A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How come if you dig down far enough, you hit water? Where does that water come from?

January 5, 2007

Dear Cecil:

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?

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.

First let's finish answering your question. Springs form when the ground surface dips below the water table, allowing hydraulic pressure to push the water out. Once you've noticed moisture bubbling to the surface in spots, no great insight is required to guess there might be more where that came from. Sure enough, people have been digging wells since neolithic times. A well at the Atlit-Yam ruins in Israel is estimated to be around 8,000 years old, and one at Kissonerga-Mylouthkia in Cyprus may be a millennium or two older.

Well-digging is a handy skill — more than 95 percent of the planet's usable fresh water is underground. Roughly a million cubic miles of groundwater can be found within a half mile of the earth's surface. The High Plains aquifer in the U.S., for one, stretches from South Dakota to Texas, and in some places is more than 500 feet from top to bottom.

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.

Sometimes, though, the water (a) picks up speed and (b) takes a defined route through the earth. Now you've got an underground river. It may not resemble the aboveground kind too closely — the Pittsburgh Underground River ambles through a subterranean channel (actually an ancient riverbed) filled with coarse broken rock.

However, if groundwater dissolves enough rock, typically hollowed-out limestone or other soluble bedrock called karst, it can form something a lot like a surface river, only underground. Such rivers are often found in caves, such as the River Styx and the Lost River in Kentucky's vast Mammoth-Flint Ridge cave system. Some underground rivers are quite long. The Saint Paul River in the Philippines has an accessible length of at least five miles, and the Son Trach River in Vietnam has been mapped out to at least seven. The longest underground river currently known underlies the Yucatán peninsula in Mexico, where an extensive system of limestone caverns and passages accessible by surface pools called cenotes extends for 95 miles. A few underground rivers were normal rivers buried by humans, such as London's River Fleet, a built-over Thames tributary, and Seoul's Chonggyechon River, which was recently restored to daylight.

Still, underground rivers are rare. Most aquifers are more like underground lakes, in that they're relatively defined bodies of water underlying much, but not all, of the earth's land surface. Largely arid Texas, for example, has aquifers under 81 percent of its surface. If you need to find water there, skip the dowsers and invest in a good groundwater map.

As you're drilling, though, remember that aquifers don't necessarily refill quickly. Many large ones contain glacial runoff from the last ice age or earlier — some water under the Sahara desert predates 38,000 BC. Alarmingly, Libya is tapping the large Saharan aquifers for its Great Man-Made River project, which supplies 6.5 million cubic meters of water daily. Jared Diamond, in his book Collapse: How Societies Choose to Fail or Succeed (2005), calls the plundering of essentially nonrenewable water resources "mining," and it's not just those crazy Libyans who are doing it. The High Plains aquifer is being depleted at approximately a foot a year, with rainfall recharge averaging less than an inch per year. In some parts of Kansas the groundwater level has fallen more than 100 feet since the 1930s. Pumpage has caused 11,000 square kilometers in California's San Joaquin Valley to sink at least a foot, and some sites more than 20 feet. Not to sound alarmist, but this can't go on.

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