From floodplain to aquatic sediments: Radiogeochronological fingerprints in a sediment core from the mining impacted Sancho Reservoir (SW Spain).

The Sancho Reservoir (SW Spain) was built in 1962, about the time of maximum 137 Cs fallout, and it has been affected by acid mine drainage (AMD) particularly since the mining cease in 2001. This is a unique scenario for studying the radiogeochronological fingerprints in AMD-affected sediments deposited over the former flood plain. A sediment core sampled in 2011 was analysed for bulk density, 137 Cs, 239 Pu, 240 Pu, 210 Pb, 226 Ra, 228 Ra, 234 Th (238 U) and 40 K, and studied with various radiometric dating models. Bulk density revealed unsteady compaction and likely depositional events. The activity concentrations of 226 Ra, 228 Ra, 234 Th (238 U) and 40 K were uniform down-core, but declining overall in the upper 0-25cm, revealing changes in provenance except for 238 U, which increased in the top 10cm likely due to its supply by AMD. The AMD fingerprint was also found in the 239+240 Pu/137 Cs activity ratio, which increased in the top sediment layers. The 137 Cs and 239+240 Pu profiles show well defined peaks at the same depth, with inventories being about four times higher than the expected integrated atmospheric deposition in the area. The unsupported 210 Pb (210 Pbexc ) showed a complex non-monotonic profile interrupted at several sections, particularly around the 137 Cs peak. The whole dataset cannot be interpreted in terms of continuous sedimentation processes. Based upon correlated features in the bulk density and 210 Pbexc profiles, a series of depositional events (likely linked to peaks in the rainfall records) have been identified in the core. These events date back to the period comprised since the construction of the dam until its increase in height in 1972, which likely displaced upstream the main depositional area of riverine loads, as inferred from sediment trap data. The CRS (with a reference date) and (a piecewise) CIC models have been used for complementing and discussing the chronology.