No abstract

Concentrations of natural and man‐made radionuclides including 40K, 137Cs, and some of the decay products of 238U and 232Th were determined in soil. Samples were taken at a set of permanent radiation monitoring stations and from random points across the state of Louisiana. The concentrations of the radionuclides were found to be correlated with soil suborder: the lowest values found in the Udults of the northern portion of the state and the highest concentrations found in the Aquepts of the flood plains and the Hemists of the coastal marshes, suggesting that processes such as weathering, transport of solids, and dissolution may be fractionating these elements into soils of water environments. The concentration of K was found to vary from 3180 to 24 900 mg kg−1; U varied from 2.3 to 4.0 mg kg−1; and Th from 8.0 to 24.1 mg kg−1. Cesium‐137, the fission product, was found only in the top 15 to 20 cm of the soils with no statistically significant aerial distribution across Louisiana.

A new type of soil sampling protocol is suggested which has applicability to surveys of environmental radioactivity. Often there is a higher concentration of radionuclides in the lower B horizon than in the upper A horizon of well-drained, well-developed soils. It is recommended that soils be sampled pedologically, i.e., by soil horizon, rather than incrementally in depth. An example is given from a site in northern Louisiana where surface and/or the classical 15 cm ("six-inch") deep sampling could lead to erroneous conclusions regarding anthropogenic contamination.

The environmental radiation exposure rate has been measured using thermolumineseent dosimetry at 26 monitoring stations in Louisiana over a period of 2 yr. Sets of LiF dosimeters that are essentially tissue equivalent in their energy response characteristics were left at each station for 90 d, replaced, and the exposure they received determined. The chips were exposed to a standard source three times during the 2‐yr period to assure their accuracy. On at least two occasions during the study period, samples of soil were taken at each station. These samples were dried, ground, sieved for uniformity, and analyzed by gamma ray spectroscopy. The specific activity of 40K, 137Cs, and some of the decay products of U and Th were determined. The average exposure rate for all stations was found to be 19.6 ± 2.8 µC kg−1 yr−1 (76 ± 11 mR yr−1). The range in values, from 15.0 to 25.5 µC kg−1 yr−1 is generally explained by the variation of the 40K specific activity in the natural soil bodies where the stations are located. The 40K specific activity ranges from 100 to 900 Bq kg−1 across the state. The other activities are also related to soil type, but none vary as dramatically as 40K. The results obtained agree with other more limited surveys and provide a large, reliable baseline to which future studies can be referred when assessing changes in environmental radiation levels.