Aquifer diagram
Schematic diagram of a coastal aquifer system. In many places, fresh groundwater extends far out to sea and may constitute a water resource for coastal cities in dry places. U.S. Geological Survey diagram

Underground boundaries between land and sea aren’t as stark as they are to us as we stand on the beach. Water knows this. Groundwater everywhere responds slowly to changes above, even geological changes. In most of the Earth’s crust, water moves around a meter per year. This enormous contrast between surface water and groundwater may be the big fact that hydrologists appreciate more than the rest of us, even geologists, who know a lot about slow things.

Seawater moving inland is a well-known problem for coastal cities. It arises when groundwater is pumped out faster than it can be replenished, and saltwater moves into the space. In the city of Fremont, the Alameda County Water District successfully manages saltwater intrusion in its major water source, the Niles Cone, by effectively building a wall of freshwater along the outer rim of the cone.

The opposite situation, freshwater intrusion, can occur offshore. Fresh groundwater lying above sea level on land would have the pressure needed to push aside the denser, salty water of the ocean once it reached that level. And scientists have known for a long time about freshwater springs that occur offshore. In the 1970s, U.S. government geologists learned that fresh water may underlie the seafloor for a surprising distance offshore, up to 100 kilometers in places.

Since that time, we’ve learned a lot more. Offshore freshwater makes sense when we consider the long series of ice ages Earth has been going through for the last 2-1/2 million years. When the glaciers are high, the seas are lowered by as much as 60 meters, which exposes a huge area of land—very fertile land, I should add. For many thousands of years, rains and rivers put fresh water into this ground. Between glacial periods (as we are today), the sea floods in to drown that land, chewing up its soils and forests in the advancing surf. But the groundwater, now offshore, remains fresh because the forces trying to push it out (density differences) are very small compared to the forces that put it in (gravity).

The review paper in the December 5 issue of Nature pointed out that offshore fresh groundwater occurs around the world and could be significant for many countries that have short water supplies. Many news outlets treated this as a “discovery,” which in science journalism usually means only that reporters (or their editors) hadn’t heard of it before. What should Californians make of this news?

The answer is that we’ll need to support a lot of scientific exploration to learn what’s off our coast. The geophysical techniques available are limited in what they can tell us, so we’ll need a systematic program of borehole drilling to map out the possibilities. This expensive research has usually been for the benefit of oil and gas producers. It’s conceivable that petroleum companies could take an interest in producing water from their offshore leaseholds, though it’s pretty unlikely as a widespread strategy.

But let’s look on the bright side and imagine how it could be. California wouldn’t have to deal with the uncertainties of international water law or maritime law, so that’s good. Freshwater trapped since the height of the ice age could be pumped ashore and treated for use by cities, which have a strong economic interest in being independent of changes in weather and climate.

A prototype that might point this way is being planned by the water supplier for Monterey. California American Water has plans (and money) for a desalination plant that would treat seawater. Its scheme would drill sideways under the seafloor from a land-based rig and pump saltwater from beneath the Monterey Bay National Marine Sanctuary. The agency has applied for a state grant that will get the drilling started late this year.

If projects like this could tap fresh or even brackish water from offshore instead of seawater, the costs of desalination would be dramatically lower. Expect to hear about this topic from time to time. But I expect things to proceed at roughly the speed of groundwater itself.

Could We Find Tomorrow’s Water Supply Under the Ocean? 6 February,2014Andrew Alden

  • Illuminating post, thank you. Do we have any guesses how much freshwater is trapped under the oceans, as a percentage of the planet’s fresh water?

  • Pal Pauer

    More than all of the “Sweet-Water” in our lakes and rivers! Please see,

  • Andrew Alden

    Wow, that’s a new variety of crackpottery, and I thought I’d seen it all.

    The Nature paper estimated half a million cubic kilometers in offshore aquifers, but that has only one significant digit and it doesn’t represent the useful, accessible fraction. It’s about five times the volume in the world’s freshwater lakes.


Andrew Alden

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as “Best of the East Bay” from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present.

Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

Sponsored by

Become a KQED sponsor