Jasechko, and 10 co-authors + laboratory people, reported in 2017 on their large project that evaluated the vulnerability of groundwater to contamination that they claim is closely related to its age. Groundwater that infiltrated prior to the Holocene have been documented in many aquifers and are widely assumed to be unaffected by modern contamination (but this depends on the geological conditions, such faulting, jointing, etc.). However, the global prevalence of these ‘fossil’ (using "fossil" is pejorative and should be referred to as "old" ) groundwater and their vulnerability to modern-era pollutants remain unclear (does this lack of clarity imply that the authors remain "unclear" on their conclusions?).
The authors analyzed groundwater carbon isotope data (12C, 13C, 14C) from 6,455 wells around the globe. The authors show that "fossil" groundwater comprise a large share (42–85%) of total aquifer storage in the upper 1 km of the crust, and the majority of waters pumped from wells deeper than 250 m (750 feet). However, half of the wells in the authors' study that are dominated by fossil groundwater also contain detectable levels of tritium, indicating the presence of much younger, decadal-age waters and suggesting that contemporary contaminants (like which? If only referring to isotopes as the "contaminant", the issue of sample and lab contamination can not be ruled out. This would negate the claims of "fossil" groundwater contamination.) and may be able to reach deep wells that tap fossil aquifers. The authors conclude that water-quality risk should be considered along with sustainable use when managing fossil groundwater resources. (This last sentence sounds like a subtle form of fear mongering by naive academic researchers who have limited field experience on matters that have not been fully investigated and explained).
[ It should be noted that this paper is otherwise quite comprehensive and useful, assuming the data can be relied upon.]
[A PDF of the above paper (here)]
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