Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 2005
Publication Date: April 28, 2006
Citation: Kimoto, A., Nearing, M.A., Zhang, X.J., Powell, D.M. 2006. Applicability of rare earth element oxides as sediment tracers for coarse-textured soils. Catena 65:214.221. Interpretive Summary: Rates of soil erosion are quite variable from place to place in the field. In order to most effectively develop plans for conserving soil we need to know where on the field the erosion is most critical. Some of our models of soil erosion give conservation planners estimates of this variability (of where the greatest erosion occurs in the field), but data to verify such models is severely limited. The purpose of this study was to develop a method whereby sediment particles could be tracked so that we can identify where the particles originate, how far they move, and which ones leave the field. Thus we developed a method using special chemicals called Rare Earth Elements, which bind strongly to soil and sediment particles and can be measured in sediment and soil samples. This kind of data is unique in the world and an important advance for the science of soil erosion and conservation. The Rare Earth Element technique has been applied previously in loessial soils of the eastern US, but coarse textured western rangeland soils present unique problems. In the coarse textured soils, the Rare Earth Elements do not bind equally on all soil particle sizes, and since soil erosion preferentially moves the different soil size materials, this presents a problem for the technique. In this study we worked out a method to account for this preferential binding of the Rare Earth Elements so that the method is effective for these soils. The impact of the work will be better spatial information on soil erosion and contaminant transport which will reduce costs and improve effectiveness of conservation plans and programs.
Technical Abstract: Rare earth elements (REE) have been used successfully for tracing sediment movement, but applications have been only on loessial soils. Soils with heterogeneous texture present problems of preferential binding as a function of particle size. The objective of this study was to examine the applicability of REE as a tracer for gravelly, sandy loam soils by measuring the leaching and binding properties that are important for function as a sediment tracer. Five REE oxide powders were mixed with soils from southeast Arizona, and then leached with deionized water. The REE oxides bound well with the soil material, however, REE concentrations associated with fine particle sizes were greater than those of sand-sized particles, which exhibited lesser aggregate formation. The results suggest that, in the presence of preferential transport, sediment tracking may be incorrect if calculated by concentrations of REE in the whole sediment. In order to use the REE oxides as a tracer on these types of soils, either the REE will need to be measured for separate sediment size groups, or the REE will need to be mixed and bound with separated size classes of material prior to spreading of the tracer on the ground. We showed that doing so, using four size groups, could produce a maximum error of approximately 4% error due to selective sediment sorting. The non-uniform binding of REE oxides with different size classes does not preclude the use of REE, but may increase the time and cost of the technique for these soils.