Use of rare earth elements to trace soil erosion and sediment movement

Erosion control planning and evaluation of spatially distributed erosion models require knowledge of rates of soil loss and sedimentation on different landscape elements and slope positions. A method for obtaining spatially distributed information on sediment movement employing rare earth element (REE) oxides was proposed. The performance of REEs as tracers was tested using a 4 by 4 meter laboratory plot with a silt loam soil and at 10% slope. Five slope sections were labeled with REE oxides in a powder form. After a series of simulated rainfalls, erosion rates on different slope positions predicted from REE were within 4 to 40% of the rates calculated using DEMs obtained by a laser scanner. The largest loss occurred on the upper-middle part of the slope. The tracer enrichment ratio was 1.7. In the field experiment, 6 REEs were used to tag different morphological units of a 0.68 ha watershed with a silt loam soil and an average gradient of 8%. Background concentrations of REEs in the soil were characterized by a uniform spatial distribution with coefficient of variation of less than 7%. To quantify the mean background value of REE in the soil with an allowable error of 5% at a 95% confidence level, 10 random, independent samples were needed. While the average soil loss on the watershed during the time of the experiment was 6.1 t ha−1, local rates varied between 46 t ha−1 of loss to 50 t ha−1 of soil gain due to deposition. While landscape elements with convergent flow experienced considerable deposition, these areas also had the greatest net soil loss, indicating a great turnover of sediment. The shoulder slope with lesser slope gradients remained relatively stable, and net deposition occurred on the toeslope area adjacent to the outlet of the watershed. The multiple REE tracing technique offers a simple and a precise way of tracing soil movement, identifying multiple sources and sinks of sediments, and determining net soil losses at various locations on the watershed