Pleistocene to Holocene Evolution and Connectivity of Groundwater Flow Systems Using Multi-Tracer Approaches

Abstract

Groundwater accounts for the majority of accessible freshwater on Earth, provides drinking water for over two billion people, and supplies over 40% of agricultural demands for water. Numerous aquifer systems around the world, in semi-arid regions, are undergoing substantial depletion due to water demands that exceed modern recharge. Many of these aquifers contain ‘fossil’ groundwater (> 12,000 years), generally considered to be non-renewable because it recharged in the geologic past under different climatic conditions. However, others have suggested that groundwater age should not be the sole indicator of sustainability, as age is also a function of aquifer size and travel distance from the recharge area. To better understand what information groundwater age provides about sustainability, susceptibility to contamination, and whether these systems are still responding to past climates, this study uses multiple age tracers of varying timescales and paleoclimate indicators at semi-arid sites in Tucson, Arizona and Saskatchewan, Canada. Additionally, groundwater age data from nine other regional aquifer systems around the world were compiled to further assess modern recharge rates, aquifer response times, and groundwater velocities.In the Tucson Basin, 3H, 85Kr, 39Ar, 14C, and 4He were applied to constrain groundwater age distributions from modern to tens of thousands of years. Effects of groundwater mixing were deconvolved using lumped parameter modeling, and mean groundwater ages are then used in conjunction with noble gas temperatures (NGTs), the first-ever measurements of Kr and Xe isotopes for the region that reconstruct water table depths (WTDs), and stable water isotopes to identify distinct climatic periods. We find a period of enhanced aridity and reduced recharge from 2.9–1.6 ka, after which NGTs cooled and WTDs quickly rebounded from 1.6–0.34 ka, possibly capturing effects from the Little Ice Age. Recharge was somewhat continuous after this point through today, with tracers identifying groundwater components as young as ~12 years. In Saskatchewan, groundwater in the upper aquifers of a glacial intertill aquifer ranged from modern to 15.5 ka, generally correlating with recharge during/after the final retreat of the Laurentide Ice Sheet and the onset of the Holocene. In the deeper buried valley aquifers, groundwater ages ranged from 43.1–73.5 ka and displayed unique geochemical and isotopic signatures, suggesting that they are isolated from overlying and underlying units. While the intertill system appears to have minimal connectivity to the deep subsurface, it is apparent that the upper aquifers are interconnected to one another, as well as the surface. In the latter case, the presence of groundwater < 60 years and anthropogenic contaminants indicates the upper aquifers are susceptible to recent surface contamination on the order of decades. In the synthesis study, while aquifers ranged from 22–1500 km in size, fossil groundwater was ubiquitous and identified in all ten aquifer systems examined. We find that aquifer response times vary by about three orders of magnitude, largely due to aquifer system size. A significant strong linear correlation between minimum aquifer response times and maximum age of groundwater along the flow path reveals that time to reach near steady-state in these systems is approximately 13% that of transport times. Finally, intercomparisons of aquifer response times, groundwater velocity, modern recharge rates, and groundwater ages reveals that aquifers with < 10 mm/yr of recharge are somewhat disconnected from modern climates regimes. Overall, this study shows the importance of using multi-tracer approaches when considering water resource problems of sustainability and susceptibility and that relying on groundwater age alone may lead to an incomplete understanding. Further, we show that many aquifers of varying size are still responding to past climatic perturbations and are disconnected from modern climate regimes, an important consideration for water resources managers.