Cesium-137, a tool for the assessment and the management of erosion risks

Cesium-137 (137Cs) is a radioisotope that was artificially introduced in the environment through the extensive high-atmosphere bomb tests that took place in the 1950s and 1960s. Due to its environmental behavior in soils, this isotope is a particularly interesting soil movement indicator. Its spatial redistribution reveals global soil movements that were initiated some 35 years ago. The rates and spatial extent of soil loss and deposition areas can be established, and thus soil movement budgets are easily estimated, at scales ranging from small plots to watersheds. Generally, soil movements are estimated by comparing the activity of 137Cs (expressed in Bq.m-2) of cultivated soils to that of so-called reference sites (old prairie or forest soils) assumed to be uneroded. Different models to translate soil 137Cs variations into soil movements have been suggested over the years (equations 3 to 5). The authors used this technique in Quebec and France. The sampling of 63 cultivated fields near Quebec City, led to an average soil loss estimate of nearly 6 Mg.ha-1.yr-1. The erosion rates varied according the land use and slope steepness (Table 2). The results of this study also suggest that redeposition is an important component of the overall erosive process, particularly under low grade slopes and protective plant cover conditions. From 137Cs measurements obtained on a 180 ha unit watershed near Vierzy (France), it was estimated that 45% and 14% of the watershed area had experienced a net soil loss and gain respectively (Figure 4). The net sediment output was evaluated at less than 1.9 Mg.ha-1.yr-1. In a still on-going study, 137Cs is used to shed some light on the origins of the sedimentation problem impairing the waters of Boyer River (Québec). Soils from cultivated fields, stream bank soils and sediments from the river bed are counted for 137Cs. It was estimated that 78% of the counted bottom sediments originated from cultivated fields, while the other 22% was from eroding banks. The literature review along with examples provided by the authors highlight that the 137Cs technique may be an interesting complement to more conventional methods for assessing erosion and sedimentation