Origin of arsenic in Late Pleistocene to Holocene sediments in the Nawalparasi district (Terai, Nepal)

A sedimentological and geochemical study was carried out to explore the origin of arsenic contamination in sediments in Nawalparasi district, in the western Terai of Nepal. The investigation tools include major, trace and rare earth element analyses of core sediments, as well as ¹⁴C datings, and O, C isotopic analyses on mollusk shells. The results show that black schists from the Lesser Himalaya highly contributed to the detrital input in Parasi during the Pleistocene–Holocene transition because of focused erosion related to rapid uplift and high rainfall along the Main Central Thrust zone. In addition, aquifer silts, sands, and most of the brown clays underwent a certain degree of chemical weathering and physical reworking, and show possible inputs from the Siwaliks during the Late Holocene. A possible correlation between late Quaternary climate regimes and the concentration of arsenic in sediments is suspected, with arsenic preferentially concentrated during the drier periods of the last 25 kyr BP. The process of arsenic eluviations in sandy and silty sediments can explain the lower arsenic concentrations in sediments during humid periods. During the drier periods, seasonal precipitation was smaller and temperature was lower, leading to wet (less evaporative) soils in swampy environments. This environment favoured the development of aquatic plants and bacteria growing within in the moist land areas, enhancing the strong weathering of initially suspended load particles (micas and clays), which were preferentially deposited in quiet hydraulic environments. These sorting and weathering processes presumably allowed the arsenic to be concentrated in the finest sediment fraction.