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An artesian basin is an aquifer in which water within permeable rock is kept under pressure between 2 layers of impermeable rock.

I don't know if any artesian basin exists like this in real life, but i haven't heard of any.

However, given that an artesian basin is under pressure between 2 layers of impermeable rock, would it be possible for it to pass underneath a sea without becoming salty due to the impermeable rock?

The sea it would go under is about 150-265km wide. Both sides have mountains on them & the area underneath one of those mountains is a subduction plate boundary.

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  • $\begingroup$ Your Wikipedia link does not say that about wells, not basins… unless all wells are to be compared to NSW Basin. Are they? $\endgroup$ Sep 18, 2023 at 22:04
  • $\begingroup$ Whose built world is this? Even if it was unfeasible in real-Earth geology, how many readers would even understand that, let alone doubt a half-way well written description? $\endgroup$ Sep 18, 2023 at 22:05
  • $\begingroup$ @RobbieGoodwin i don't know why but the wikipedia wage for artesian wells & artesian basins are the same. I'm mostly worldbuilding just for a personal project that i might make an online wiki for at some point, it's not for a novel. It's mostly an alternate earth type thing, most of the rules of the universe stay the same but the technological development, nations & geography are largely unrecognisable to our own earth. My main goals are more to create a coherent economy & geography for this world than write a story in it. $\endgroup$
    – OT-64 SKOT
    Sep 19, 2023 at 1:07
  • $\begingroup$ Thanks for sharing that much. $\endgroup$ Sep 19, 2023 at 11:50
  • $\begingroup$ For interest: The Great Australian Artesian basin mentioned in @Richard Kirk's answer has issues with aresnic content. Your bores could have various random "contaminants" if desired to make the story more interesting. These could be negative or positive in effect as desired. $\endgroup$ Sep 21, 2023 at 10:34

5 Answers 5

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In support of what you're looking for...

On a clear September day in 2015, after 10 years of working to get funding, my colleague Kerry Key and I stepped aboard the R/V Langseth, a research ship docked at the Woods Hole Oceanographic Institution in Massachusetts. We were about to lead a 10-day expedition to map a deposit of fresh water, size unknown, hidden 100 meters (about 330 feet) under the rocky seafloor. (Source)

Freshwater aquifers can and do exist beneath the sea floor. And frankly, the more shallow the sea, the easier it would be to rationalize the aquifer.

A continent does not stop at its shoreline; it extends well offshore as a rocky underwater shelf. The shelf ends at a steep slope that transitions sharply to deep oceanic seafloor. The rock and sediments that make up the world's continental shelves are not dry. Some rocks crack, allowing seawater to penetrate. And most shelves are covered by layers of sedimentary rock, which are like hard sponges with small, interconnected, water-filled pores.

Sediments at or just below the seafloor are typically 40 to 50 percent porous. The weight of the ocean above pushes water down into the sediment as far as it can go. Geoscientists still debate the maximum depth, but it can be at least several kilometers, although the seepage decreases rapidly with depth as the increased pressure closes up cracks and pore spaces. The rock's permeability—the ease with which water can flow through it—depends on how extensively its various pores are interconnected.

Because the shelf is a continuation of the continent, models of groundwater flow in land along the northeastern U.S. coast suggest there could be substantial amounts of fresh water hidden within the rocks and sediments below the continental slope's seafloor. But there are competing hypotheses about how such water might get there—and remain there. (Ibid.)

@L.Dutch's answer, which I upvoted, makes a point, but it might not be the crisis he's suggesting.

It's easy to rationalize an undersea fresh water aquifer — but only when the ground beneath the aquifer isn't so messed up that it can't reasonably retain the water. Consider this image from the quoted article:

enter image description here
Credit: Julia Ditto; Source: “Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA,” by Denis Cohen et al., in Ground Water, Vol. 48; January-February, 2010 (reference). Image sourced from Rob L. Evans, "Found: Giant Freshwater Deposits Hiding under the Sea" (July 1, 2023).

In all those examples you'll see that the flow of rainwater from the land is contiguous. If you break that up with a fault line, you should expect the aquifer to end at the fault. Worse, the fault line represents a way for salt water to enter the undersea strata, meaning the aquifer will push less into the undersea region than it might otherwise have done.

I note that you describe the fault line (the subduction layer) under the mountains... not under the ocean. From a practical perspective, that means the water is running down the mountain and being absorbed into the undersea strata. Since the fault line is behind the aquifer, you don't have a problem. What you won't have is an aquifer on the other side of the mountains unless you allow sufficient rainfall on that side (in a continental context, the mountains could believably cause a continent-side rain shadow).

And a word about undersea artesian wells...

This is more believable than you might think, but not because of pressure. Again, from the linked article:

Records of fresh water being found offshore go as far back as the 1800s. Fishers off Florida have occasionally reported “boils” of water on the ocean's surface, which they assume leaked upward from below. In some cases, they sampled the water and it did not taste salty; fresh water is less dense than seawater, so it rises. (Ibid.)

In other words, you don't need pressure. You only need a leak and enough volume of fresh water that the press of salt water doesn't inundate the leak, causing that region of the aquifer to turn salty. Permeable, impermeable, kinda doesn't matter. By definition an aquifer forms because the water stops going down, not because it can't go up. In other words again, it means you can use the idea of impregnable rock as a story concept, but it's unnecessary for the science to work. All you really need is an easier path for the freshwater to enter the aquifer from land than for the saltwater to enter in the sea.

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There is one point which I see as a big no no with respect to what you describe:

the area underneath one of those mountains is a subduction plate boundary.

This is a simplified tectonic map of the Adriatic sea, which resembles the sea you describe:

enter image description here

The presence of a subduction plate boundary will lead to faults which will break the 2 layers of impermeable rocks needed to maintain the basin separated from the salty water above it.

With those faults fresh water in the basin will get mixed with salty water from the sea.

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In the seventies, the London-Brabant massif was thought to have an aquifer that spanned the English Channel / La Manche in this way. It probably doesn't. It is possible, but for the reasons cited in several other answers, it is unlikely. Seawater will enter, and fresh water will leave if there are any faults.

The Great Artesian Basin in Australia is big enough to span the gap you require. If the basin was filled on the surface with salt water, this would be doing what you want. However, if you want an ocean that divides two land masses, then we need a 'U'-shaped or saddle-shaped deformation, or some fault-based graben that managed to seal itself, both at the faults and at either end. The 'neck' of the great Artesian Basin might work this way, but the subduction zone you mention makes a concave warp at the sea-bed rather unlikely.

If the impermeable rock was flat, but sloped, a bit like the aquifer found off the eastern US recently, and you drilled a deep well on the subduction zone side, you might get the artesian well you wanted. This is much more likely but the well would be very deep. For a 200 km span, and a 1% slope, you would have to drill down 2 km. You might be able to tweak the parameters in your story to make this work.

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  • $\begingroup$ OPs additional constraint about tectonic plates still makes it impossible though. The faultlines would break the continuous aquifier $\endgroup$
    – Hobbamok
    Sep 18, 2023 at 13:44
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    $\begingroup$ @Hobbamok You may be right. All depends on what the impermeable layer is. If you have something like London clay, it may be very flat, and bend and re-seal rather than fault. There are still rather too many 'what if's for my liking. $\endgroup$ Sep 18, 2023 at 15:15
  • $\begingroup$ on the other hand, this is worldbuilding so as long as it's theoretically possible a bit of handwaving can be applied imho $\endgroup$
    – Hobbamok
    Sep 18, 2023 at 16:33
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There is a deep fresh water artesian well on Spitbank fort just offshore from Portsmouth England. And probably on the other three sea forts as well.

https://en.wikipedia.org/wiki/Spitbank_Fort

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It's definitely possible, there are several areas off my island where you can dive down and drink fresh water coming up from the seabed. We did it as children for fun, you just cover your mouth with a cloth to stop from sucking sand and it was fresh water.

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    $\begingroup$ For interest - what island is that ? $\endgroup$ Sep 21, 2023 at 10:27
  • $\begingroup$ @RussellMcMahon Savaii $\endgroup$
    – Kilisi
    Sep 21, 2023 at 10:33
  • $\begingroup$ @Ki That sounds like an extremely interesting 'trick'. Presumably there are aquifers which could be tapped for freh water for general use. Is this in a particular area or in many plaves around the island? 35 countries so far - I haven't visited Samoa yet. I hope to. I'm a member of a church with a majority of Samoan members. . $\endgroup$ Sep 21, 2023 at 11:49
  • $\begingroup$ @RussellMcMahon I only know of two places I have done it, both within the same area, but I'm sure there are more. And yes, it is pretty cool. There was also a couple of springs nearby on land so I assume they're all from the same source. Samoa is a nice place to visit, but nothing you can't see better and cheaper elsewhere thats more geared towards tourists. $\endgroup$
    – Kilisi
    Sep 21, 2023 at 23:23

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