306-10 Source-to-sink evolution of lithogenic assemblages in a Himalayan river and its implications for arsenic transport

Abstract:

The geochemical processes governing arsenic enrichment in Himalayan-fed floodplains and deltaic aquifers of Southeast Asia are strongly influenced by the redistribution of major and trace elements during transfer of sediments from the Himalayas to downstream alluvial plains. However, the role of source-rock lithology, weathering, and fluvial transport in controlling downstream sediment geochemistry for arsenic mobilization in the Himalayan headwaters remains less understood. In the present study, we investigate the source-to-sink evolution of river sediments along the ~400 km long Beas River, in the western Himalayas, using sediment grain-size analysis, quantitative X-ray diffraction (XRD), major and trace element geochemistry (ICP-MS), river and pore water chemistry, weathering indices, and multivariate statistical analyses. Sedimentary concentrations ranged from Fe (14 – 37 g kg ^-1 ),Al (8 – 23 g kg ^-1 ), Ca (4 – 92 g kg^-1 ), Mn (208 – 515 mg kg⁻¹), Ni (13 – 42 mg kg⁻¹), Co (5– 16 mg kg⁻¹), and As (3 – 27 mg kg⁻¹), with systematic downstream variations reflecting changes in sediment provenance and transport processes. Strong positive correlations among Fe–Al (r = +0.98), Fe–Mg (r = +0.99), Fe–Co (r = +0.98), Fe–Ni (r = +0.99), and Ni–Co (r =+0.94), indicate that these elements are transported as coherent lithogenic assemblages rather than as isolated elemental phases. However, As showed a moderate positive correlation with Mn (r = +0.51) and showed a weak correlation with Fe and other lithogenic elements. Arsenic has distinct separation from the Fe-Al assemblage in PCA plot reflecting its conservative behaviour and role of secondary processes regulating its mobility in the basin. XRD refinement shows that river sediments are dominated by quartz (32–79 wt %), together with variable abundances of plagioclase, K-feldspar, muscovite, and minor calcite. Progressive downstream quartz enrichment coupled with reduced carbonate contributions indicates systematic mineralogical evolution during fluvial transport, providing a mechanistic framework for the observed geochemical patterns. These results indicate that the Beas river sediments naturally host arsenic and its transport is governed by the interplay of adsorption–desorption reactions, redox-sensitive dissolution and precipitation of Fe–Mn oxides, mineralogical transformations during weathering, hydraulic sorting of sediments, and continuous water–sediment interactions. The results establish a direct source-to-sink link between Himalayan weathering, downstream evolution of lithogenic mineral assemblages, and the geochemical framework that governs arsenic transport and enrichment in downstream alluvial and deltaic environments.

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