Contrasting origins of spring water in a 'water tower' of Northeast Asia: Evidence from stable isotopes and tritium

Mountainous regions are often described as 'water towers' because they are important sources of freshwater that sustain downstream water supply and ecosystems. With the increasing demand for groundwater extraction and climate change, detailed knowledge of the recharge patterns of mountain spring systems is required. Mountain spring recharge can be challenging to determine due to its various potential sources such as snowmelt, local rainfall or deeper, older groundwater. In this study, we used the stable (delta 18O and delta D) and radioactive (3H) isotopes of water to distinguish the recharge sources and flow paths of 13 springs in the Changbai Mountain area in Northeast China ('CBM springs') as well as nine springs located in a nearby area to the west ('western springs'). Our results showed that the sources of spring water were highly variable across the region, with high-elevation meltwater a dominant source for the CBM springs, while lower-elevation meltwater and local rainfall were the primary sources of the western springs. The western springs were fed by young waters (3H higher than 7.1 TU; young water fractions between 0.66 and 1) that exhibited high seasonal variability (0.6 to 4.1 %o increase in delta 18O between spring and summer), whereas the CBM springs had more depleted and less variable isotopic compositions (-2.6 to 1.2 %o change in delta 18O between spring and summer) and more complex age distributions (3H between 0.6 and 10.7 TU; young water fractions between 0.06 and 1). Our study provided insights into the vulnerability of CBM springs to climate change and western springs to increased human activities, particularly groundwater extraction.