In spite of the fact that scientists first discovered the presence of freshwater seabed aquifers in the 1970s, most people are not aware that vast quantities of fresh to slightly brackish (TDS concentration less than 10 g l−1) water are sequestered in passive continental margins around the world. Emplaced during the last glacial maximum when sea levels were much lower than today, these hidden caches of offshore fresh groundwater have been confirmed along the coasts of every continent, with the northeast United States and northern Europe being the most intensely studied. Some offshore aquifers sequester isolated pockets of freshwater, while others have hydraulic connections to land-based aquifers or receive recharge from terrestrial flows. Estimates based on testing and modeling predict that seabed aquifers may contain as much as 500,000 km3 of usable water, which is equivalent to one hundred times the volume of land-based groundwater extracted globally since 1900. For the most part, these aquifers are relatively close to shore (within 100 kilometers) and in shallow waters (depths not more than fifty meters below sea level), making them reasonably accessible.
In spite of the quantity and quality of offshore freshwater, the presently low commercial value of freshwater limits the possibility of achieving sufficiently attractive profit margins to entice private investment. Public money is not available either, since terrestrial freshwater scarcity has not yet reached the point where governments are willing to finance development of unconventional and potentially costly reserves. Indeed, even scientific research into these aquifers has not received much support – at the moment only the MARCAN project funded by the European Research Council is actively exploring offshore aquifers as a component of the effect that coastal groundwater has had on marine topography. If the dire predictions about looming freshwater crises turn out to be accurate, then the law of supply and demand suggests that the commercial value of freshwater will increase exponentially, since without freshwater no terrestrial species can survive. At that point, both the private and the public sectors may feel compelled by political, economic and humanitarian pressures to tap reservoirs of freshwater lying just offshore.
Under the globally accepted international law regime as manifested by the UN Convention on the Law of the Sea (UNCLOS), exclusive ownership and control over a seabed aquifer belongs to the relevant coastal state when the resource lies within that state’s domestic continental shelf, with the exclusivity extending to a distance of two hundred nautical miles from the low-tide shoreline. While the exclusive right to resources within domestic waters is well-established, UNCLOS does not address governance of seabed natural resources that straddle one or more political boundaries, and states have generally negotiated mutually acceptable, bilateral arrangements regarding transboundary resources. When the time comes to negotiate rights to transboundary offshore aquifers, states may choose to follow the pattern of unitization and joint development agreements that have become commonplace in the oil and gas industry. Under these structures, states appoint a single operator to conduct exploration and extraction activities while splitting revenues and expenses according to pre-agreed percentages, terms and conditions. In addition to providing templates for bilateral agreements, the hydrocarbon industry can also contribute technical expertise, since techniques honed for offshore drilling and submarine pipelines could easily be utilized to access and extract offshore freshwater.
A state wishing to develop seabed aquifers will need to be cognizant of the environmental impacts of its activities and the limitations contained in any treaties to which it is a party. Thanks once again to offshore oil and gas development, the environmental effects of seabed drilling are well known. Noise and light pollution from exploration and extraction equipment can affect marine denizens, especially mammals dependent on ecolocation. In addition, the drilling process brings chemically-enhanced cuttings from the subsoil to the sea floor; these cuttings are generally deposited on the sea floor near the drilling site, where they alter ecosystems by smothering and burying sedentary species. In addition, just as extraction of water from terrestrial aquifers can cause land subsidence, extraction from offshore aquifers may cause seabed subsidence in the continental shelf. Global and regional treaties require signatories to protect and preserve marine species and biodiversity, but enforcement of those obligations is often lacking.
In addition to economic, legal, technical and environmental issues, a state planning to utilize offshore freshwater must also weigh policy considerations. If development of offshore freshwater should come to be viewed as reasonable and even necessary due to insufficient quantities and/or quality of land-based freshwater, difficult choices will have to made about how much water to extract for present needs and how much to leave in situ for posterity. States and other stakeholders will also have to determine whether decisions regarding allocation should be made at the local, national, regional or international level(s). Currently, agriculture is responsible for 70 Percent of global freshwater use, yet domestic and municipal needs may become more urgent in the future, especially in large coastal cities. Seabed aquifers could be developed to meet those domestic and municipal needs. Vital ecosystems may have become compromised by water scarcity, and utilizing offshore freshwater to satisfy human requirements may preserve greater quantities of land-based freshwater for non-human species. In addition, humanitarian and economic demands may compete for priority: some offshore aquifers are co-located in the same submarine basins as hydrocarbon reserves, and drilling for one resource could contaminate the other.
Mainly for commercial reasons, utilization of the freshwater in offshore aquifers will not take place in the very near future. However, given the enormous quantities that are both available and accessible and in light of the dire predictions of future scarcity, the question is not if, but rather when, seabed freshwater will be tapped. Under current structures, proper stewardship of freshwater has been more of a dream than a reality. When faced with extracting and distributing the planet’s last reservoirs of freshwater during a time of great need, one can hope that our very clever species will act with prudence, wisdom, fairness and compassion.