Investigating Hydrological Processes and Groundwater Dynamics in the Upper Godavari Catchment, India, Using Environmental Tracers

The Upper Godavari (UG) catchment, India, faces critical water quantity and quality challenges driven by high evaporation rates and spatially variable rainfall, which significantly affects the catchment's hydrodynamics and threatens agriculture-based livelihoods. Understanding groundwater recharge processes and the impact of contaminants is essential for effective groundwater management. This study employs stable isotopes and tritium, alongside major ion geochemistry, to investigate hydrological processes in the riverine belt of the Western Ghats, characterized by Deccan basaltic terrain. Groundwater, surface water, and precipitation samples were collected in pre- and post-monsoon seasons (2022-2023). Post-monsoon stable isotope signatures indicate recharge predominantly from Indian monsoonal precipitation, while pre-monsoon stable isotopes reflect evaporation. The stable isotope signatures of the shallow aquifers imply rapid recharge and higher vulnerability to evaporation and contamination from agricultural runoff. In contrast, the stable isotope signatures of the deeper aquifers suggest older, more distant recharge sources with minimal recent contribution. Surface water closely resembles isotopically lighter monsoonal precipitation, and this plays a key role in recharging shallow aquifers. Tritium (³H) concentrations in groundwater (0.64 to 7.6 TU) locally exceed the annual average tritium concentration in modern rainfall (~7 TU), and locally higher values are observed post-monsoon and lower values pre-monsoon. This implies that most of the water in the subsurface is derived from recent rainfall with low transit times. Lumped parameter models (LPM) were used to estimate the mean transit times (MTTs) of groundwater, which ranged from <2 to 40 years. Older MTTs (25-40 years) were observed during the pre-monsoon season, reflecting slower recharge dynamics than the post-monsoon period (1.5 – 20 years). A mass-balance mixing model determined the contribution of each NO₃ source to the UG catchment. Results from the mixing model indicated that NO₃ from the irrigation return flow contributed 90%, and the other NO₃ sources contributed 8% in groundwater. These findings demonstrate the value of a multi-tracer approach in unraveling the hydrological complexities of the Upper Godavari catchment. The relatively young groundwater indicates high recharge rates, underscoring the catchment's resilience in sustaining water resources. However, this also highlights its vulnerability to decadal climatic variations and contamination risks. By elucidating recharge mechanisms, contamination pathways, and groundwater depletion patterns, this study provides insights to support sustainable water management strategies tailored to the dynamic hydrogeological conditions of the region.