Given that they're 2.2 kilometers underground, it looks like they can take advantage of the water table, the place where rocks are saturated with stored groundwater. The water table often holds aquifers, which can be accessed with only a bit of drilling.
Is groundwater accessible 2.2 kilometers down? Well, they're certainly below the water table. I've found a map of the water table depth of Wisconsin (chosen at random), and it shows that the water table is seldom more than 50 feet below the surface:
It is important to note that the water table doesn't mark the area where the water is stored; it simple marks the highest point of stored groundwater.
We do have a problem, though: How deep does groundwater go? 2.2 kilometers is pretty far down, relative to some sources of groundwater. For a good survey on aquifer depth, I chose this report (Ashworth & Hopkins (1995)), a summary of various studies of aquifers in Texas. They identified nine major aquifers in the state. Here they are, along with the estimated water depth.
- Ogallala: 600 feet
- Gulf Coast: 3,200 feet
- Edwards: 600 feet
- Carrizo-Wilcox: 3,000 feet
- Trinity: 900 feet
- Edwards-Trinity: 800 feet
- Seymour: 360 feet
- Hueco-Mesilla Bolson: 9,000 feet (Hueco), 2,000 feet (Mesilla)
- Cenozoic Pecos Alluvium: 1,500 feet
2.2 kilometers is about 7,200 feet, so most of these aquifers wouldn't be deep enough. Even the lower depths of some of the large ones contain dissolved solid minerals, that could be hazardous to human health. The USGS National Water-Quality Assessment (NAWQA) Program has a variety of maps and data detailing water quality nationwide. Various sources of terror for groundwater include
- Miscellaneous "dissolved solids"
- More miscellaneous pesticides
The water quality can vary by region. For example, this map of nitrate concentration in the High Plains (from (Gurdak et al. (2009)) varies widely, do to agricultural use and other sources. Location is everything when it comes to water quality.
A nationwide map of aquifers can be found here.
There's one last thing we have to address: How will these people access the groundwater? I would wager that they have decent drilling equipment, if they're that far down, so digging a well system (vertical or horizontal) shouldn't be too hard.