Submitted to: Catena
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2003
Publication Date: 2/2/2004
Citation: Polyakov, V.O., Nearing, M.A. 2004. Rare earth element oxides for tracing sediment movement. Catena. 35:255-276. 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 erosion occurs. 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 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 part of the project involved testing the method in the laboratory using a rainfall simulator over a 4 meter square soil box. The results showed clearly that we are able to measure where the erosion occurred in the box and how much of the sediment from the upper portions of the box is re-deposited on lower sections. We were also able to tell where the sediment came from that was eroded off the end of the soil box. This work may have significant value when applied to field scale erosion studies because it will give scientists a capability that we have never had before: we will be able to actually track the movement of sediment on a field, rather than only measuring what leaves the field. Then we will be able to test our soil erosion models to see if they really are able to predict where soil erosion is serious in a field. In the end this method will give us the knowledge to help conservationists develop better soil conservation strategies for protecting America's ability to produce food on a sustainable level for generations.
Technical Abstract: The development of soil conservation plans and evaluation of spatially distributed erosion models require knowledge of rates of soil loss and sedimentation on different landscape elements and slope positions. Characterization of soil erosion rates and patterns within watersheds is important for the understanding of erosion processes and landscape evolution. Experimental data that show spatial translocation of soil on slopes are limited. A method for obtaining spatially distributed information on sediment movement employing rare earth element (REE) oxides was proposed. Five REE oxides in powder form were uniformly mixed with the soil on different parts of a 10% slope in a four by four meter soil bed. Particle translocation was measured during 8 simulated rainfalls at 60 mm h-1 intensity. A laser scanner was utilized to obtain digital elevation models (DEM) of the soil surface that were used as the reference data to compare with the tracer method. REE concentration in soil samples taken from the bed and sediment sampled from runoff was determined by Inductively Coupled Plasma Mass-Spectrometry. Erosion rates for different slope positions estimated from REE concentrations correlated with those calculated from the DEMs with relative differences for different slope sections of 4 to 40%. The enrichment ratio for this type of tracer was 1.7. The amount of sediment produced on different parts of the slope varied, with the greatest erosion occurring on the upper-middle part of the slope. The experiment showed that the multi-element tracer method provided a satisfactory way to study soil erosion distribution on a uniform slope.