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[Theifofdreams]

While desperately digging for the caverns so I could get some cave spider silk for my moody dwarf, I stumbled upon a damp sand wall, and promptly had my dwarfs dig in new areas.
Shortly after, however, I noticed something. The damp sand wall was actually leaking water, and the area around it began to fill with water. As the sand is located nearly directly below a river, I'm confused as to whether this is an aquifer, or if the sand is literally filtering the water through it.
Anyone know the !!Science!! on this?
(Fortunately, regardless if it is an aquifer or not, I'd ordered separate staircases dug out, so the area being flooded is on a branch off from the main staircase, and then 3 levels down. Now I have an area for a well.)

[Wannazzaki]

Aquefier.

[Theifofdreams]

That's what I thought. (The wiki had nothing to say on the matter, so I figured I'd ask.)
Thanks for confirming that for me, though.

[Wannazzaki]

BASICALLY.  It's a rough wall. No getting around that. Unless you puncture it, the river remains in the river. I think sand + river is guarantee aquefier due to the nature of how sand is supposed to behave in the world. I could be wrong, but i dont think i am.

[Theifofdreams]

Hm... well, I'll poke around the area abit more, do some !!Science!! on this, and post the exact result.
So there is that, and the unlimited amount of water I now have.

On the other hand, my cloth-maker has gone insane, and Berserked.
Oh well, she was didn't have any friends or family, so no real loss.

[Loud Whispers]

Yup. Aquifer. Or you hit an actual part of the river.

[Nan]

Basically, in DF, all sand, soil and stone is completely and utterly impervious to water. While to a layman, an aquifer looks like water-bearing sand, the scientific explanation is that aquifer sand actually encapsulates millions of miscroscopic wormholes to the plane of water, this is why a single tile of aquifer - even if isolated - can produce and absorb an infinite amount of water. Attempts to mine out, isolate and weaponize utilize these microscopic wormholes have failed, because any disturbance of the sand substructure causes the wormholes to collapse. Exactly why these microscopic wormholes have formed in layers near the surface is still not known. However it is speculated that the delicate arrangement of sand/soil particles required to maintain the wormholes is only stable under a certain amount of pressure. Too little pressure, and the wormholes "blow out", too much and they simply collapse. And since the wormholes are technically empty space, any kind of substrate where the particles are too finely grained or compacted, is incapable of forming them.

[Theifofdreams]

See, I haven't actually mined through the wall yet. The squares contain, in order, a damp stone wall, a damp sand wall (which is dark brown) and a damp sand wall (which is light brown).
Immediately south of that are a rough marble wall, the mined out square, and another marble wall.
Both marble walls were dry right up until the draining started, as was the marble wall that was mined out.
I'm not able to mine out the sand wall to check if it's the river or not because my miner went off to mine something else, and only came back to that order after the water was 5/7 in that square.
If the square I mined through had been damp, I'd have just assumed "aquifer" on the spot.

[dragginmaster]

Df hack reveal tells all? Could u have punctured on a diagonal which would result in a low pressure leak?

[Minnakht]

One fun thing to remember is that if you really have an aquifer there, then it can be the tile on the z-level above. Even if that sand is not actually aquiferous or something, the tile above it can be, and thus water flows from the ceiling while you wonder what's wrong with your walls.
A damp wall by itself doesn't mean a leakage - only that there most likely will be some if you dig it out.

...this, or I have no idea, really.

Screenshots would be nice.

[Theifofdreams]

I... am way too lazy to put in the effort to make screen shots, but i can throw up a map

#.123##
111711
_____1
111111

This is all one Z-level. the level above is unmined in that location.
1=Rough hewn damp stone wall
2= Rough hewn damp sand wall (Dark brown)
3= Rough hewn damp sand wall (light brown)
7= water origination point
_= Water filled hallway
# are placeholders.

If the river bed is directly adjacent, I haven't mined into it, so I can't see it. As the water is now filling the halls, I cannot mine into it directly, either, without much effort.