The Distribution of Uranium and Thorium Decay‐Series Radionuclides in the Environment—A Review

Natural radioactivity has played an indispensable role in the development of the earth to the form and shape that we now observe. Energy from radioactivity powers the large‐scale processes that move the continents and cycle the elements. Most of the energy due to radioactivity comes from members of the three decay‐series, two of which are parented by isotopes of uranium, 238U and 235U, and one by 232Th. These decay‐series cascade to produce radioactive offspring such as Ra, Rn, and Po, but the ultimate fate of all members of these series is to end as a stable isotope of Pb. Rocks or minerals that provide sites for the long‐lived decay‐series parents, U and Th, are destined to be associated with the other members of the decay‐series. The rocks that tend to host U and Th include some crystalline rocks such as granite and quartz‐conglomerate metamorphics, and certain sedimentary rocks such as organic shales, sandstones, carbonates, and phosphorites. All materials at the earth's surface eventually are broken down to their constituent parts and then, usually by the agency of water, transported to environments where they again may be geochemically sorted. The sorting processes at or near the surface of the earth operate on radioelements just as they do on other elements. The result is that the various radioelements are not simply dispersed but rather tend to accumulate in certain environments or rock types. Thus, the behavior of the radioelements found in the natural environment is based on their geochemistry, half‐life, and the nature of their surroundings.