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environments that may not be evident from existing geologic maps. The mineral-resource analysis portion of this project builds on each of the other sections and, like the other sections, contains new results that will require many geoscientists to reexamine their existing concepts of mineral resources in Nevada.

Each of the researchers is addressing part of the overall problem of analyzing and predicting Nevada's mineral resources. The interdisciplinary nature of the research for this cooperative venture has amplified and reinforced each part of the project and has led to products not otherwise obtainable.

Uranium in the United States

By Joseph S. Duval

Background

Uranium-238 is a radioactive element that occurs naturally in trace amounts in all types of rocks and soils. Uranium is chemically mobile in oxidizing environments and many of the rockforming and surface processes result in uranium concentrations that are characteristic of a particular rock or soil. For this reason, a map of the uranium distribution in rocks and soils can provide information useful to understanding geologic and soil formation processes. Uranium is itself of interest as a fuel for nuclear energy, but because it is often associated with other elements, it also can be useful in mineral exploration applications.

The USGS, in cooperation with the U.S. Environmental Protection Agency, has compiled aerial gamma-ray data obtained during the National Uranium Resource Evaluation (NURE) Program sponsored by the U.S. Department of Energy to create a map showing surface concentrations of uranium-238 for the conterminous United States.

Health Hazards and Uranium
Concentrations

Because uranium produces radioactive decay products, its presence in the environment in various forms presents health hazards. Radon-222 in indoor air, for example, is known, under certain conditions and at certain exposure levels, to cause lung cancer. Radium-226 in water supplies has been shown to cause other forms of cancer. These health risks underscore the need to have the best possible picture of where uranium occurs in the geologic environment.

The data on the map showing uranium distribution for the conterminous United States are grouped into intervals of 0.5 parts per million (ppm) equivalent uranium (eU). The term "equivalent" is used because radioactive disequilibrium can occur in the uranium decay series, and the gamma-ray measurement is not a direct measure of the uranium. Where blank areas exist on the map, no data are available.

States and regions having large areas of low values (less than 1.5 ppm eU) are Oregon, western Washington, northern California, northwestern Nebraska, northern Maine, Michigan, Wisconsin, Minnesota, Florida, and the outer coastal plains of New Jersey, Delaware, Maryland, North Carolina, South Carolina, and Georgia. In the Western States, these low values generally occur in areas underlain by igneous rocks having low original uranium content. In the coastal areas of the Southern and Eastern States, the low values occur in areas underlain by marine sands of low uranium content. In the North-Central States, the low values are caused by high soil moisture that attenuates the gamma-ray signal.

The higher concentrations of uranium are associated with granites, granitic metamorphic rocks, black shales, phosphatic rocks, and Tertiary rhyolites (between 63 and 2 million years old). Most of the granites and granitic metamorphic rocks are distributed in the Western and Eastern States with only a few occurrences in the Central States. Some of the granites having the highest uranium concentrations are the Conway granite in New England and the Silver Plume granites in Colorado. Phosphatic rocks in Alabama, Mississippi, and Flor

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ida have elevated uranium concentrations. Tertiary rhyolites in southwestern New Mexico, western Utah, Nevada, Arizona, southwestern California, Idaho, and Wyoming also have high uranium concentrations. Some of the more radioactive black shales occur in Utah, Ohio, and Kentucky.

The distribution patterns seen in Ohio and Kentucky provide information relevant to soil formation processes associated with glaciers. In central Ohio, the uranium is widely dispersed as a result of glacial processes, but, below the glacial boundary in Kentucky and southern Ohio, the elevated uranium values are more localized and occur in areas underlain by the Devonian black shales. Some of these areas have concentrations greater than 5.0 ppm eU. Similar processes have affected the uranium distribution in the New England States. In Maine, the areas of highest uranium concentrations are underlain by rocks described as two-mica granites. The areas to the south and east of the granitic

intrusions also have elevated uranium concentrations (greater than 2.5 ppm eU) and presumably the surface soils (of which the concentrations are being measured by the gamma-ray data) contain materials displaced by glacial scour from the granitic rocks. Other locally higher uranium concentrations in Maine occur along the Atlantic Coast and in northcentral Maine and are also related to twomica granites.

Radon Potential

Uranium maps have been especially useful in understanding the source and presence of indoor radon. Because an aerial gamma-ray system is measuring the gamma-ray flux from the decay of Bismuth-214 (a radon-222 decay product) in the ground, the aerial gamma-ray data provide estimates of the concentrations of radon-222 in the soil gas. The USGS has conducted field studies in Montgomery County, Md., of the appar

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Distribution of uranium-238 in the conterminous United States (ppm, parts per million; eU, equivalent uranium).

... surface gamma-ray

data can be used

to estimate the radon

in the soil gas.

ent surface concentrations of uranium in the soils by using gamma-ray measurements and of radon in the soil gas. The results indicate that the surface gammaray data can be used to estimate the radon in the soil gas. Other studies compared the average indoor radon in counties in New Jersey to the average apparent surface concentrations of radium [1 picocuries per gram (pCi/g) radium = 0.333 ppm eU], and the results indicate that the average radium concentrations can be used to estimate the average indoor radon levels in New Jersey. Average indoor radon levels for houses in townships in Washington, Oregon, and Idaho were also compared to surface concentrations of radium. The results of this study also suggest a predictive relation. Scientists caution, however, that townships underlain by highly permeable soils (soils through which water percolates greater than 6 inches per hour) constitute a distinct subset compared to townships having less permeable soils. Highly permeable soils apparently increased the average indoor radon for a given concentration of radium by about a factor of 6.

All of the above results indicate that the map of the surface concentrations of uranium can be used to estimate average

indoor radon levels, but only in a relative sense. In other words, an area in a particular region of the country having 4 ppm eU should result in higher average indoor radon levels than another area in the same region having 2 ppm eU. But because of differences in soil conditions, climate, and house construction, 4 ppm eU in New Jersey would not be expected to result in the same average indoor radon levels as 4 ppm eU in Colorado. Areas having inherently permeable soils should tend to have higher average indoor radon levels than areas having less permeable soils if the areas have similar uranium concentrations.

The compilation of the NURE aerial gamma-ray data to produce a map of the surface concentrations of uranium for the conterminous United States has resulted in a reasonably accurate representation of the distribution of uranium in the surface rocks and soils, and general agreement exists between mapped geology and the patterns seen in the uranium distribution. Because of the relation between uranium and radon, this map is also useful as a tool for estimating indoor radon levels. This map, however, cannot be used to directly estimate the radon levels because of the effects of permeability, housing construction, and differences in soil weathering profiles. With this map as a starting point, local and regional planners and managers have an important tool to use in identifying areas that warrant additional study to determine the radon potential of a specific area. This map also should prove useful for understanding other geologic, geochemical, and soil formation processes.

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National Mapping Program

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Highlights

Mapping
Modernization

By Eugene C. Napier, Ernest B.
Brunson, and K. Lea Ginnodo

When the U.S. Geological Survey was established in 1879, topographic mapping was conducted as an adjunct to geologic studies. Soon, topographic maps were recognized as having intrinsic value as well as value for many other applications. From this beginning, the concept of preparing standard topographic maps for multipurpose use was conceived, and the first standard quadrangle map was completed in 1881. Since that time, the USGS's topographic mapping program has undergone several transitions as users' needs for more detailed map information increased and as mapping procedures and technology evolved. The current transition began in 1979 when the USGS formally initiated the Digital Cartography Program.

The widespread use of computers and associated technology has generated new and increasing demands for map

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information in computer-compatible form. Since 1979, user requirements for accurate and current cartographic data in both digital and graphic forms have accelerated beyond the capacity of the National Mapping Division to produce them.

... The USGS has

undertaken an extensive

mapping modernization

effort....

To respond to this increasing demand, the USGS has undertaken an extensive mapping modernization effort that includes major technical and programmatic transitions. The objective of this modernization effort is to develop and implement advanced digital cartographic systems and production procedures in the USGS's cartographic production centers by the mid-1990's. This modernization will make it easier to revise maps more frequently and to prepare and maintain more up-to-date cartographic products.

Tasks underway to accomplish this ambitious goal include expanding and improving mass digitizing capabilities; modifying data structures to support increased content and access requirements; developing digital revision capability; developing digital productgeneration capabilities for standard, derivative, and digital products; improving quality control; and supporting advanced spatial analysis and applications. When these tasks are completed and the digital cartographic systems are fully operational in the mid-1990's, the USGS will be able to meet increasing requirements to keep current the Nation's 57,000-plus primary-scale maps

(fig. 1).

A Primary Mapping Economic Analysis, completed in fiscal year 1989, showed that there are significant economic benefits to computer-based map revision. Further, the analysis showed that the current target of revising maps on a 10-year cycle is conservative and that there would be greater benefits if the revision cycles were shorter.

The mapping modernization effort represents a substantial and timely program change in the USGS. It appropriately takes advantage of state-of-the-art mapping technology and will result in a highly responsive digital cartographic

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