Fossil Groundwaters: Relict of Past Climates or Flow System Size?

Abstract:

Many aquifer systems 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 that water traveled from the recharge area, and in many cases ‘fossil’ groundwaters are part of extensive aquifer systems. Using groundwater ‘age’ as the exclusive measure of sustainability thus propagates the myth that pumping ‘fossil’ groundwater is always unsustainable, and that pumping ‘modern’ groundwater is always preferable. This study aims to determine if older groundwaters are ‘fossil’ simply because they are part of a large or long subsurface flow system with relatively continuous recharge; or, if paleo-recharge rates were substantially higher than today and modern recharge is negligible? To begin to address this question, this study synthesizes published groundwater ‘age’ datasets from well-constrained regional aquifer systems in semi-arid regions, such as the Nubian Aquifer, Great Artesian Basin, and Black Mesa Basin, and compare these ‘ages’ to hydraulic head distributions, distance from areas of recharge, and other geochemical tracers that can be used as qualitative indicators of residence time. These groundwater systems all 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.