Importance of Springs and Groundwater in the Hydrological Dynamics of Mountain Basins in Southern Chile

Mountain systems are experiencing significant impacts from changes in precipitation and temperature, directly affecting natural water reservoirs like glaciers and snow. Projections suggest that glaciers in some ranges may vanish entirely by the end of this century. Currently, the loss of glacier mass is already altering streamflow and impacting dependent ecosystems. Snow cover has decreased globally, especially at lower elevations, due to more precipitation falling as rain rather than snow. This has dramatically changed the hydrology of mountain basins in the last decade, raising concerns about the sustainability of global water resources.

Groundwater recharge in mountains occurs through two main pathways: shallow flows replenishing aquifers formed by coarse sediments like slope deposits, and deep flows. These surface aquifers significantly contribute to runoff, especially during dry periods, by buffering streamflow. Studies highlight their critical role in hydrological systems.

In the wet Andes, groundwater plays a vital role in downstream flows amidst changing climatic conditions. However, the impact of cryosphere recession and altered precipitation patterns on mountain aquifer recharge and groundwater discharge remains poorly understood. This gap is particularly evident in the Southern Glaciological Zone, where mountain recharge processes lack comprehensive research.

Through isotopic monitoring, researchers analyzed glacial melt contributions in the Allipén headwater basin of the Wet Andes. Measurements of stable isotopes (¹⁸O and ²H) were taken seasonally across snow, glacial melt, lagoons, groundwater, and streamflow in nested basins. Using the MixSIAR model, the study showed that groundwater was the primary contributor in most basins, with its share ranging from 56% to 62%. Ponds followed with contributions of 13.5% to 23%, while glacier thaw accounted for 11% to 18%, despite glaciers covering just 1.5% of the basin area.

Hydrogeochemical analyses from springs, rivers, and wells across the basin provided insights into groundwater dynamics and rock-water interactions, especially in the volcanic context of the Allipén sub-basin. Measurements of physical-chemical parameters and major ions (e.g., Ca²⁺, Na⁺, HCO₃^-) revealed significant geological influences on water composition. Igneous and volcanic areas contributed higher sodium and potassium levels due to silicate alteration, while sedimentary zones showed higher sulfate concentrations from detrital material leaching.

A PHREEQC model confirmed processes like mineral dissolution and ion exchange, highlighting increased ion concentrations downstream due to prolonged water-rock contact. The findings emphasize the importance of understanding water-rock interactions and their influence on groundwater flow and chemistry. This study underscoring the role of geology in shaping water resource sustainability.