You might not know it, but water is almost always underneath your feet. It's called groundwater, and it's hidden in the tiny spaces between particles of soil and the cracks, crevices, and pore spaces in solid rock. Taken all together, the volume of groundwater is huge: estimates range from 30 to 100 times more than is in all Earth's rivers, lakes, and streams.
With all that water underground, no need to worry about running out, right? Wrong. Groundwater can be too deep to reach or too salty to drink, or there may be too little in one spot to make drilling worthwhile. And especially in dry places, the groundwater has taken a long, long time to accumulate. When it's gone, it's gone, and it may not collect again for many thousands of years.
"Water is the true wealth in a dry land."
-- U.S. author Wallace Stegner, Beyond the Hundredth Meridian
When we withdraw groundwater faster than rainfall or surface water can soak in to replace it, the ground can sink beneath our feet--literally. Subsidence, as this sinking is called, is happening all over the world, including in California's San Joaquin Valley. This 30-year-old photo shows that the land surface sank about nine meters (30 feet) in a little over half a century. The ground in this valley is made up of compacted peat, a material that holds water like a sponge--until it's pumped dry.
Water Flowing Underground
Groundwater doesn't usually exist as underground rivers or streams, but it still flows--often at a snail's pace of centimeters per day. Flowing groundwater sometimes comes in contact with manmade chemicals in the soil. These can include leaks from underground gasoline and chemical storage tanks.
By the Numbers
One gallon (3.8 liters) of gas can contaminate 1,000,000 gallons of groundwater (3,700,00 liters).
Groundwater picks up substances that occur naturally in the surrounding rock, including microorganisms and inorganic materials. When those substances are harmful, so is the water. In Bangladesh, millions of deep wells were drilled to end use of bacteria-laced surface water. It worked: for instance, infant mortality dropped sharply. But many of the wells turned out to contain high levels of naturally occurring arsenic--an extremely toxic chemical element with serious health effects. Today, experts are working to develop inexpensive methods of removing arsenic from the water.