Testing the Use of 210Pbex to Study Sediment Connectivity in a Mediterranean Mountain Basin with Badlands

摘要

Basin management demands a sound understanding of sediment dynamics, particularly in Mediterranean mountain catchments with badlands, which affect water bodies and freshwater ecosystems. Connectivity has emerged as a framework for understanding how sediments move between geomorphic zones. We analyse the feasibility of excess lead-210 (210Pbex) to study sediment connectivity in a 4-km2 Mediterranean mountain basin with badlands (Vallcebre research catchments, Eastern Pyrenees) by applying mass-balance models for hypothesis generation and experimental testing. Badlands in the area are weathered by freezing during the winter and are eroded in summer by high-intensity showers. The eroded sediments may remain deposited within the streams from months to years. Application of 210Pbex balance models in our basin suggests: (i) a saw-tooth seasonal pattern of badland surface 210Pbex activities (increasing from October to May, and depleted in summer) and (ii) a downstream increase in sediment activity due to fallout lead-210 accumulation in stream-bed sediment deposits. Our experimental results (obtained during 2013 and 2014) showed low sediment 210Pbex concentrations, illustrating the fresh-rock origin of sediments, but also hampering their study due to high measurement uncertainty (especially for sediments with d50 > 20 μm) and dependence on sampling methods. Suspended sediment 210Pbex activity reproduced the simulated seasonal activity patterns for the badland surfaces. However, sediment activity decreased downstream, suggesting that flushing of fine sediments by flooding prevents fallout lead-210 accumulation on the stream bed. Overall, high sediment connectivity was revealed between the badlands, streams and outlet of the basin, as well as the fast transmission of 210Pbex by the finest sediment fraction. © 2017 John Wiley & Sons, Ltd.