the regional area covered by that chapter. Regional maps that show amounts of precipitation, runoff, physiography, geology, and ground-water withdrawals by county are included in this overview. Each chapter contains block diagrams, cross sections, and isometric diagrams, such as the one shown in figure 2, that show the relation of each aquifer or aquifer system to underlying and overlying hydrogeologic units. Figure 2 shows aquifers in semiconsolidated and consolidated deposits; some chapters also have a regional map that shows aquifers in unconsolidated deposits. Discussions of each principal aquifer within a regional area are accompanied by maps that show the location and extent of the aquifer, the thickness of the aquifer, the potentiometric surface of the aquifer, and the quality of the water in the aquifer. Stratigraphic Figure 2. All or parts of nine principal aquifers, shown here from shallowest to deepest, are exposed at land surface in the four States (Alabama, Florida, Georgia, and South Carolina) described in area 6 (modified from map HA 730-G) of the Ground Water Atlas of the United States. A tenth aquifer, the Intermediate aquifer system in Florida, is completely buried. charts list the geologic units that compose the aquifer. Cross sections detail the relation of the aquifer to the geology of the area, and flow-direction arrows superimposed on the cross sections show the movement of water in the aquifer. Representative hydrographs illustrate the response of water levels in the aquifer to changes in the amount of precipitation or ground-water pumping. Where data are available, the maps depict the change in water levels over time as the aquifer is developed. Pie diagrams portray the amount of water withdrawn for specific categories of water use. Special conditions caused by ground-water use, such as sinkholes that form in response to ground-water pumping, waterlogging caused by irrigation, and large water-level declines or intrusions of saltwater caused by excessive ground-water withdrawals, are discussed and illustrated. Color is used in the Atlas to emphasize the information presented in each illustration and for consistency among chapters. A set of five uniform map scales has been chosen so that the maps can be directly compared within and among chapters. Photographs illustrate special features or conditions. Because USGS ground-water data are so extensive, no new data are being collected for the Atlas; illustrations and interpretations already published are merged, joined, modified, and simplified as necessary. Data bases and interpretive results of the studies conducted as part of the USGS Regional AquiferSystem Analysis (RASA) program are major sources of information for the compilation of the Atlas. Some of the results of these studies are combined into nationwide summaries in the Atlas. The regional syntheses of the RASA studies are supplemented where necessary by material from smaller scale studies. If appropriate illustrations are not available, maps and illustrations are constructed from data files. Summary.-The Ground Water Atlas of the United States is designed for a varied readership. Accordingly, the level of writing is aimed at readers who have some technical knowledge but are not hydrologists. Technical jargon is kept to a minimum, and technical terms are defined in simple language. The Atlas does not present a comprehensive description of all that is known about each aquifer or aquifer system; rather, it presents, within a single book, the most important aspects of the geohydrology, ground-water flow system, water quality, and use of water withdrawn from the Nation's principal aquifers. Water Movement Through Soil at a Low-Level Radioactive-Waste Site in the Amargosa Desert By Brian J. Andraski bout 1.5 million cubic feet of filters, equipment, clothing, and other waste contaminated by radioactive nuclides are generated every year in the United States by electric utilities, universities, hospitals, and industrial facilities. These wastes are called low-level radioactive wastes because the hazard from radiation is both relatively low and short-lived (from 100 to 500 years). At present, low-level radioactive wastes are disposed of in shallow land burial trenches at three locations (Barnwell, S.C.; Beatty, Nev.; and Richland, Wash.) in the United States. Beginning in 1993, two of these three sites (Barnwell and Beatty) will be closed, so several new facilities for the disposal of such wastes will have to be sited and built in the late 1990's. Arid regions appear to be the ideal choice for shallow land burial of these wastes because low precipitation and high evapotranspiration, which are typical of arid regions, can reduce the potential for contact between water and buried waste. Also, thick, unsaturated zones may provide a natural barrier to the migration of radionuclides to the water table. The processes affecting movement of water through soil at arid sites are not well understood, however, because only recently has there been a need to understand the soilwater regime and the movement of water through soil in arid, nonirrigated sites. This need has been underscored by the ongoing proposals to use arid sites for the disposal of several types of waste. Currently, little information is available about the natural soil-water flow systems at arid sites, and even less is known about how the construction of a wastedisposal facility alters the natural environment of the site. Beatty Study Site.-Detailed field investigations at the site in the Amargosa Desert near Beatty, Nev., are part of the USGS program of research on low-level radioactivewaste disposal. Through this program, the USGS assesses the suitability of existing burial sites and develops site-selection and facilitydesign criteria for use by the U.S. Nuclear Regulatory Commission (NRC) in their selection of future burial sites. The Beatty facility, which is in one of the most arid parts of the Aerial view of the undisturbed (fenced, upper left) and disturbed (fenced, center) study sites, which are adjacent to the low-level radioactive-waste facility (behind the fence across the road at the upper right of photo) in the Amargosa Desert near Beatty, Nev. figure). At the undisturbed site, a vertical shaft provides access for installation and retrieval of instruments in the upper 45 feet of the vegetated soil. At the disturbed site, two small-scale test trenches (each about 14 feet in length, depth, and width) were constructed in the same manner as those of the burial facility. The trench covers and the undisturbed soil between the trenches are kept free of vegetation. At the undisturbed site, near-surface soil is usually dry; however, the amount of water in the soil fluctuates in response to precipitation and evapotranspiration and ranges from about 1 to 12 percent moisture. Below a depth of 3 feet, the amount of water in the soil depends in part on differences in soil texture, and the moisture content ranges from about 4 to 14 percent. Measurements of soilwater-potential and vapor-density gradients in the upper 30 feet of soil indicate that the direction of water movement varies; below the 30-foot depth, however, the measurements indicate that the direction of water movement is generally upward. Estimates of the volume of water flow, in either liquid or vapor form, are typically less than 10 inches over the area per year. At the disturbed site, construction of the test trenches produced a backfill material that was generally drier (3 to 4 percent moisture) and significantly more homogeneous than the US ECOLOGY, INC. undisturbed soil. During the first year after the test trenches were built (September 1987 to September 1988), precipitation at the site totaled 6.2 inches (152 percent of the 10-year average). By the end of the first year, the amount of water stored in the near-surface soil of the unvegetated trenches and in the unvegetated soil between the trenches showed a net increase of as much as 64 percent over initial amounts, whereas no net increase occurred in vegetated soil at the undisturbed site. Precipitation at the site totaled 1.5 inches during the next 2 years. In spite of these extremely dry conditions, the amount of water stored in the near-surface soil of the unvegetated trenches and in the unvegetated soil between the trenches still showed a net increase of from 5 to 15 percent over initial amounts. In contrast, the amount of water stored in the vegetated soil at the undisturbed site was 15 percent less than the initial amount. By the end of the third year (1990), water that infiltrated the trench covers and water from the undisturbed soils adjacent to and beneath the trenches still had not come in contact with the simulated waste. Results of Investigation.-Results to date (1991) from these ongoing studies have clearly demonstrated that the natural system impedes water permeation at depth at this arid site. In addition to the natural climatic regime (that is, precipitation and evaporation), soil and vegetation are extremely important factors influencing the soil-water regime at the Beatty site. The stratified alluvial soils provide natural barriers to water movement. Because vertical flow of water is impeded, plants can extract and transpire water that accumulates in the root zone. Creosote bush, an evergreen shrub that is extremely drought tolerant, can deplete the soil of what little water is available. The limits on the downward movement of water through the soil caused by these near-surface conditions are reflected in the consistently low soil-water content that is measured at depth. The amount of water or vapor moving through the soil is quite low, and the moisture moves upward or downward in response to soil-water-potential and soil-temperature gradients. The construction of the burial trenches and removal of the native vegetation significantly altered the natural environment of the site. Trench construction eliminated the natural barriers to water movement originally present in these alluvial desert soils. Two points should be considered regarding the absence or presence of natural barriers to water flow: (1) the lack of such barriers in the trench backfill could increase the potential for deep percolation of water and (2) the presence of these barriers in the soil adjacent to a trench could result in lateral flow of water from the soil into the trench. Lack of vegetation increases the potential for water to percolate toward the buried waste. Application of Findings to Waste Sites in Arid Regions.-Regulations of the NRC require that near-surface, low-level radioactive-waste sites "shall be capable of being characterized, modeled, analyzed, and monitored." The 1 year of preoperational monitoring required by the regulations for purposes of characterization and subsequent modeling of any proposed commercial lowlevel radioactive-waste site is generally limited to natural site conditions. Results from the studies near Beatty, Nev., demonstrate that the installation of a disposal facility markedly alters the natural environment. These changes must be considered when a disposal facility is designed and the effectiveness of a proposed site and facility for long-term isolation of waste is studied. Based on the predominantly low soilwater potential measured to date at both the undisturbed and disturbed study sites, the movement of moisture in the vapor phase is likely to be an important transport mechanism. Transport of radioactive constituents in the vapor phase, such as tritium and carbon14, may be a major pathway for the migration of contaminants at arid sites. Unsaturatedzone monitoring programs, which are designed for arid disposal sites, need to include monitoring of both liquid and vaporphase transport mechanisms as potential pathways for the release of radionuclides. Summary.-The movement of water through soil at arid sites is extremely complex and is influenced by several interacting factors and physical processes. Factors such as climate, vegetation, and soil properties significantly affect the processes that control the movement of water through soil. Depending on specific but often transient conditions, water movement at arid sites may be predominantly either liquid or vapor flow and may occur in response to soil-water-potential and temperature gradients. Results from ongoing studies near Beatty, Nev., are being used to better define the mechanisms and soil properties that control the movement of water through the soil at that arid site. Additional studies of the processes that control directions and rates of water moving through the soil throughout the unsaturated zone are needed. Such studies are critical to evaluating the importance of potential contaminant-release pathways at arid, low-level radioactive-waste disposal facilities and to obtaining a better understanding of the overall hydrologic cycle in arid regions. |