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Rivers flowing across the Himalayas are important water resources and deliver large amounts of sediment to regional and downstream ecosystems. However, the geochemistry of Himalayan river sediments has been less studied. Surface sediment samples collected from a typical trans-Himalayan river, the Koshi River (KR), were used to investigate the distribution, pollution status and potential sources of heavy metals. Heavy metals did not show significant spatial differences between the upstream and downstream areas of the river, but Cd and Pb displayed higher values in the upstream area. The average heavy metal concentrations in the KR sediments are comparable to the natural background values and are lower than the sediment guidelines. Pollution assessment using the geo-accumulation index (Igeo), enrichment factor (EF) and pollution load index (PLI) suggested negligible anthropogenic disturbances except for slight contamination by Cd, Pb and Cu at a few sites. Principal component analysis revealed that Cr, Co, Ni and Zn were primarily from the parent rock and that Cu, Cd and Pb were derived from both natural and anthropogenic sources. Despite contrasting environmental settings and human activities in the upper and lower reaches of the river, the heavy metals concentrations in the KR sediments showed consistency with natural backgrounds and negligible contamination. The geochemistry of river sediments is a useful indicator of environmental changes, and long-term observations of the geochemistry of trans-Himalayan river sediments are needed to understand the impacts of intensified climate change and human activities on the Himalayan environment. Image 1 • First evaluation of heavy metals pollution in Koshi River sediments were conducted. • The distribution of heavy metals in the up- and downstream portions were uniform. • Only Cd, Cu and Pb indicated potential anthropogenic disturbances at a few sites. • Heavy metals in the surface sediments were mainly related to watershed lithology. • Trans-Himalayan river sediments may indicate regional environmental changes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Integrated dataset of major ions and trace elements for Nepal Himalayan rivers basin. • Rock weathering is the primary factor controlling ion concentrations. • Anthropogenic impacts become significant in densely populated lowland basins. • Water in most rivers is safe for drinking, except for densely polluted urban Bagmati. • A uniform and comparable Himalayan Water Quality Index is proposed. Rivers originating from the Himalayas play a crucial role as primary water sources for a significant proportion of the population in Asia downstream. This paper presents a comprehensive synthesis of hydrogeochemical data, incorporating major ions and trace elements from 153 papers spanning 2000 to 2023, with a detailed analysis of 20 papers specific to Nepal Himalayan rivers. To bridge the research gap due to sparse data, we aimed to enhance inclusivity by amalgamating both seasonal and one-time datasets, computing mean values for singular instances, and employing grand means for multiple seasonal data points. Our findings highlight varied research attention across different river basins, with the eastern Koshi Basin receiving extensive attention, while the middle Gandaki and Bagmati Basins garner moderate attention, and data scarcity observed in the western Karnali Basin. Predominant ionic composition, notably Ca2+ and HCO 3 –, reflect lithological influences, primarily driven by rock weathering. Although anthropogenic impacts remain minimal in most basins, certain lowland areas with higher population densities exhibit increasing significance. Trace element contamination, notably Cd, Cu, Sr, and Zn, poses ongoing threats, both from natural and anthropogenic sources. Water quality assessments conducted for agricultural and drinking suitability indicated good status, except in the densely polluted urban Bagmati stem. Health and ecological risks are low, as determined by pollution indices such as Pi, NPI, HI, and CI. The Bagmati River's middle segment, with high population density and moderate agricultural activity, shows pronounced anthropogenic footprints, while downstream areas exhibited greater impacts. Additionally, this article proposes the potential use of a Water Quality Index tool to comprehensively assess water quality across all parameters in Himalayan water bodies, providing a roadmap for future environmental conservation in the Himalayas. This pioneering review represents a significant effort to understand Himalayan rivers geochemistry on a large geographical scale and emphasizes the need for integrated observation and consideration of river water quality across the entire Himalayan region, from high mountains to lowlands. [ABSTRACT FROM AUTHOR]

• Hg in meltwater runoff is controlled by total suspended particle. • Hg levels in RGB have significant seasonal and diurnal variation characteristics. • Proglacial lakes have a sedimentation effect on runoff Hg transport. • The potential risk of Hg release from glacial lake outburst increases under changing climate. Glaciers in the Himalayan region have been receding rapidly in recent decades, drawing increasing concerns about the release of legacy pollutants (e.g., mercury (Hg)). To investigate the distribution, transport and controlling factors of Hg in glacier-fed runoff, from June 2019 to July 2020, a continuous monitoring and an intensive sampling campaign were conducted in the Rongbuk Glacier-fed basin (RGB) on the north slope of Mt. Everest in the middle Himalayas. The total Hg (THg) and methyl Hg (MeHg) concentrations were 1.56 ± 0.85 and 0.057 ± 0.025 ng/L, respectively, which were comparable to the global background levels and were mainly affected by the total suspended particulate matter (TSP). In addition, THg and MeHg showed significant diurnal variations, with peak values appearing at approximately 17:00 (upstream) and 19:00 (downstream). Based on the annual runoff and average Hg concentration, the annual export fluxes of THg and MeHg were estimated to be 441 g and 16 g, respectively. The yields of THg and MeHg in the RGB were 1.6 and 0.06 μg/m2/year, respectively. Currently, the annual Hg export of meltwater runoff in the Himalayan region is approximately 337 kg/year. When flowing through the proglacial lake, the THg concentrations decreased by 32% and 15% in the proglacial lake water and in the outlet, respectively, indicating that proglacial lakes had a sedimentation effect on the Hg transport. The Hg export from meltwater runoff in the Himalayas will likely increase considering the meltwater runoff has been projected to increase in the future. Nonetheless, emerging proglacial lakes may exert ambiguous effects on the glacier exported Hg under changing climate. Proglacial lakes could lower the levels and amounts of Hg in the glacier runoff, whereas the outburst of proglacial lakes could lead to an instantaneous release of Hg stored in lake waters and sediments. Our analysis shed light on the environmental impact of glacier retreat in the Himalayas and highlighted the need for integrated monitoring and study of Hg in glacier runoff and glacial lakes. [Display omitted] [ABSTRACT FROM AUTHOR]

Glaciochemical records serve as one of the best archives and as good proxies to indicate regional and global anthropogenic influences. The Himalayas, with fragile ecosystems and pristine environments, hold the third largest reservoir of glacier ice and represent an ideal region to investigate trace metal pollution using glacier records. Limited studies on glacially recorded trace metals in the Himalayas usually collect samples from individual glaciers and report a few trace metals in different seasons. We provide a comprehensive and in-depth understanding of the glacially recorded trace metals in the central Himalayas regarding their spatial distribution, seasonal variability, and anthropogenic signals. We analyzed six representative metals (Cu, Zn, Cd, Cr, Pb, Co) and found that the trace metal concentration range largely varied between the studied metals and sampling sites. The Zn metal concentration is higher, attributed to the contribution of natural sources (e.g., forest fires, dust storms) and anthropogenic sources, including industrial and traffic-related emissions. The Pb concentration showed striking seasonality due to the relatively natural input of local material during the monsoon season and the regional and long-range transport of anthropogenic sources during the non-monsoon season. There was a clear spatial variation in certain trace metals, such as Cu, Zn, and Pb, showing decreasing trends with increasing elevation. The enrichment factor (EF) results showed that Zn metal was highly enriched, followed by Cu and Cd, indicating that Zn metal was relatively highly susceptible to intensified human activities. The seasonal paradox between the enrichment factor and metal concentration revealed that the EF of the monsoon season was usually higher than that of the non-monsoon season, and vice versa regarding metal concentrations primarily associated with metal deposition in regional climate regimes, particularly atmospheric circulation. We suggest that the analytical method can influence the trace metal concentration and EF calculation, resulting in a previously unrecognized bias in the seasonality of trace metals. Future research should prioritize stable isotopes of trace elements (e.g., Pb, Cu) in glaciers that would provide valuable information in identifying the potential source of anthropogenic inputs and the degree of extent affecting the glaciochemistry of the Himalayas. • Glacial records of atmospheric trace metal pollution over the central Himalayas were summarized. • Zn had the highest concentration at most sampling sites and Pb showed the most striking seasonality. • Trace metal concentrations were lower in monsoon season and higher in non-monsoon season and vice versa for enrichment factors (EFs). • Analytical methods can influence the trace metal concentration and EFs that bring bias in the seasonality of trace metals. • Isotope studies are needed to further isolate the sources of trace metals. [ABSTRACT FROM AUTHOR]