Indra Sekhar Sen

Coseismic changes in the hydrogeochemistry of hot springs link meteoric water circulation to seismic activity in an uplifting orogen. However, the impact of tectonic structure on meteoric water circulation through the crust and, conversely, the influence of meteoric water on seismicity within the Himalayan orogeny remains poorly constrained. In this study, we investigate deep meteoric water circulation and its relationship with regional tectonics and seismic activity by analyzing the stable isotopic compositions ( ¹⁸ O/ ¹⁶ O and ² H/ ¹ H, expressed as δ ¹⁸ O and δD) and water chemistry of hot springs located across three major tectonic boundaries. We integrate geothermometry‐derived reservoir temperatures with the spatial distribution and hypocentral depths of earthquakes to investigate the dynamics of deep meteoric water circulation in the Himalayas. We find that the maximum depth of the meteoric water circulation is ∼3–5 km, which is shallower than earthquake hypocenter depths (5–25 km). The data set further reveals that meteoric water circulation is deeper in extensional settings or in the presence of an additional heat source. Although meteoric water circulation is typically shallower in compressional settings, it can still be significant when thrust faults are tectonically active. We conclude that deep meteoric water circulation in the Himalayas generally does not reach earthquake hypocentral depths and that circulation depths are strongly controlled by the subsurface tectonic structure and fault activity.

Smallholder farmers produce approximately one-third of the world’s food supply, but face persistent challenges from soil degradation, limited access to affordable inputs and growing climate variability. Utilizing enhanced rock weathering (ERW) offers a pathway to address these constraints while generating durable carbon dioxide removal (CDR). Commercial-scale deployments by Mati Carbon during the 2024 summer monsoon rice growing season in Chhattisgarh, India applied basalt on over 2,000 acres ( 810 ha) farmed by more than 600 smallholders. Key agronomic findings from paired field measurements indicate a median yield increase of 22.9%, corresponding to estimated revenue gains of $303 per farmer, or approximately 20% of baseline household income. Inference via a simplified mass-balance model applied to soil geochemical data constrains the first-year weathering extent at τ = 0.27 (68% CI: 0.14–0.39), yielding an initial CDR potential of 1.6 (0.8–2.3) tons CO2 per acre (4.0 (2.1–5.7) t CO2/ha). Together, these results demonstrate that ERW can simultaneously deliver measurable livelihood benefits and climate mitigation under real-world smallholder conditions. Importantly, CDR generates monetizable carbon removal outcomes that are economically separate from on farm productivity gains, enabling climate finance to underwrite the costs of soil restoration without requiring farmer co investment. Combined measurement of CDR potential and yield response provides the empirical foundation for scaling ERW in smallholder systems. Establishing these outcomes demonstrates a replicable pathway for climate-financed agricultural development.

Predicted alterations to the hydrological cycle due to higher temperatures at the end of this century will impact riverine processes such as weathering, erosion, and sediment transport. The Himalayan River basins, with their steep slopes, heavy rainfall, and increasing river engineering projects, are excellent sentinels for monitoring climate change and human impacts on rivers. However, few attempts have been made to capture the river mountainous catchment interaction over shorter time scales (weeks to months) to capture pulses of enhanced chemical weathering rates and other riverine processes. Here, we present a weekly time‐series record of dissolved inorganic constituents near the mouth of the Alaknanda and Bhagirathi rivers—the two headwater rivers of the Ganga River—with a weekly resolution during 2018–2019. We report new estimates of discharge‐weighted concentrations and fluxes. We found chemical weathering rates of 98.2 ± 54.0 and 32.2 ± 20.4 t/km²/year and CO₂ consumption yields by silicate weathering of 3.7 ± 1.5 × 10⁵ and 1.8 ± 1.2 × 10⁵ mol CO₂/km²/year for the Alaknanda and Bhagirathi basins, respectively, which are significantly higher compared to the global chemical weathering rate and CO₂ consumption yield of ∼24 t/km²/year and ∼1 × 10⁵ mol CO₂/km²/year, respectively. We find that the concentration‐discharge relationship shows both chemostatic and dilution trends, with the Tehri dam strongly influencing the hydrology of the Bhagirathi River. The alkalinity‐DIC framework reveals that the Himalayan weathering acts as a net source of atmospheric pCO₂ over a timescale of 10⁵–10⁷ years. Further, we show that a sampling campaign spreaded well throughout the year is imperative in reducing uncertainties in flux estimation.

The Indian Summer Monsoon (ISM) and meltwater from the Himalayan are the two most important sources of water in the Indian subcontinent. However, the impact of ISM on Himalayan glaciers and subsequent stream hydrology remains largely unknown. To provide new insight into the impact of rainfall on glacial hydrology, here we present hydro‐meteorological and time‐series observations of meltwater stable water isotope compositions from the snout of the Chorabari glacier in the Upper Ganga Basin, Central Himalayas across the ablation season corresponding to 2019. We observe that rainfall events (>2 mm d⁻¹) on the glacier enhance discharge driven by ice meltwater in River Mandakini. Energy balance calculations reveal that one of the drivers behind enhanced ice meltwater contribution could be rain‐induced melting of the glacier where rainfall on the ice surface melts the glacier producing up to 13% of the total discharge at the glacier snout. Further, rainfall on glacier surface have other control on glacial processes—for example, snow metamorphism, ice flow dynamics such as short‐term acceleration in ice speed flow, and reorganization of the englacial and subglacial drainage network—that are poorly studied and needs further investigation. We conclude rainfall events on the glacier have a complex control on mountain hydrology. This study, therefore, provides an interpretative framework that calls for additional assessments of the direct and indirect impact of rainfall in glacial hydrology.

Microplastics (MPs)-i.e., plastic particles less than 5 mm in length-are becoming a growing environmental concern due to their potential ecotoxicological impacts on aquatic ecosystems. In India, MPs contamination is a significantly growing problem due to increased plastic production as well as its low rate of recycling. As a result, MPs research work in India has gained considerable attention in the last two decades. The objective of this study is to conduct a comprehensive review of the existing scientific literature on MPs in freshwater ecosystems (e.g., lakes and rivers) of India. A bibliographical search was used to conduct the literature review across a number of databases including ScienceDirect, Google Scholar, and ResearchGate. We found that in comparison to the marine ecosystem the source, transport, and fate of MPs in freshwater ecosystems of India are still underexplored, and we found only 18 relevant papers. This review work reveals that there is no standard procedure for separating MPs from water and sediment samples, and as a result, comparing the results was a challenging task. The larger MPs (>500 μm) in water and sediments were identified most commonly using the attenuated total reflection (ATR) Fourier Transform Infrared (FTIR) spectroscopy technique (ATR-FTIR), whereas smaller-sized MPs (<500 μm) were identified using FTIR fitted with a confocal microscope, also known as μ-FTIR imaging or chemical imaging. We found that white-colored fibers and fragments of polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE) were the most common polymer types in the freshwater ecosystems of India. Although research on MPs in freshwater ecosystems of India has gained momentum over the past decade, the literature review reveals a limited understanding of the impact of MPs' weathering patterns, the role of biofouling, and the role of water hyacinths on freshwater ecosystem services in India. Furthermore, the fluxes of MPs to the Indian oceans are not constrained, and atmospheric transport in high-altitude mountains, which have already been made fragile by climate change, has not been fully investigated. This study, therefore, calls for additional assessments of MPs in freshwater ecosystems-particularly in the central parts of India.

Silicate and carbonate weathering reactions consume atmospheric CO₂ depending on the type of weathering agents, namely carbonic (H₂CO₃), sulfuric (H₂SO₄), and nitric acids (HNO₃), and have potential climate implications. However, the importance of HNO₃ in weathering processes in the Himalayan glacierized basins has not been examined yet but is critical to better constrain the concomitant short (10⁶ years) variability in the carbon cycle as it can drive negative feedback to a climate. By analyzing time‐series hydro‐geochemical data of proglacial meltwater in the Ganga headwaters of Central Himalaya, we demonstrate that the weathering rate of carbonate minerals is increased 1.06 times when the role of HNO₃ is considered together with H₂CO₃ and H₂SO₄ in comparison to the role of H₂CO₃ and H₂SO₄. However, we also observe that the CO₂ drawdown rate decreases 1.13 times and 1.06 times when the role of all three acids is considered in silicate and carbonate weathering reactions, respectively, compared to the CO₂ drawdown rates linked to the role of H₂CO₃ and H₂SO₄. Moreover, the involvement of HNO₃ in chemical weathering can reduce the inorganic global carbon sink by releasing CO₂ into the ocean‐atmosphere system. We conclude that HNO₃‐mediated chemical weathering reactions are important processes that alter the geologic carbon cycle of high‐altitude glacierized Himalayan catchments as well as on a global scale.

Understanding the impact of land use/land cover (LULC) change on hydrology is the key to sustainable water resource management. In this study, we used the Soil and Water Assessment Tool (SWAT) to evaluate the impact of LULC change on the runoff in the Rur basin, Germany. The SWAT model was calibrated against the observed data of stream flow and runoff at three sites (Stah, Linnich, and Monschau) between 2000 and 2010 and validated between 2011 and 2015. The performance of the hydrological model was assessed by using statistical parameters such as the coefficient of determination (R²), p-value, r-value, and percentage bias (PBAIS). Our analysis reveals that the average R² values for model calibration and validation were 0.68 and 0.67 (n = 3), respectively. The impacts of three change scenarios on stream runoff were assessed by replacing the partial forest with urban settlements, agricultural land, and grasslands compared to the 2006 LULC map. The SWAT model captured, overall, the spatio-temporal patterns and effects of LULC change on the stream runoffs despite the heterogeneous runoff responses related to the variable impacts of the different LULC. The results show that LULC change from deciduous forest to urban settlements, agricultural land, or grasslands increased the overall basin runoff by 43%, 14%, and 4%, respectively.

Himalayan glaciers are invariably covered by supra‐glacial debris. Of these glaciers, the Chhota Shigri Glacier (CSG) in the western Himalaya has minimal debris cover (3.4%), yet has a comparable melt rate to other Himalayan glaciers. Utilizing osmium isotopic composition, and major and trace element geochemistry of cryoconite, a dark colored aggregate of mineral and organic materials on the surface of the ablation zone of the CSG, we show that the surface of CSG is essentially free of anthropogenically emitted particles, contrary to many previous findings. Given this and the overall lack of debris, we conclude that the high melt rate of CSG is primarily related to the increase of the Earth's near‐surface temperature linked directly to global warming. Therefore, the future meltwater supply for glacial‐fed rivers originating from Lahaul and Spiti region would be most vulnerable for >50 million population living downstream and requires immediate attention.

The Indo-Gangetic Plain (IGP) is one of the most highly polluted regions of the world, yet the temporal pattern of transport of anthropogenic aerosols from this region to the Himalayas is poorly constrained. On the basis of the seasonal variation of planetary boundary layer heights, air mass back trajectory analysis, and year-long time-series data for ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, and ¹⁴³Nd/¹⁴⁴Nd from aerosols collected over a high-altitude station, we demonstrate that anthropogenic Pb transport to the glacierized catchment has a seasonal pattern. The Pb isotope data reveal that during winter, the thinned boundary layer traps up to 10 ± 7% more coal-derived Pb in the IGP. In contrast, in nonwinter months, a thicker boundary layer and enhanced subtropical westerly winds result in efficient Pb transport to the Himalayas. As Pb isotope ratios are robust conservative chemical tracers and Pb is predominantly derived from anthropogenic sources, these observations suggest that enhanced transport of anthropogenic aerosols to the glacierized catchment of the Himalayas coincides with higher near-surface temperatures in the summer, creating positive feedback that enhances melting. Our results further suggest that >50% of Pb in the Himalayan aerosols originates from the resuspension of historic Pb derived from phased out leaded gasoline, highlighting the importance of legacy Pb stored in the Indo-Gangetic Plains.