Tag Archives: Fresh

Aquifer.

Largest freshwater aquifer of its kind found off the U.S. Northeast coast

A gigantic, relatively-fresh-water aquifer has been discovered just off the U.S. Northeast coast.

Aquifer.

Yellow crosses and the yellow dashed line show the inferred spatial extent of the low-salinity aquifer system.
Image credits Chloe Gustafson, Kerry Key, Rob L. Evans, (2019), Nature.

The aquifer is contained by the sediments of the seafloor and seems to be the largest of its kind (freshwater aquifer underneath a body of saltwater) that we’ve found so far. The aquifer stretches at least from the shore of Massachusetts to New Jersey, extending more or less continuously out about 50 miles to the edge of the continental shelf, a new study reports.

Thirsty?

“We knew there was fresh water down there in isolated places, but we did not know the extent or geometry,” said lead author Chloe Gustafson, a PhD candidate at Columbia University’s Lamont-Doherty Earth Observatory. “It could turn out to be an important resource in other parts of the world.”

The first clues that an aquifer rests in this area came in the 1970s when wells drilled off the coastline in search of oil sometimes hit fresh water. At the time, it was debated whether these were isolated pockets of water or a larger continuous body. About 20 years ago, study coauthor Kerry Key, now a Lamont-Doherty geophysicist, helped fossil fuel companies develop techniques to use electromagnetic imaging of the sub-seafloor to look for oil in this area. More recently, he decided to see if the same approach can be turned to spotting freshwater deposits in the area.

In 2015, he spent 10 days on the research vessel with Marcus G. Langseth and Rob L. Evans of Woods Hole Oceanographic Institution, taking measurements off the coast of southern New Jersey and the Massachusetts island of Martha’s Vineyard, where scattered drill holes had hit fresh-water-rich sediments. Their data indicated that these were not scattered pockets of water, but a more or less continuous structure extending from the shoreline far out through the continental shelf — in some areas as far as 75 miles. For the most part, the aquifer horizon spans from between 600 feet below the ocean floor to about 1,200 feet.

Based on the readings, the team is confident that the aquifer spans not just New Jersey and much of Massachusetts, but also the coasts of Rhode Island, Connecticut, and New York. All in all, they report, the aquifer holds an estimated 670 cubic miles of fresh water. So how did all this fresh water get there? The team was two hypotheses.

Aquifer section.

Conceptual model of offshore groundwater. Arrows denote groundwater flow paths.
Image credits Chloe Gustafson, Kerry Key, Rob L. Evans, (2019), Nature.

Some 15,000 to 20,000 years ago, toward the end of the last glacial age, most of the Earth’s fresh water was locked up as ice. In North America, these ice sheets extended through northern New Jersey, Long Island, and the New England coast. Since all of that water was solid ice, sea levels were much lower, exposing large surfaces of the continental shelf that today are submerged. As the climate warmed and the ice started melting, outflowing water formed huge river deltas on top of the shelf, and fresh water got trapped there in small pockets, eventually becoming submerged under the sea bed. This is the more traditional hypothesis as to how freshwater bodies can form beneath the ocean.

However, the team has reason to believe that the aquifer is still being fed by modern runoff from dry land. Rainfall and water infiltrating from other sources percolate through onshore sediment, Key explains, and is likely pumped towards the aquifer by the cyclical motions of the tide. This hypothesis is supported by the fact that the aquifer is generally freshest near the shore and saltier the further out you go — suggesting its water gradually mixes with that from the ocean.

This water is still less salty than ocean water. Fresh water usually contains less than 1 part per thousand of salt, and this is about the value found undersea near land; on the aquifer’s outer edges, it rises to 15 parts per thousand. Typical seawater is around 35 parts per thousand salt. As such, if water from the outer edge of the aquifer would be pumped out, it would need to be desalinated — but this would still be cheaper than processing seawater, according to Key.

“We probably don’t need to do that in this region, but if we can show there are large aquifers in other regions, that might potentially represent a resource,” he explains.

Key cites southern California, Australia, the Mideast, or Saharan Africa, as some of these regions, adding that the group hopes to expand its surveys there.

The paper “Aquifer systems extending far offshore on the U.S. Atlantic margin” has been published in the journal Scientific Reports.

Saltwater electrolysis.

New process can make hydrogen fuel out of seawater without destroying the devices

Researchers from Stanford University have developed a process to make hydrogen fuel using only electrodes, solar power, and saltwater from the San Francisco Bay.

Saltwater electrolysis.

A prototype device used solar energy to create hydrogen fuel from seawater.
Image credits Yun Kuang et al., (2019), PNAS.

Hydrogen fuel holds a lot of promise as the energy source of the future. It’s clean, doesn’t emit anything, it’s energy-dense, and it’s beyond abundant — if only we were able to develop a way of retrieving the element from its chemical constraints. A new paper describes a way to do just that, starting from saltwater.

Salt of the water

Why is this news? Well, it simply comes down to quantities — the Earth has a lot of saltwater, but not very much fresh water. Methods of producing hydrogen fuel from the latter have already been developed, but the fact of the matter is that fresh water is valuable. We need it to drink, we need it to wash, we need it to grow our crops and, as our planet’s population increases, we run a very real risk of not having enough water for everyone. So it doesn’t make sense to use it for energy — we need it elsewhere.

“You need so much hydrogen [to power our cities and economies that] it is not conceivable to use purified water,” said Hongjie Dai, J.G. Jackson and C.J. Wood professor in chemistry at Stanford and co-senior author on the paper. “We barely have enough water for our current needs in California.”

Salty water, in contrast, is plentiful — which also means cheap. There’s enough of it that we can turn it into hydrogen without upsetting the natural balance of Earth’s ecosystems too much. Hydrogen fuel also doesn’t emit carbon dioxide as it ‘burns’, only water. Given our current troubles with man-made climate change and habitat destruction, both are very appealing qualities.

Saltwater does, however, come with a major drawback. It’s not that hard to split water into hydrogen and oxygen, and we’ve known how to do it for a long time now. Just take a power source, connect two wires to and place their other end (or some electrodes if you want to be fancy about it) in water. Turn the power on, and you’ll get hydrogen bubbles at the negative end (cathode), and oxygen bubbles at the positive end (anode). This process is called electrolysis.

So far so good. But, if you try the same thing with saltwater, the chloride ions in salt (salt is a mix of chloride and sodium atoms) will corrode the anode and break down the system pretty quickly. Dai and his team wanted to find a way to stop those components from breaking down in the process.

Their approach was to coat the anode in several layers of negatively-charged material, which would repeal chloride, thus prolonging the useable life of the electrolysis rig. They layered nickel-iron hydroxide on top of nickel sulfide, over a nickel foam core. The nickel foam acts as a conductor, carrying electricity from the power source, while the nickel-iron hydroxide performs the electrolysis proper, separating water into oxygen and hydrogen.

As this happens, the nickel sulfide becomes negatively charged, protecting the anode. Just as the negative ends of two magnets push against one another, the negatively charged layer repels chloride and prevents it from reaching the core metal. Without the coating, the anode only works for around 12 hours in seawater, according to Michael Kenney, a graduate student in the Dai lab and co-lead author on the paper.

“The whole electrode falls apart into a crumble,” Kenney said. “But with this layer, it is able to go more than a thousand hours.”

Another bonus this coating brings to the table is that it allows for electrolysis to be performed at much higher currents. Previous efforts to split seawater had to use low current, as higher values promote corrosion. The team was able to conduct up to 10 times more electricity through their multi-layer device, which helps it generate hydrogen faster. Dai says they likely “set a record on the current to split seawater.” By eliminating the corrosive effect of salt, the team was able to use the same currents as those in devices that use purified water.

The team conducted most of their tests in controlled laboratory conditions, where they could regulate the amount of electricity entering the system. But, they also designed a solar-powered demonstration machine that produced hydrogen and oxygen gas from seawater collected from San Francisco Bay. Dai says the team pointed the way forward but will leave it up to manufacturers to scale and mass produce the design.

“One could just use these elements in existing electrolyzer systems and that could be pretty quick,” he adds. “It’s not like starting from zero — it’s more like starting from 80 or 90 percent.”

In the future, the technology could be used to generate breathable oxygen for divers or submarines while also providing power. And, perhaps, it could also be used in space exploration to limit the need for water purification systems — at least as far as power and oxygen are concerned.

The paper ” Solar-driven, highly sustained splitting of seawater into hydrogen and oxygen fuels” has been published in the journal Proceedings of the National Academy of Sciences.