Using cesium-137 to quantify sediment source contribution and uncertainty in a small watershed

Knowledge of sediment provenance is critical for precision conservation planning and calibration of soil erosion models. The objectives are to evaluate the ability of Cs-137 to apportion sediment source contributions, quantify uncertainty of the estimates, and estimate desirable sample numbers. We collected 50 surface soil samples from overland in a watershed (15.6 km(2)), 28 subsoil samples from gully bank, and 43 sediment samples in channels. The Cs-137 activity was measured by gamma spectrometry. Proportion means were calculated by solving a linear mixing model. Uncertainty was estimated by the first-order approximation and Monte Carlo (MC) simulation. The Cs-137 tracer was conservative in the fraction of <63 mu m. The Cs-137 activities differed between sources in the whole watershed at P = 0.05. The mean proportions of the <63 mu m fraction (relative error) predicted for the watershed using the mixing model were 0.42 (35%) for the overland source and 0.58 (25%) for the gully source, with a 95% confidence interval of +/- 0.145 for both. Overland source contributed most uncertainty to the proportion estimates, followed by sediment, with minimum from gully. The MC simulation predicted the same mean proportions, but with relative errors being <3% for both. Compared with the first-order approximation, MC underestimated uncertainty of the means due to the large sample number used in simulation. In general, 30 to 50 samples are needed for each source and sediment to generate reliable estimates using Cs-137. Estimated source proportions of the fine sediment may be converted to proportional contributions of total soil erosion by adjusting by particle sizes and sediment delivery ratio. No adjustment or weighting should be made directly to the mixing model unless conservativeness of the tracer is violated. Published by Elsevier B.V.