Understanding spring aquifer dynamics through observational data patterns, stable isotopes, and hydrochemistry – An account of experimental pilots in the Indian Himalayas.

Springs are a critical water source for the people of the Indian Himalayas, sustaining their domestic and agricultural requirements. However, spring discharges are declining and flow regimes have transitioned into ephemeral or intermittent systems, primarily due to rainfall variability, landuse transformations and anthropogenic actions. Consequently, water scarcity has become a major challenge. To ensure water and livelihood security, quantifying spring water responses and understanding aquifer recharge is paramount for maintaining spring ecosystem services. Experiments were conducted to enable site-evidence-based intervention designs for spring rejuvenation through a robust management framework. In this study, we operationalize pilot observatories in the Uttarakhand Himalayas, India and integrate the application of hydrological time-series analysis, stable isotopes and water chemistry to understand spring watershed behaviour. We instrumented springs (A_1, P₁, P₂, P₃) for high-resolution hydrological monitoring. Spring hydrodynamics was assessed by employing Hydrograph and flow recession analysis. Autocorrelation and cross-correlation functions were used to estimate the system memory and reveal rainfall and spring interdependence. Isotopes and water quality were sampled bi-monthly at selected springs/streams (S₁-S₁₇, St₁-St₅) since December 2021. Through isotope analysis and reflecting on the geochemical evolution of springwater along flow paths, attempts were made to understand the origin (recharge) of water types.  

Inferences show that spring A₁ indicates intricate flow networks and slow flow velocities, while P₁, P₂, and P₃ springs are characteristics of transmissive fractures. A low value of recession coefficient ‘α’ for A₁ depicts diffused fracture system compared to P₁, P₂, and P_3, which indicates rapid aquifer emptying and a well-interconnected flow network. Correllograms for A₁ decline (r_xx(k) value) steadily show a high memory of 120 days, while P₁, P₂, P₃ exhibit shorter system memory and poor drainage flow network. A better storage capacity and homogeneity of underlying geology for A₁ are revealed compared to P₁, P₂, and P₃. Isotope values range from -8.1‰ (S₁₂) indicating anthropogenic forcing at recharge zones to -9.7‰ (S₆), representative of natural recharge conditions. The characteristic δ¹⁸O-δD regression line has shallower slopes than Global Meteoric Water Line, indicative of multiple moisture source mixing. S₆ is monitored for intervention planning and shows isotopic values distinctive of high elevations and far transport of water-bearing clouds. Two hydrochemical facies HCO₃-Ca and mixed HCO₃-Ca-Mg, were determined from the Piper ternary diagram which indicates carbonate rock geology and flow evolution through pathways.

The research aims to improve the understanding of mountain hydrological processes and drivers of groundwater fluxes. Such an integrated-approach permits detailed process understanding and limits erroneous interpretations. Policymakers can extend the results across the Indian Himalayas to inform management decisions and frameworks. 

Keywords: Springshed hydrodynamics, systems memory, long-term observatory, spring aquifer, Himalayas