Rare earth elements in suspended and bottom sediments of the Mandovi estuary, central West coast of India: influence of mining

Abstract

Rare earth elements (REEs) in the suspended particulate matter (SPM) of the Mandovi estuary indicated that the mean total-REEs (∑REE) and light REE to heavy REE ratios are lower than that of the average suspended sediment in World Rivers and Post-Archean average Australian shale. High ∑REE were associated with high SPM/low salinity and also with high SPM/high salinity. Although the ∑REE broadly agree with SPM levels at each station, their seasonal distributions along transect are different. SPM increased seaward in the estuary both during the monsoon and pre-monsoon, but consistently low at all stations during the post-monsoon. The mean ∑REE decreased marginally seaward and was <25% at sea-end station than at river-end station. Spatial variations in ∑REE are maximum (64%) during the pre-monsoon. Strong to moderate correlation of ∑REE with Al, Fe and Mn in all seasons indicates adsorption and co-precipitation of REEs with aluminosilicate phases and Fe, Mn-oxyhydroxides. The ratio of mean ∑REE in sediment/SPM is low during the monsoon (1.27), followed by pre-monsoon (1.5) and post-monsoon (1.62). The middle REE- and heavy REE-enriched patterns with positive Ce and Eu anomalies are characteristic at every station and season, both in SPM and sediment. They also exhibit tetrad effect with distinct third and fourth tetrads. Fe-Mn ore dust is the most dominant source for REEs. However, the seasonal changes in the supply of detrital silicates, Fe-Mn ore dust and particulates resuspended from bottom sediments diluted the overall effect of salinity on fractionation and distribution of REEs in the estuary.