Track soil particles moving through watershed

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Soil erosion of croplands, rangelands, and urban areas is one of the most common problems that negatively affects the environment. Soil particles carried by rainwater along slopes may transport a variety of harmful agricultural and industrial chemicals, cause the soil to lose fertility, and inundate streams. Quantifying the movement of soil through watersheds is a complex task, which requires sophisticated sampling and modeling methods. At the present time, there is a limited number of reliable field techniques capable of tracking soil movement through a watershed, and the experimental data is lacking. One method for obtaining such information is through the use of sediment tagging.

Scientists at the USDA-ARS Southwestern Watershed Research Center in Tucson, AZ used oxides of five elements from the lanthanide group, also called rare earth elements, to tag and monitor the movement of soil particles. These elements are found naturally in the soils in very small quantities. When artificially imbedded into soil aggregates, the elements can be detected and quantified using mass spectrometry analysis. A technique to imbed tracers into soil aggregates was developed. Tagged aggregates were placed in different parts of a small watershed in southern Arizona in 2005 and were allowed to move with surface runoff over a period of several years.

Using an extensive sampling scheme, the tagged aggregates were later recovered elsewhere on the watershed and in runoff water. Knowledge of the amount of tagged soil applied at various locations and the amount recovered allowed the researchers to construct a mass balance of soil movement for the entire area. They wanted to characterize the relationship between topography and the soil erosion process and assess its severity. The researchers were also interested in adapting this new method for use in a semiarid rangeland environment.

Results from the study were published in the September–October Soil Science Society of America Journal. According to the authors, the tracing technique using rare earth elements proved to be a useful tool for measuring soil erosion in a rangeland watershed with coarse soil. It was found that watershed channels were eroding at a much greater rate compared with slopes, which will ultimately result in a deepening and expansion of the channel system. It is plausible that the erosion patterns observed in this study are indicative of the long-term erosion rates, the authors say.

“This study provides a unique set of data,” concludes Viktor Polyakov, one of the authors of the study. “We utilized a method to track erosion, translocation, and re-deposition of sediment in a small watershed, thus allowing a complete, spatially distributed sediment balance to be made as a function of landscape morphology. This is a valuable tool that may help to develop more precise management practices aimed at minimizing soil erosion.”

Further research is needed to investigate the feasibility of applying these tracers in other chemical formulations and in other soils and environments and linking the obtained results with soil erosion modes.

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