Variation in δ18O and δ2H values of rainfall, surface water, and groundwater in the Sukhna Lake basin in northwest India

Sukhna Lake in the Himalayan foothills is a vital wetland with important ecological functions in north India. In the present study, we have collected water samples from rainfall (n=482), surface water (n=146), and groundwater (n=404) during 2011-2017 for isotopic analysis (delta O-18 and delta H-2) to understand the recharge processes in the Sukhna Lake basin. The delta O-18 and delta H-2 values of rainfall show seasonality across the study area due to the different vapor sources. The moisture source for precipitation is the Arabian Sea during the monsoon season (July-September) and from a westerly source during winters (December-January). The delta O-18 and delta H-2 values of the surface water bodies show an evaporative signature across the study area. We also observed spatial and depth-related variations in the delta O-18 and delta H-2 values of groundwater. Here, we categorized groundwater samples into two depth zones based on the surface topography, isotopic values of groundwater, and aquifer characteristics. The groundwater zone-1a-b (depth:<65 m below ground level (bgl)) shows active recharge processes because of rainfall recharge and seepage from surface water bodies. We observed the dominance of precipitation recharge in groundwater zone-2 (depth:>150 m bgl) due to the regional groundwater flow pattern. We suggest two groundwater flow patterns present in the study area: local flow system is limited to zone-1a-b, whereas the regional flow pattern can be seen in zone-2. The present study provides new insight into the study area's recharge mechanism to understand the hydrological processes in any lake basin.