Worldwide, billions of people depend on groundwater for their water needs from wells, deep boreholes and natural springs. Even surface water in rivers and lakes is directly connected to that moving sluggishly below the surface. In fact the surface water level marks where the water table coincides with the land surface. From season to season the water table rises and falls and so too do river and lake levels, depending on fluctuations in rainfall, snow melt, evaporation and extraction. Where it is present, vegetation plays a role in the hydrological cycle, through transpiration from roots through stems and leaves, from which it is exhaled by minute pores or stomata; effectively plants are able to pump water through their tissues to a height of up to a hundred metres. Groundwater, like that at the surface, moves under gravity roughly parallel to the slope of the land surface from the place where precipitation infiltrates soil and rock. But the deeper it is the slower the flow and the less it is in direct contact with surface processes to be replenished by infiltration. Wells and boreholes rarely penetrate deeper than a few hundred metres, so that the vast bulk of groundwater is never used. Indeed most deep groundwater would not be drinkable or suitable for irrigation since over millennia or longer it dissolves material from the rock that contains it to become saline. In some deep sedimentary aquifers it may actually be composed of seawater trapped at the time of sedimentation.
The pore spaces in sandstones and fractures in limestones, the most common aquifers, are not the only conduits for groundwater. Crystalline igneous and metamorphic rocks are generally full of minute fractures resulting from their tectonic history. The deepest mines in crystalline basement, such as the gold mines of the Johannesburg area in South Africa, penetrate almost 4 km below the surface, yet are by no means dry and have to be pumped to stave off flooding. The water is a brine containing sodium and calcium chloride with high concentrations of dissolved, reduced gases such as hydrogen, methane and ethane (C2H6). Studies of the proportions of oxygen isotopes in the water reveal that the water in the fractures is very different from that in modern rainwater: this fluid is completely isolated from the modern hydrological cycle and is very old indeed. Just how old has now been determined (Warr, O. et al. 2022. 86Kr excess and other noble gases identify a billion-year-old radiogenically-enriched groundwater system. Nature Communications v. 13, Article number 3768; DOI: 10.1038/s41467-022-31412-2).
Brine extracted from a borehole in the floor of the Moab Khotsong gold/uranium mine also contains the noble gases helium, neon, argon, krypton and xenon. Noble gases are present in today’s atmosphere, so conceivably they may have originally entered the brine in rain water that seeped along fractures. However, when their isotopes are measured their proportions are very different from those in air. There are excesses of 4He, 21Ne, 22Ne, 40Ar, 86Kr and several isotopes of Xe. These isotopes are emitted during the radioactive decay of uranium, thorium and 40K, the main heat producing isotopes in the crust and mantle. Oliver Warr of the University of Toronto Canada and geochemists from Oxford University UK, Princeton University and the New Mexico Institute of Mining and Technology US, and the Sorbonne France show that originally atmospheric noble gases have been enriched in these radiogenic isotopes. Their present isotopic proportions therefore give clues to the time when air dissolved in groundwater was trapped in the host rock more than a billion years ago. A complicating factor is that the host rocks themselves are dated at about three times that age. They suggest that the fractures systems were initiated by the Vredfort asteroid impact at 2.0 Ga to form aquifers, but they became isolated from hydrological circulation around 1.2 Ga and now now contain the world’s oldest groundwater.
One of the implications of the study is that such trapped water may be present at depth in the crust of Mars, despite its current aridity. Another is that, because the fluid contains hydrogen, sulfate ions and hydrocarbon gases, it can potentially support organisms that use them to power their metabolism and reproduce. In 2008 microbes were found living in similar ancient groundwater 2.4 km below the surface in the Kidd Creek Mine, Canada, at a level of around 5 thousand cells per millilitre (50 times less than in surface water). They are powered by reduction of sulfate ions to sulfide. In 2008 another peculiar discovery in the deep biosphere emerged from the Mponeng gold mine near Johannesburg, South African (the world’s deepest) in the form of a living sulfate reducing bacterium Desulforudis audaxviator. DNA analysis of the ancient water revealed that it was the sole inhabitant, a biological mystery confirmed by later deep-biosphere studies in Death Valley, USA, and Siberia.
See also: Researchers uncover life’s power generators in the Earth’s oldest groundwaters, EurekaAlert, 5 July 2022; Mantle link with biosphere, July 2009