Fossil groundwaters: Relict of past climates or flow system size?

Chandler Noyes1, Jennifer C. McIntosh1
1Department of Hydrology and Atmospheric Sciences, University of Arizona

Many aquifers undergoing depletion contain ‘fossil’ groundwater (>12,000 years old), leading to the idea that pumped groundwater will not be replenished because it was recharged during wetter periods in the geologic past. However, groundwater ‘age’ is a function of distance from the recharge area, and ‘fossil’ groundwaters are often found in extensive aquifer systems. Using groundwater ‘age’ as a measure of sustainability propagates the myth that pumping ‘fossil’ groundwater is unsustainable and pumping ‘modern’ groundwater is always preferable. This study aims to determine if groundwaters are ‘fossil’ because they are part of a large subsurface flow system with relatively continuous recharge; or, if paleo-recharge rates were substantially higher than today. We synthesize groundwater ‘age’ datasets from regional aquifer systems in semi-arid regions, including the Nubian Sandstone Aquifer, Great Artesian Basin, Continental Intercalaire Aquifer, North China Plain, and Tucson Basin, and compare these ‘ages’ to hydraulic head distributions, distance from recharge areas, and geochemical tracers that can be used as qualitative indicators of age. These groundwater systems contain a spectrum of groundwater ages, indicating continuous groundwater recharge over the past million years, rather than episodic replenishment. The relationship between long-term recharge rates and those of the last 12,000 years remain unclear because of a lack of groundwater ‘age’ data covering the interval between modern and ‘fossil’ groundwaters. To examine the latter occurrence, a full suite of geochemical, isotopic, and noble gas tracers has been collected along a transect in the Tucson Basin to define groundwater age distribution more adequately.

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