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More than one-third of the United States is estimated to have high geologic radon potential. A high radon potential area is defined as an area in which the average indoor radon screening measurement is expected to be 4 pCi/L or greater. Geologic terrains of the United States with high radon potential include:1. Uraniferous metamorphosed sediments, volcanics, and granite intrusives that are highly deformed and often sheared. Shear zones in these rocks cause the highest indoor radon problems in the United States.2. Glacial deposits derived from uranium-bearing rocks and sediments and glacial lake deposits. Clay-rich tills and lake clays have high radon emanation because of high specific surface area and high permeability due to desiccation cracking when dry.3. Marine black shales. The majority of black shales are moderately uraniferous and have high emanation coefficients and high fracture permeability. 4. Soils derived from carbonate, especially in karstic terrain. Although most carbonates are low in uranium, the soils derived from them are very high in uranium and radium.5. Uraniferous fluvial, deltaic, marine, and lacustrine deposits. Much of the nation's reserve uranium ores are contained within these sedimentary deposits, which dominate the stratigraphy of the western U.S.

As part of the special interest series, Public Issues in Earth Science, published by the U.S. Geological Survey (USGS), this Circular describes the importance of the earth sciences in the investigation of environmental problems. The report focuses on geochemistry-the study of the amounts, distribution, and cycling of chemical elements in the Earth and atmosphere-and how this science helps to evaluate critical issues that relate to our fragile environment. The mission of the USGS is to provide geologic, topographic, and hydrologic information that contributes to the wise management of the Nation's natural resources and promotes the health, safety, and well-being of our people. Part of this task includes characterizing the Nation's geochemical environment and understanding the dynamic processes responsible for change in that environment. One of our greatest assets at the USGS is our long tradition of excellence in unbiased earthscience research. Part of that legacy is our commitment to supply the geochemical information necessary to confront urgent environmental challenges. Geochemistry provides information on the distribution of chemical elements to help us define and understand environmental problems. This information then allows us to provide answers for their resolution and possible remediation. This Circular presents an overview of geochemistry and its application to various case studies that illustrate the use of geochemistry in examining environmental problems. Some new and exciting areas of environmental geochemistry are discussed, involving r(.)ck/water, soil / water, and plant/ soil investigations. These investigations focus on many of our natural resources including minerals, soil, water, air, and vegetation and examine environmental concerns-such as acid precipitation, mine drainage, and sources of contamination-from a "systems" or "holistic" approach. As the primary Federal earth-science agency, the USGS leads in the collection, interpretation, and dissemination of earth-science information. This report helps to define one area in which the USGS is an active participant-the application of geochemistry to environmental concerns.

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