Estimating Surface/Subsurface Sediment Mixing in Karst Settings Using 7Be Isotopes

This study shows that the cosmogenic radionuclide beryllium-7 can be used to track sediment movement through caves. The activities of beryllium-7 and cesium-137 were measured in two different karst settings at both surface and subsurface sites before and after storm runoff events. At one site, Be-7-enriched sediment was detected up to 1.5 km along a stream conduit after a moderate storm event; however, the activity of Cs-137 was too variable to show a meaningful pattern. The percentages of surface sediment that was found ranged from 0 to 52% along the entire 3 km cave stream and from 33 to 52% along the upper 1.5 km. At the other site, as much as 96% of the sediment initially discharged at the spring during a storm event was fresh surface material that had traveled into and through the cave stream. Moreover, during the 4 day runoff event, approximately 23% of the total suspended sediment flux was estimated to originate from surface erosion with 78% being reworked sediment from within the cave. The data in this study show that cosmogenic radionuclides with multiyear half-lives are too long-lived to track sediment origins in the caves; whereas, Be-7 with a 53.2 day half-life, can be used to track movement of sediment along cave streams.