Use of fallout tracers 7Be, 210Pb and 137Cs to distinguish the form of sub-surface soil erosion delivering sediment to rivers in large catchments

Fallout radionuclides (FRNs) 137Cs and 210Pb are well established as tracers of surface and sub-surface soil erosion contributing sediment to river systems. However, without additional information, it has not been possible to distinguish sub-surface soil erosion sources. Here, we use the FRN 7Be (half-life 53days) in combination with 137Cs and excess 210Pb to trace the form of erosion contributing sediment in three large river catchments in eastern Australia; the Logan River (area 3700km2), Bowen River (9400km2) and Mitchell River (4700km2). We show that the combination of 137Cs, excess 210Pb and 7Be can discriminate horizontally aligned sub-surface erosion sources (rilled and scalded hillslopes and the floors of incised drainage lines and gully badland' areas) from vertical erosion sources (channel banks and gully walls). Specifically, sub-surface sources of sediment eroded during high rainfall and high river flow events have been distinguished by the ability of rainfall-derived 7Be to label horizontal soil surfaces, but not vertical. Our results indicate that in the two northern catchments, erosion of horizontal sub-surface soil sources contributed almost as much fine river sediment as vertical channel banks, and several times the contribution of hillslope topsoils. This result improves on source discrimination provided previously and indicates that in some areas erosion of hillslope soils may contribute significantly to sediment yield, but not as topsoil loss. We find that in north-eastern Australia, scalded areas on hillslopes and incising drainage lines may be sediment sources of comparable importance to vertical channel banks. Previous studies have used the combination of 137Cs, excess 210Pb and 7Be to estimate soils losses at the hillslope scale. Here, we show that with timely and judicious sampling of soil and sediment during and immediately after high flow events 7Be measurements can augment fallout 137Cs and 210Pb to provide important erosion source information over large catchments. Copyright (c) 2013 John Wiley & Sons, Ltd.