Environmental magnetic characterization of the Alaknanda and Ramganga river sediments, Ganga basin, India

The Ganga basin represents a network of complex river systems, experiencing rapid changes due to natural and anthropogenic activities. The Alaknanda and Ramganga rivers lying adjacent to each other in the NW Himalayas represent two unique sub-basins of River Ganga having distinct physiography, climate, physical and chemical processes and land-use patterns. The present study employs mineral magnetic technique to unravel the controlling factors that influence alteration in sediment source, composition and transport processes along the routing pathway. The variations in magnetic mineralogy and concentrations provide a suite of information discriminating the sediment sources and transport mechanisms. The results show dominant ferrimagnetic mineral assemblage in the Alaknanda basin whereas an admixture of ferrimagnetic and antiferromagnetic minerals in the Ramganga basin. The high spatial variability in concentration of magnetic minerals in Alaknanda basin indicates the role of felsic and mafic rocks weathering as well as anthropogenic contribution in sediment magnetism. The tributaries flowing through peculiar lithology and anthropocentric polluted areas influence the magnetism of main river channels. The loss of mafic weathering signatures in downstream sediment of the Ramganga floodplains is due to reconstruction of man-made dam at Kalagarh, which traps upstream sediment load. The magnetically hard antiferromagnetic minerals in sediments of Ramganga floodplain region compared to mountainous region indicate its possible source either from pollution due to extensive land use for industrial and agricultural practices or from more oxidative weathering under low sedimentation and low energy environment. The findings signify new aspects of sediment sources, accentuate anthropogenic perturbation and demonstrates the applicability of environmental magnetism for developing sediment dispersal-mixing and pollution model for the larger Ganga basin.