Use of 7Be and 137Cs for sourcing sediments in catchment studies of erosion and deposition: A modelling application

Full text: The use of 137Cs as an environmental tracer for work on erosion 'has become widespread in recent decades'; due to its convenient half-life of 30.2 a, and the relatively high post-bomb levels of the isotope. Also, 7Be has recently come into vogue as an environmental tracer for sedimentation studies. The much shorter half-life of 7Be (53.3d) suggests a suitability for application to small catchment studies. To 'infer absolute rates of erosion and deposition from measurements of the concentrations of radionuclides in the soil it is necessary to measure independently, or make assumptions about, processes which would otherwise have a confounding effect. For instance, soils high in organic matter, or with a high percentage of very fine material may be expected to be more effective in scavenging both caesium and beryllium. In addition, soil particles which have been remobilized my be expected to have higher concentrations of these tracers because of their greater opportunity for scavenging. If the concentration of these tracers is measured in sediment samples collected, for example, at the outlet of a weir of a small catchment, measured values will reflect not only the concentrations in the exposed soil, but also the relative contributions of sheet, rill and gully erosion. Particle size specificity and amount of organic matter present will also play a part. For beryllium especially, the amount of time from the last rainfall, and the concentration in the rainfall (which may vary with time) will also have an effect. Knowing the distribution of both tracers in the soil, and measuring the particle size distribution and organic matter content will then allow inferences to be made about the mechanisms of erosion that are taking place. Surface soil profiles in two small (7 ha) experimental catchments and in adjacent runoff erosion plots given different tillage treatments were surveyed for concentrations of the environmental isotopes 7Be and 137Cs. Sediment samples collected from runoff from the catchments and plots were analysed for particle size and organic matter content, and the concentrations of the above isotopes measured by gamma ray spectroscopy. These sediment characteristics were then related to different sources of sediment in the catchments and plots to determine the dominant type of erosion-deposition process affecting sediment detachment and transport. The results were used to verify the predictions of a topographically based analytical model of the erosion-deposition process. Although concentrations of 7Be in the top 5 mm of soil were barely measurable, sediment samples were consistently enriched in Be by up to two orders of magnitude. The degree of enrichment was particularly pronounced in the organic fraction, but was present across the entire range of particle size fractions. This is to be expected from the selective transport of detachable surface material on the soils where cosmogenic 7Be is deposited and strongly absorbed. Very high levels of 7Be enrichment were found where deposition and remobilization of soil was a feature of the sediment collected. Caesium-137 showed much lower levels of enrichment in the transported sediment, reflecting lower deposition rates, extended period of deposition and deeper distribution through the top 50-100 mm of the soil profile. Caesium profiles were of the expected shape, decaying from moderately high values near the surface to very low values at depth, with some variations observed in areas of deposition. The most noticeable common feature ws the shallowness of the profiles, with few profiles extending more than 50 mm below the soil surface for both undisturbed and eroded sites. Small rainfall events produced small amounts of sediment with generally high 7Be and 137Cs concentrations, and high Be/Cs ratios. Events of intermediate size produced considerably lower concentrations of 7Be and also lower Be/Cs ratios; whereas large events resulted in very low concentrations of both caesium and beryllium. The high concentrations, especially of beryllium, observed in the smaller samples of sediment were related to the concentrating effect alluded to above, where only previously mobilized surface material was collected. Larger events were able to mobilize previously unmobilized material from deeper within the soil profile. Variations in the Be/Cs ratio between samples were related to the different effective depth of scouring, and the different source areas of sediment production. (author)