Natural and mining legacies in Nalón river floodplain deposits (NW Iberian Peninsula)

Abstract

<p>Nalón River is one of the most important rivers draining the Northern watershed of Cantabrian Mountains (NW Iberian Peninsula). This drainage basin was subject to an intense coal mining activity during the 19th and 20th centuries. The resulting products of this mining activity (mainly those derived from coal washing) have been incorporated into the stratigraphic sequence of floodplains. This makes Nalón river a unique opportunity to study accretion rates in floodplains and to investigate the rate of sand sediment propagation and dispersion in gravel-bed rivers (two processes whose study is complex and elusive), as well as to study the spread of coal residues and other contaminants in the fluvial environment.<br>With these aims, we accomplished a stratigraphic survey of floodpain deposits in the lower alluvial plain of Nalón river. We identified a total number of 17 stratigraphic levels in a 3-m thick floodplain sequence. Some of these levels correspond to low-energy deposits (fine sand), while others correspond to high-stage flood deposits (very coarse sand and gravel), with abundant   sediment structures such as parallel lamination, cross-bedding and ripples. Samples taken from the floodplain sequence were prepared for grain-size analysis, organic petrography determinations (16 samples) and geochemical analysis (24 samples). Geochemical analysis measures the total concentrations for 63 elements by inductively coupled plasma-mass spectrometry (ICP-MS). ?). Additionally, we took 7 samples for luminescence (OSL) dating and one (small wood pieces) for 14C.<br>Coal is very abundant throughout the stratigraphic section, which suggests that sedimentation was simultaneous with the development of intensive mining that has developed in the last 150 years. On the other hand, there are several dams upstream the study section, the oldest one built in 1952. These dams have undoubtedly interrupted sediment continuity, so mining-derived sediments might not have represented a significant contribution to floodplain deposits after dam construction. However, the upper levels of the stratigraphic section contain abundant coal particles, which suggests that these levels pre-date dams (1952) or that the upper levels incorporated material derived from riverbed and bank erosion.<br>The objectives of this study are twofold: 1) to  analyse coal and  their associated  contaminants incorporated into fluvial sediments in the Nalón river; and 2) to reconstruct the sedimentary history of the alluvial plain, while exploring the use of coal and geochemical data as sediment markers.</p>