Novel Application of Isotopic and field-based survey for uncovering spring hydrogeology for sustainable management

Freshwater springs are the primary source of drinking water for people residing in remote and inaccessible mountainous terrains. Additionally, springs play a crucial role in maintaining the base flow of Himalayan rivers during the lean season. However, half of the Himalayan springs are experiencing a noticeable decline in discharge and are drying up due to current climate change and anthropogenic influences. The rejuvenation and restoration of freshwater springs have long been a challenging task due to the presence of complex geological terrain, along with fractured aquifers that connect across multiple watersheds. Previous studies have focused more on the watershed approach, lacking aquifer-targeted springshed management. This study integrates isotopic (δ¹⁸O, δ²H and ³H) and field-based hydrogeological and geophysical resistivity surveys to uncover multifaceted hydrogeological elements, including recharge sources, recharge altitudes, surface and subsurface conduits, and hydrological processes controlling spring flow for springshed management.

In the present study, 240 samples were collected from 120 springs for pre-monsoon and post-monsoon seasons, and 66 samples of precipitation were collected on an event basis throughout the year. The Local Meteoric Water Line (LMWL) for the study region has been developed as δ²H = 8.02*δ¹⁸O + 11.79 and found equivalent to GMWL. The isotopic values of the springs (δ¹⁸O ranges from -10.6‰ to -5.2‰ and δ²H ranges from -68.0‰ to -35.9‰) coincide with seasonal precipitation signatures (δ¹⁸O ranges from -16.23‰ to +2.82‰, δ²H= -125.0‰ to 20.5‰), indicating that the springs are recharging from the Indian Summer Monsoon. The precipitation exhibited an isotopic lapse rate of -0.4‰ and -4.1‰ for δ¹⁸O and δ²H, respectively, for a 100m increase in altitude. The recharge elevation of all springs, calculated from the isotopic lapse rate, lies within the altitude range of 1386 to 2194 m in the study area. These locations can be utilised for artificial recharge interventions, such as the construction of check dams, ponds, and trenches, as per the suitability.  

The hydrogeological survey reveals that freshwater spring discharge is influenced by gravity flow along local-scale geological discontinuities, including fracture zones, joint networks, and minor fault and fracture interactions that have developed within the rock mass. In contrast, geothermal springs are channelised across major regional geological discontinuities, such as the MCT. The geophysical resistivity survey in the springshed of the different springs is capable of mapping the subsurface conduits and pathways in the fracture-dominant lithology. These results provide site-specific guided intervention, along with validation of the isotopic results. The proposed methodological integration of stable isotopes and field-based hydrogeological and geophysical surveys can successfully aid in the investigation of complex mountain hydrology. The study can help policymakers, the government, and other stakeholders in the successful implementation of targeted recharge interventions for springshed management.