I just got back from my vacation to Martha’s Vineyard, and I have to wonder–where do islands like that get fresh water? That particular island is a good 5 miles from the mainland, and both it and that part of the mainland are primarily made of sand. I imagine that it would be hard to dig wells into sucha sandy location. And there are no mountains to speak of, so spring water wouldn’t be used. It can’t be shipped in, as the sheer volume would be prohibitive.
Here’s a clue: Some roads have signs saying “Freshwater collection area”, which is a cryptic way of saying to not salt icy roads. But where would they collect it from? It’s all salt marsh.
At a depth about 70 feet below sea level is an aquifer consisting of bands of water bearing sand and gravel. About 145 billion gallons of it, with 12% being recharged annually (I assume by rain and snow melt).
Probably “voluntary” in the way paying the IRS is “voluntary.” It only means they count on you to do your part without having to send out the tax collector. As soon as you fail to volunteer, well, then it’s no longer voluntary.
Why is the water not salty there? I was just in St Augustine Florida visiting the old fort which is righ on the bay but there were freshwater wells within the walls.
On an island you can dig down into the ground stopping before you reach the actual sea level, and there is fresh water “floating” on top of the supply of salty water. Normally you dig places with lots of plants, because most plants aren’t real tolerant of salty water, so that’s a clue.
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Saltwater is denser than freshwater, so that the freshwater deposited on the island as rain will “float” as a lens over the salt water without mixing too much. If too much freshwater is removed, the saltwater will intrude into the island’s aquifer.
Concrete rain catchment basins (look kind of like a raised, sloping parking lot) are also seen, like one per neighborhood, e.g. on smallish Caribbean* islands (especially because there’s a rainier season and a drier season).
I was stationed at a LORAN station on San Salvador, Bahamas for a year. We had a rain catchment and wells for fresh water. Our Corpsman did a daily check on the water in the “day” tank to make certain it was safe for drinking. And we only were permitted to take “sea showers”*.
*sea shower = 1) Turn on the shower and get wet all over. B) Turn off the water, soap up, and scrub. III) Turn on the shower and rinse off.
An extreme example of an island you wouldn’t guess has “fresh” water is Clipperton in the Pacific.
A thin, virtually barren atoll surrounding a large lagoon. Yet the lagoon is ~fresh water. All supplied by rain in the middle of nowhere. The excess of rainfall rate over evaporation rate is quite small, but big enough over long periods of time.
