自流井

托福听力：自流井（只听题干部分）

geographer
artesian spring

One type of natural springs geographers are interested in is artesian springs. Hiking through the woods, some of you may have been surprised to see water flowing from an opening in the ground that was nowhere near a stream or river. That may have been an artesian spring. To help you understand why water might flow like this from underground, I’d like to explain the two basic conditions that are necessary for their formation. The first condition is that water must be contained in an aquifer. An aquifer is an underground layer of rock or sediment that has pores or holes in it. And this porous through rock allows water to flow through it freely. The aquifer must be inclined so that the upper end of it is exposed to the air at the surface of the ground. Rain water enters it through the exposed end and travels downward to the lower portions of the aquifer. The second condition is that above and below the aquifer there must be layers of non-pores rock or clay. These are called aquicludes and they will block or hinder the flow of water. Aquicludes prevent water from draining out of aquifers. So let’s go back to our artesian springs. They are usually located above ground, near the lower end of inclined aquifers. Artesian springs are those places with some hole or crack extends from the ground surface down through the aquiclude and into the aquifer. Now the rain water that had drained into the aquifer from its exposed upper end created a buildup of pressure at the lower end. So if there is a crack in the rock, a crack that runs from the aquifer to the surface, then the pressure pushes the water up through it. And water comes trickling out of the artesian spring.