Groundwater recharge sources inferred from stable isotopes and electrical conductivity in aquifers of the Ghaggar River Basin in northwestern India

The Ghaggar River basin lies between the Himalayan-fed Yamuna and Sutlej River systems in northwest India. The basin is underlain by alluvial deposits that comprise a major aquifer system that is a vital water source for irrigation in this agriculturally important region. Groundwater depletion here is a matter of great concern due to the indiscriminate pumping for irrigation over the last 4-5 decades. A better understanding of this groundwater resource requires knowledge of the sources of groundwater recharge. However, there have been no systematic surveys of groundwater sources to aid identification of groundwater flow systems. In this study, 166 groundwater samples were collected from various sources such as; tube wells, piezometer stations, and hand pumps drawing water from different screening depths (6-365 m bgl) in the study area. Samples were analyzed for their stable isotope compositions (δ¹⁸O and δ²H). Hierarchical cluster analysis was carried out for δ¹⁸O and δ²H data along with electrical conductivity data, and six clusters (from C1 to C6) were identified. Importantly, systematic spatial and vertical variations are observed in the isotopic signature of the groundwater samples. Samples taken from the depths < 80 m bgl showed a broad range of isotopic values and grouped into four clusters (C1 to C4). In contrast, samples from the depth > 80 m bgl showed a narrow range of isotopic value and grouped in two clusters (C5 and C6). Maximum samples in C1 and C2 clusters are spatially distributed in the upstream part of the Ghaggar Basin, and C3 and C4 are in downstream part of the basin, while samples of C5 and C6 clusters are spatially distributed throughout the basin. The isotopic values of groundwaters in cluster C1 and C5 show the influence of evaporative enrichment during the recharge process. The isotopic values of cluster C2 and C6 are very close to the precipitation value suggesting rain as the primary recharge source. Clusters C3 and C4 have isotopic values that are consistent with local recharge by canal water abstracted from the Sutlej River. This study not only reveals the spatial variation of recharge sources of groundwater in the Ghaggar River basin but also provides a method to identify the nested flow systems by using stable isotopes and electrical conductivity.