Water isotopes and 222Rn in disentangling the impact of macroscale climatic controls and microscale hydrological processes in groundwater-wetland ecosystems in the lower Gangetic plain, Indian Peninsula

It is well established that nearby unconfined aquifer systems are typically hydraulically connected to shallow lakes and wetland systems. This connection between surface waters and adjacent groundwater systems has significant implications for the effective protection and management of the high environmental values often associated with lake and wetland habitats. Understanding the behaviour and interactions of groundwater and stagnant water bodies in these systems is crucial. This study explores the seasonal dynamics and interactions between groundwater, stagnant water bodies (SWBs), and surface water in the lower Gangetic floodplain of eastern India, using isotopic mapping (δ¹⁸O values) and ²²²Rn values.

The δ¹⁸O values of groundwater were found to vary seasonally and spatially across the region. Groundwater near the River Ganges exhibited lower δ¹⁸O values, which increased as the distance from the river increased. The lowest δ¹⁸O value (-9.0‰) occurred in the post-monsoon season, and the highest value (-0.8‰) was observed during the monsoon. The higher δ¹⁸O values in monsoon were likely influenced by irrigation, which introduced water with higher isotopic values into the groundwater. This seasonal fluctuation reflects the impact of land use and agricultural practices on groundwater composition.

The SWBs showed different δ¹⁸O patterns, with values varying seasonally due to factors like evaporation, rainfall, groundwater seepage, and the size of the water body. The mean annual δ¹⁸O value of SWBs was -0.20‰. Larger SWBs, which are less affected by evaporation and more likely to be connected to the aquifer, exhibited lower δ¹⁸O values (from -3.0 to 0‰). In contrast, smaller SWBs showed higher δ¹⁸O values due to stronger evaporation effects. A significant correlation was observed between the δ¹⁸O values of SWBs and rainfall, with a one-month lag. This suggests that the SWBs are primarily influenced by the hydrological cycle, with the addition of rainwater during the monsoon season lowering the δ¹⁸O values.

The study also examined groundwater recharge using data from three boreholes located 15 km from the River Ganges. These boreholes tapped different aquifers at varying depths, revealing seasonal fluctuations in δ¹⁸O values. Shallow boreholes (20 meters) exhibited higher δ¹⁸O values, reflecting recent rainfall and evaporation. Deeper boreholes (30 meters and 50 meters) showed more negative δ¹⁸O values, suggesting recharge from different water sources. These variations highlight the influence of different recharge events and the seasonal patterns of groundwater interaction with surface water.

Radon (²²²Rn) levels were measured in the boreholes to assess groundwater-surface water interaction. The radon data suggested that lithology (rock and soil types) played a significant role in groundwater composition. Borehole-1, located in silty clay, showed higher radon levels than the other two boreholes in silty sand, reflecting differences in uranium content in the soil. In conclusion, this isotopic mapping study reveals the complex, seasonal interactions between groundwater, river water, rainwater, and SWBs in the lower Gangetic floodplain, with important implications for water resource management and environmental protection.