FRESH WATER AGES IN GLOBAL ARID AND MOUNTAINOUS REGIONS (Invited Presentation)

Rivers and groundwaters in arid and mountainous areas provide drinking and irrigation water for billions of people. Thus, understanding fundamental recharge and runoff is especially important to issues of sustainability, but also the degree to which water sources may be vulnerable to pollutants that may affect human health and water quality. For instance, the age of water influences its vulnerability to anthropogenic contaminants. However, these water ages remain poorly constrained in many arid and mountainous regions. Here we analyze carbon, hydrogen and oxygen isotope data in rain, snow, groundwater and streamflow to better constrain the age of fresh waters. First, we show that fossil groundwaters more than 12,000 years old dominate many semi-arid aquifer systems, and likely comprise more than half of global groundwater stored in the uppermost 1 km of the crust. Groundwater replenishment rates are known to be very low in many (semi)arid regions, suggesting that pumping these fossil groundwaters is unsustainable. Second, we show that wells tapping fossil aquifers often pump a mixture of fossil groundwater and much younger decades-old water, rendering these fossil waters potentially vulnerable to contaminants close to the land surface. Third, using a global database of streamflow oxygen isotope compositions, we show that streamflow contains smaller proportions of recent rain or snow in mountainous regions than in flatter terrain. Our results imply that watersheds in mountainous regions may be better equipped to store rain and snowmelt for multiple months prior to its release into streams.