Evaluation of Temporal and Spatial Sediment Dynamics in Agricultural Fields Using Lanthanide Tracers

A fully integrated hill-slope scale hydrologic monitoring project was performed to evaluate sediment transport mechanisms by combining a network of sensors monitoring the temporal and spatial occurrence of runoff with lanthanide (rare earth element (REE) oxides) tracer analysis and radiometric (Be-7, Pb-210, Cs-137) fingerprinting. These methods have been combined on a hill-slope with a Plano silt loam soil at Arlington, WI under two tillage orientations (contouring and up-and-down the slope) and multi-year (2005-2007) data were collected from a series of natural rainfall-runoff events. The major goal is to determine the spatial patterns of runoff and sediment movement for different agricultural management systems. To delineate hydrologic active areas and sediment source areas during an event, an array of surface runoff sensors was placed in each plot. This presentation will specifically focus on the use of REE-tracer methods for studying sediment transport dynamics, including preparation, application, and sampling of tagged-soil. The hillslope (40 ft) was divided into three segments within which different types of soil-REE (Gd, Nd, Pr)-oxide mixtures were applied. Both surficial soil and edge-of-field samples were collected to quantify translocation of tagged-soil within the field and off-site sediment transport, respectively. Surficial soil samples were collected from nine bands located 60 cm apart covering the entire hillslope. The redistribution of REE-tagged soils and the REE concentration in suspended sediments are being used to delineate source regions and determine characteristic transport distances. Preliminary data indicate a good correspondence between the sedigraph peaks and contributions from the various REE segments. Our results suggest the suitability of using REE-tagging technique to acquire information on spatial and temporal patterns of sediment movement in agricultural fields. Radiometric fingerprints using concurrent measurements of multiple environmental isotopes are being used to determine relative areal extents of the hillslope subjected to interrill vs. rill erosion.