TRANSPORT OF REE-TAGGED SOIL PARTICLES IN RESPONSE TO THUNDERSTORM RUNOFF

Fundamental to evaluating the success of agricultural management practices and understanding the delivery of sediments to downstream waters is the transport distance of eroded particles. The downslope transport of rare earth element-tagged soil particles eroded during a spring thunderstorm was studied at both a natural prairie and an agricultural field in southwestern Iowa. Soils are a silty loam developed on loess. Natural soils were tagged with the rare earth elements Eu, Tb and Ho to ~1000 ppm via coprecipitation with MnO2. Tagged material was replaced in three target locations; surficial soil samples were collected following precipitation and runoff; and rare earth element concentrations were determined by inductively coupled plasma mass spectrometry. Diffusion and exponential models were applied to the concentration-distance data to determine particle transport distances. The results indicate that the concentration-distance data are well described by the diffusion model, but the exponential model does not simulate the rapid drop-off in concentrations near the tagged source. Using the diffusion model, calculated particle transport distances at all hillside locations and at both the cultivated and natural prairie sites were short, ranging from 3 to 73 cm during runoff from a single storm. REE concentrations were below detection limits at a weir at the downstream end of the field. The short transport distances indicate that the majority of particles were retained on the landscape and were not delivered to the stream. Erosion control methods implemented in the watershed should be reflected quickly in downstream waters.