Fluxes and storage of fine-grained sediment and associated contaminants in the Na Borges River (Mallorca, Spain)

Abstract

[eng] Samples of suspended and fine channel bed sediment were used to examine the spatial and temporal variability in the amount of fine-grained sediment and associated contaminants temporarily stored in the Na Borges River (319 km2) in Mallorca and the relationship of such channel storage to the fluxes of fine sediment and associated contaminant through the system. This Mediterranean groundwater-dominated river drains a predominantly agricultural catchment, although urbanisation during the twentieth century has changed the catchment hydrology. A re-suspension technique was used to obtain estimates of channel storage at monthly intervals during the 2004-2005 hydrological year at eight locations along the main stem of the stream (i.e. 26 km). The estimates of fine sediment storage ranged between 0 and 13,000 g m− 2, with a mean value of 2400 g m− 2. Only Cu exceeded the critical threshold (36 μg g− 1), established by existing guidelines for the contaminant content of fluvial sediment. The results demonstrated significant spatial and temporal variability, in response to the influence of urban point sources, agricultural practises, seasonal groundwater interactions and the first-flush effect. The amount of fine sediment entering storage during the study period was 515.2 t, representing the net increase in storage over the study period. As a result, the mean specific storage was 21 t km− 1. Suspended-sediment load and temporary fine sediment storage are the two basic components of the channel sediment budget that interact to determine sediment transport through a channel system, and they can therefore be used to compute the total input of sediment and associated contaminants to the system. Accordingly, storage values were compared with estimates of suspended sediment load and associated contaminant load values at three measuring stations along the river. During the study period, storage in the main channel system represented 87% of the sediment input and 68% of the contaminant input, indicating that deposition was more important than transport. The low gradient of the main channel and the low return period (i.e. 0-2.5 years) of the flood events that occurred during the study period meant that remobilised bed sediment and associated contaminants were redeposited downstream rather than being flushed to the catchment outlet as suspended sediment. Furthermore, the river bed is dry during the summer months, allowing sealing and crusting processes to stabilise the sediment deposited during the wet season and thereby reduce its availability for remobilisation at the beginning of the next wet season. Together, these factors promote sediment deposition and storage, with the result that sediment progressively accumulate over several hydrological years until a major flood event (i.e. return period ≈ 5 years) evacuates the stored sediment.