Chemical and Isotopic Characterization of the stable isotopes of water in the deep aquifers of the middle Ganga basin

Water is the foundation of human survival, ecological stability, agricultural productivity, and industrial development. In regions such as the Indo-Gangetic Plain, where population density, agricultural dependence, and industrial expansion are exceptionally high, groundwater constitutes the most reliable and extensively used freshwater resource. The Middle Ganga Basin of Uttar Pradesh, located within one of the most fertile yet environmentally stressed regions of India, has witnessed a rapid increase in groundwater abstraction over recent decades, driven by irrigation demands, urban growth, and industrial activity. At the same time, surface water resources have been increasingly degraded by pollution, inadequate wastewater management, and anthropogenic pressures, enhancing the dependence on groundwater and raising concerns regarding its long-term sustainability, quality, recharge potential, and vulnerability to climate change. In this context, the present study, titled “Chemical and Isotopic Characterization of the Stable Isotopes of Water in the Deep Aquifers of the Middle Ganga Basin”, provides an integrated assessment of groundwater recharge potential, hydroclimatic trends, groundwater level dynamics, water quality status, and isotopic characteristics of groundwater to develop a comprehensive understanding of the subsurface hydrological system of the Middle Ganga Basin in Uttar Pradesh. The study area forms part of the largest river basin in India, the Ganga Basin, and is drained primarily by the Ganga, Sharda, and Ramganga rivers. Geologically, the region lies within the Indo-Gangetic alluvial plain, bounded by the Himalayas to the north and the Vindhyan range to the south, and is characterized by thick alluvial deposits, varying soil textures, and diverse geomorphological features. These factors strongly influence groundwater occurrence, movement, recharge, and storage. The basin supports a large agrarian population, with nearly half of Uttar Pradesh’s population directly dependent on agriculture, while agriculture’s contribution to the state’s Gross State Domestic Product has declined, necessitating increased groundwater extraction to maintain productivity. The study recognizes that continued exploitation, combined with climatic variability and land-use changes such as deforestation and urbanization, poses significant risks to groundwater sustainability. A major objective of this research is the delineation of Groundwater Recharge Potential Zones (GWRPZ) for the Middle Ganga Basin in Uttar Pradesh. Using an integrated geospatial and hydrogeological approach, thematic layers including rainfall distribution, drainage density, lineament density, soil type, slope, land use/land cover, and geomorphology were analyzed to identify areas favorable and unfavorable for groundwater recharge. The results reveal that less than 33% of the study area is suitable for effective groundwater recharge, with the most favorable zones concentrated in the Terai region of northern Uttar Pradesh. These areas benefit from higher rainfall, relatively flat terrain, permeable soils, and the presence of lineaments that facilitate infiltration. In contrast, the southern parts of the basin exhibit higher drainage density, clayey and impermeable soils, and reduced vegetation cover, leading to lower infiltration and recharge potential. To address the imbalance between extraction and recharge, the study identifies 92 potential sites along third-order streams where artificial groundwater recharge structures could be developed to utilize excess surface runoff and enhance subsurface storage. In addition to recharge assessment, the study conducts a detailed analysis of hydroclimatic variables and groundwater level trends to evaluate the impacts of climate variability and anthropogenic pressures on groundwater resources. Thirty years of hydroclimatic data (1991–2020) were analyzed using non-parametric statistical methods, including the Mann–Kendall trend test and Sen’s slope estimator, to detect trends in rainfall and temperature and to forecast future conditions for the period 2021–2044. The results indicate spatially heterogeneous rainfall trends across the basin. Rainfall in zones closer to the Himalayas exhibits relatively higher values but shows a declining trend in some zones, while other zones demonstrate marginal increases. Specifically, rainfall is decreasing at rates of approximately 2.1 mm/year and 0.5 mm/year in zones 1 and 3, respectively, while zones 2 and 4 show increasing trends of about 0.5 mm/year and 0.4 mm/year. Groundwater level trends reveal a concerning decline of approximately 10 cm per year in zones 2, 3, and 4, while zone 1 shows relatively stable conditions. The average daily temperature across the basin is increasing at a rate of 0.0068°C per year, indicating a warming trend that has implications for evapotranspiration rates, crop water demand, and groundwater recharge. The study further establishes that groundwater level fluctuations are influenced by a combination of rainfall variability, temperature, rural and urban population density, and evapotranspiration, with geographic factors such as proximity to the Himalayas playing a significant role in controlling local hydroclimatic conditions. To complement the hydroclimatic and recharge analyses, the study evaluates groundwater quality with respect to drinking and irrigation suitability. Chemical analyses reveal that groundwater in much of the Middle Ganga Basin is hard to very hard, primarily due to elevated concentrations of calcium and magnesium ions derived from the dissolution of aquifer minerals of geogenic origin. While natural geological processes are identified as the dominant source of groundwater mineralization, anthropogenic activities contribute locally to groundwater quality degradation. Drinking water quality assessment indicates that approximately 74% of the basin area, covering about 71,084 km², falls within the good-quality category, while only about 4% is classified as very poor. Irrigation suitability assessments using indices such as Sodium Adsorption Ratio (SAR), Kelly’s ratio, Percentage Sodium, and Permeability Index suggest that groundwater is generally suitable for agricultural use, with notable exceptions in districts such as Jaunpur and Unnao. Health risk assessment highlights the presence of heavy metals, particularly iron, aluminum, and arsenic, as significant contributors to non-carcinogenic health risks, with children identified as the most vulnerable group. However, cancer risk analysis indicates that the Incremental Lifetime Cancer Risk (ILCR) values remain below the critical threshold of 10⁻⁴ for both adults and children, suggesting a relatively low carcinogenic risk under current exposure conditions. A distinctive and crucial component of this study is the application of isotope hydrology to characterize groundwater recharge sources, flow dynamics, and residence times. Stable isotopes of water (δ²H and δ¹⁸O), along with d-excess values, were analyzed to trace the origin and evolution of groundwater in shallow and deep aquifers. The results show that d-excess values exhibit weak linear relationships with latitude, longitude, and depth, indicating the influence of multiple recharge sources and complex subsurface flow paths rather than a single dominant control. Isotopic signatures suggest that groundwater in the basin is recharged by a combination of rainfall and surface water, with the relative contribution varying spatially across aquifers. Tritium (³H) data reveal that deeper aquifers generally contain older groundwater, with estimated ages ranging from approximately 25 to 45 years, while shallow aquifers typically contain younger water. However, anomalously old groundwater ages exceeding 60 to 100 years were observed in some shallow aquifers, likely due to low permeability, poor terrain features, and longer subsurface travel times. Several sites with tritium values below 1 TU were identified, indicating the need for advanced dating techniques using isotopes such as carbon, uranium, or krypton to refine groundwater age estimates. Overall, this study presents an integrated chemical, isotopic, hydroclimatic, and geospatial assessment of groundwater resources in the Middle Ganga Basin of Uttar Pradesh. The findings highlight the limited natural recharge potential of the basin, the ongoing decline in groundwater levels in several zones, the influence of climate change on hydroclimatic variables, and the complex, multi-source nature of groundwater recharge. By combining conventional hydrogeological methods with stable isotope analysis, the study enhances the understanding of subsurface hydrological processes and provides valuable insights for sustainable groundwater management, artificial recharge planning, and climate-resilient water resource strategies in one of India’s most critical and vulnerable river basins.