Gravity pulls water down how far?

December 20, 2005 3:59 PM

How much potable-ish water is water is extremely deep underground?

Having investigated how far I'd have to drill for water on land I own (not too deep...surface water within 30 feet, 110 feet to a decent well supply), I've started to wonder...barring water being pumped to the surface in the form of magma-heated steam, how much is down there much further than is reasonable feasible for us to drill for it?

A side point to this question, how far down are we likely to find large cave structures (empty or filled with water)?

posted by Kickstart70 to science & nature (7 comments total)

Here is a list of the deepest caves in the world. Short answer is that caves have been explored to over 7000 feet (2140 meters) down. It's a safe bet that there are deeper ones no one has reached.

Here is a USGS ground-water atlas of the U.S. After a brief look at it, it seems like most wells are under 100 feet deep, with some as much as 500 feet.
posted by Kirth Gerson at 5:08 PM on December 20, 2005

There is a lot of underground water in aquifers, unfortunately in a lot of areas (like the american southwest) it is getting pumped out faster than it gets replenished which creates a lot of problems: New wells are required because the water table keeps dropping, ground subsides, and surface flows are reduced downstream because new rainfall ends up recharging the aquifer rather than feeding streams and rivers.
posted by Good Brain at 6:49 PM on December 20, 2005

As far as caves go, water underground is usually NOT in caves or anything like caves. By and large, most water down there is just in the ground floating down with the rocks and the dirt. Check out the wikipedia page on groundwater.
posted by pwb503 at 7:41 PM on December 20, 2005

As far as caves go, water underground is usually NOT in caves or anything like caves.

It would help me understand your comment if you would give your definition of "cave". Hereis the first definition on dicionary.com:
A hollow or natural passage under or into the earth, especially one with an opening to the surface.
If the water isn't in anything like a cave (and lots of it is in caves), what is it in?
posted by Kirth Gerson at 3:48 AM on December 21, 2005

The water is in the pore space of rocks. Caves begin to form when the host rock is dissolved (typically limestone) and forms solution channels. Like oil, the bulk of groundwater is not actually in large hollow spaces and "underground rivers", but filling the pore space in otherwise solid sedimentary and even fractured crystalline rocks.

When you drill a well, the idea isn't to just keep going until you hit an underground cavern containing water. You are actually in search of a rock layer with high porosity and permeability (like a loosely-cemented sandstone), which not only holds a lot of water, but is able to deliver it at a good rate.
posted by daveleck at 5:29 AM on December 21, 2005

As far as how deep such units can be found, that is going to vary by geologic terrain. As long as the subsurface rock has a composition favorable to transporting water (not to mention its need to be recharged with additional water), the limiting factors become heat and pressure - not only in scenarios where there is nearby magma, but also the heat/pressure generated by the burden of billions of tons of overlying rock.

Of course, the feasibility of drilling deep wells for water comes into play. Consider the economic return on a water well vs. an oil well.
posted by daveleck at 5:38 AM on December 21, 2005

I confess my last post was hasty; I was headed out the door. I understand that the goal of water-well drilling is to reach an aquifer, not a cave.

What I was reacting to was the offhand assertion that most underground water is not in "anything like caves", which seems to me to be undemonstrable. In fact, in parts of the world that have widespread karst topography, most of the groundwater may be in caves. This UKY page discusses the difference between karstic and non-karstic aquifers. This USGS page has a map of karst aquifers in the U.S.

Most caves have significant water in them, which is not surprising, since they are mostly created by water. Some caves have so much water in them that you need a boat to travel in them. Many more are completely full of water. In some caves, the historic boundary between the vadose and phreatic zones is visible, indicating where the surface of the aquifer once was.
posted by Kirth Gerson at 6:49 AM on December 21, 2005