How come if you dig a well far down enough, you hit water? Where does that water come from, and how did it get so far underground? How did people first figure out that digging in the ground would get you water? –Will, Washington, D.C.
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Ever hear of gravity, nudnik? Water in the form of rain, snowmelt, and seepage from lakes and rivers soaks into the ground until it hits an impermeable layer, typically rock or consolidated soil called hardpan, whereupon it collects in what’s known as an aquifer. A well merely has to reach down past the upper surface of this buried water, known as the water table, and you’re up to your knickers in the stuff. Granted, the subject has its infrequently explored aspects. Here’s one: Water dowsers claim they can detect underground rivers, the idea being that groundwater flows in narrow channels and not elsewhere (which is why you need a dowser). That flies in the face of the commonsensical view that, barring some peculiarity of local geology, the water beneath your feet is distributed pretty uniformly and you can find it digging most anywhere. But you know what? Underground rivers actually exist.
All that water isn’t just sitting there. It takes a while to sink in, and what with variations in rainfall, terrain, and geology there’s often lateral pressure as well. So aquifer water can flow, usually pretty slowly if it’s going through fine sand, clay, or limestone–well under ten feet per day. It can flow a good deal faster through coarse sand, gravel, or broken rock–say, 10,000 feet per day, which works out to the breakneck pace of 0.08 miles per hour.
