Stable Isotope and Geochemical Approaches for Hydrological Characterization and Estimation of Seasonal Groundwater Inputs in the Humid to Semi-Arid Reach of the Kabini River, South India

Abstract

In tropical catchments, like the Kabini region of South India, understanding the intricate contributions of baseflow is essential for sustainable water management and ecosystem preservation. Baseflow, which is predominantly sourced from groundwater, plays a pivotal role in ensuring aquatic ecosystem health and supporting human communities, particularly in dry periods. Despite its significance, groundwater’s contributions are often overlooked and misunderstood. This study aims to understand the spatio temporal variations of baseflow contributions using hydrogeochemical tracer tools, including major ions and stable water isotopes (δ18O and δ2H). Employing a methodology based on hydrochemistry and isotopic analysis, the study executed rigorous fieldwork, resulting in a collection of 469 samples across two years from primary water sources: rain, groundwater, lakes, and rivers. Rainwater samples were gathered from five key sites, each representing distinct LULC patterns and climatic distinctions, offering an encompassing view of the region’s hydrological dynamics. To enhance the precision of rainwater sampling, we designed a custom collector, known as the ‘tube-dip-in-water collector’, specifically optimized for the Indian climate and inspired by International Atomic Energy Agency (IAEA) guidelines. A pivotal outcome of the research, facilitated by stable isotopic techniques, was the establishment of a local meteoric water line, termed as Kabini Meteoric Water Line (KMWL), defined by δ2H = 7.89(±0.11) × δ18O + 7.9(±0.37) (n= 105, R2 = 0.98, p < 0.001). Although this KMWL closely aligns with the Global Meteoric Water Line (GMWL) in terms of slope, its intercept is notably different, highlighting the unique hydrological processes at play in the Kabini catchment. Within the Kabini headwaters region, characterized by its humid climate backdrop, groundwater samples showcased unique isotopic signatures. Interestingly, the groundwater’s isotopic trend deviates slightly from the rainfall trend, implying that additional factors, potentially evaporation, shape its isotopic composition. Furthermore, a discernible cyclic pattern in groundwater δ18O values underscores the region’s pronounced seasonal hydrological oscillations. Diving deeper into the lc-excess analysis, a considerable segment of the region’s groundwater seems to stem directly from precipitation recharge. This notion is further reinforced by median values that hint at minimal evaporation, especially when contrasted against global standards. The Kabini River’s isotopic patterns highlight significant evaporation influences, as indicated by the deviation of its Local Evaporation Line (LEL) from the Kabini Meteoric Water Line (KMWL). To further understand surface water evaporation in tropical catchments, we employed the Craig-Gordon model to calibrate evaporation-to-inflow (E/I) ratios. Remarkably, the semi-arid region consistently displayed higher E/I ratios than humid region. With the integration of advanced Bayesian tools, uncertainties in isotopic and meteorological parameters were quantified. Expanding on our study of Kabini’s hydrogeochemical dynamics, tools like the Gibbs and Gaillardet diagrams helped us identify key factors affecting the basin’s water quality. Interactions between rock and water emerged as central processes, and the impact of rainfall, especially during the monsoon seasons, was evident. This understanding was supported by the Chadha and Piper diagrams, which showed a high presence of calcium and bicarbonates, indicating the typical Ca-Mg-HCO3 water type in the region. Using statistical analyses, we found strong relationships between different physicochemical parameters and stable water isotopes, suggesting common underlying processes. A notable finding was the strong link between Ca2+ and HCO−3, pointing to carbonate dissolution as a key process, with nitrate levels hinting at potential anthropogenic influences. Our examination of groundwater recharge, using the CMB method, illuminated the critical contribution of annual rainfall to the groundwater reservoir. Data from 2018 and 2019 revealed that approximately 10.5% and 8.6% of the annual rainfall, respectively, was channeled directly into groundwater recharge. This replenished groundwater becomes instrumental in sustaining the Kabini river’s continuous flow, as evidenced by the pronounced baseflow contributions, particularly noticeable post-monsoon. Conservative tracers like δ18O, Cl, and EC further established the interconnectedness between river and groundwaters. While our End Member Mixing Analysis (EMMA) offers empirical insights, comparison with studies like George and Sekhar (2022) highlights the merit of a multifaceted approach. In conclusion, this research presents a comprehensive understanding of Kabini’s hydrological intricacies, emphasizing the importance of holistic methodologies for tropical water management.