Earth’s OCS Cradles Huge Freshwater Reserves

The latest issue of Nature promises temporary relief to a planet where fresh water is quickly becoming scarcer and scarcer

Vincent E.A. Post of Flinders University in Adelaide and his coauthors Jacobus Groen, Henk Kooi, Mark Person, Shemin Ge, and W. Mike Edmunds report the surprising news that outer continental shelves worldwide contain huge freshwater and low-salinity aquifers.

Dr. Post et al. came to the conclusion that separately observed subsea freshwater deposits were not anomalous by reviewing an extensive collection of seafloor studies done for both science and oil and gas exploration. The study evinces vast meteoric groundwater reserves below earth’s oceans. Notable locations to date mainly lie off Australia, China, North America, and South Africa.

From hydrologic models, we know something about subsea groundwater accumulated and discharged through the nearshore seabed. However, its occurrence below the continental shelves has received little notice until now. The authors illustrate geology, key groundwater flow, and dissolved salt transport processes in cross-sections below the continental shelf during glacial and interglacial times. They also present a global overview of inferred key metrics for seven well-characterized vast meteoric groundwater reserves.

The useful contents of these subsea water basins range from fresh to a third saline, the upper limit of brackish. As technology has improved and reverse osmosis and desalination costs have dropped, partly saline water has become more attractive.

Short-term implications for a freshwater-starved planet may be enormous. Our coasts harbor 80% of the world’s population. New subsea sources, albeit nonrenewable, could offset water shortages in coastal cities and even relieve droughts. They could also be put to work mitigating anthropogenic land subsidence and seawater intrusion.

Lead author Post and his colleagues estimate that “the volume of this water resource is a hundred times greater than the amount we’ve extracted from the Earth’s subsurface… since 1900.” They peg the total as around 120,000 cubic miles (half a million cubic km) of freshwater. Its main origin: rainwater filtered through the ground surface into water tables that have submerged with sea level rise over the past 200 centuries. Its main origin: rainwater filtered through the ground surface into water tables that have submerged with sea level rise over the past 200 centuries.

Layers of sediment and clay contain the undersea aquifers, which greatly resemble bore basins beneath the land. The authors believe they can be accessed by either conventional offshore platforms or by horizontal drilling from islands or the shore.

Post et al. envision these freshwater supplies sustaining coastal areas for decades. By 2030, almost half the world population will be water-stressed according to UN Water, so the discovery could not come at a more opportune time.

The Australian-led international research team has added greatly to our store of knowledge and opportunities to alleviate climate change in the short run. Their work has mapped global topography and bathymetry to show all known occurrences of fresh and brackish offshore groundwater. It also indicates useful foci for further hydrologic work. The authors take the implications even farther than water study to advancement of sediment knowledge and marine geochemistry.

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