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How do wells get their water from underground rivers?
Category: Earth Science
Published: July 16, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Most wells do not get their water from underground rivers, but instead get the water from aquifers. Aquifers are layers of rock and soil with water flowing through their small pores. For the most part, there are not giant caves under earth's surface containing violent rivers of water flowing quickly through them. Instead, groundwater drips slowly and gently through the small spaces within rocks, between rocks, and between loose materials such as sand and gravel. In fact, water in aquifers can take years to centuries to flow back to the surface, as shown in the figure. A typical flow rate for water in aquifers is ten feet per year. For this reason, if a region experiences no rain for a few weeks, the wells will not immediately run dry.
New water, such as from rain or melting snow, drips down into the ground through the pores and cracks in the rocks and soil. Some of the water sticks to the dirt and rocks close to the surface and some of it continues to drip downward. The layer of ground just below the surface is a mixture of rock, soil, water, and air bubbles. When gravity pulls the water in the ground deep enough, it fills all of the possible pores and cracks, forcing the air bubbles up. At this depth, the ground becomes saturated with water. The boundary between the unsaturated ground and the saturated ground is called the water table. The exact location of the water table depends on how much new water there is, how quickly the water is flowing away, and how permeable the ground is.
If you dig a hole into the ground that ends above the water table, most of the water at this depth is stuck to bits of soil and rock, so that little water empties out into your hole. In contrast, if you dig a hole deep enough that it ends below the water table, the water in the saturated ground is pulled by gravity into the empty space at the bottom of the hole. In this case, your hole fills up with water that drips out of the holes in the rocks. But the water only fills up your hole up to the level of the water table (slightly lower actually). For water in your hole to go higher than the water table, it would have to flow up instead of down, which is not how gravity works. A "well" is simply a hole dug deep enough that it penetrates below the water table and therefore fills up with water. To retrieve the water, old wells used simple buckets on ropes. More modern wells use pumps that suck the water up the hole. Pumps can be driven manually by hand action, by an attached windmill, or by an electric motor. When digging a new well, you don't have to locate an underground river. You just have to dig deep enough that you reach below the water table.
Something interesting can happen if a layer of impermeable rock sits above a layer of water-filled permeable rock, and if the impermeable rock slopes downward. The water flowing in the lower level gets trapped by the impermeable upper level, creating a confined aquifer. As the water flows downward with no outlet, pressure builds. If a hole is dug into the ground deep enough that it reaches a confined aquifer, the pressure can be great enough to shoot water up the well without any help from a pump. Such a well is called a flowing artesian well.