NONRADIOACTIVE CESIUM AND RUBIDIUM AS ANALOGS FOR 137CS BEHAVIOR IN SOIL AT THE KONZA PRAIRIE LTER SITE, NORTHEASTERN KANSAS, USA

The radioactive isotope, 137Cs, carried in meteoric precipitation, is used to quantify soil erosion by comparing inventories of 137Cs in disturbed soils with those in similar but undisturbed soils. Mobility and spatial variability of 137Cs in soils is not well characterized, with the result that erosion calculations based on 137Cs may be difficult to interpret. We have compared the vertical, spatial, and solid-component distribution of major, minor and trace elements with the 137Cs activity at the Konza Prairie LTER site in northeastern Kansas. This grassland environment is an undisturbed analog for mid-continent grasslands that have been affected by human activities.

We used a sequential extraction technique that employs progressively more aggressive chemicals to remove components from soil, defining five elemental reservoirs of different potential mobility (exchangeable, affiliated with carbonate mineral phases, affiliated with reducible phases, affiliated with organic matter, and affiliated with silicates and other resistates). We have found that Rb and nonradioactive Cs in the organic matter reservoir have depth profiles similar to the depth profile of 137Cs activity. Although most (95%) of the nonradioactive Cs and Rb in the soils is bound in the most immobile reservoir (silicates or other resistates), the part that is mobile is progressively enriched with depth below the land surface both in organic matter and in the easily exchangeable reservoir. Enrichment, especially in the case of organic matter, is relative, because organic matter content of the soil decreases with depth in the soil. Change with soil depth in the distribution of elements among the five reservoirs defined by the extraction technique may show the transformation of elements from one reservoir to another, with implications for transport or attenuation of radioactive elements.