How does a well work? Is there a finite amount of water in a drinking well? Why is it okay to drink? Do they somehow replenish water or if they dry up that is it?
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The ground is not completely solid, as a result, water can flow through it. However, the ground is full of stuff with very little room between it for water to get through, so water moving through the ground can only flow a small amount at a time, as well as must travel in a very zigzag, and therfore long path.
When it rains, water flows from the surface down into the earth, following gravity. However, gravity is not the only force at play. The soil the water is trying to get into already has water in it. The more water is already there, the less opening there are for water to flow through. The result is that soil which has its maximum amount of water already is basically a solid object. It’s not accepting more water. There are layers to soils at different depths, the fill up at different speeds, and as a result, form barriers to water sinking deeper. The areas where these soil types transition between one another form a boundary where water can not go deeper, but it can go down. The result is water follows this boundary layer “downhill” following the contour of the earth. This is generally (but definitely not exactly) the same contour as the surface.
Occasionally, there will be bowls and valleys formed where a highly permeable (lots of water can flow through it) region of earth will dip down into a less permeable layer. Just like on the surface, water collects in this bowl or valley and has difficulty leaving, because when it follows the boundary contour down it can’t go back up, therefore the only way out is to be absorbed by and flow through the less permeable layer. Because the more permeable layer by definition has more water flowing through it than the less permeable one, the water slowly accumulates. This is the simplest form of what’s known as an aquifer, a region of earth which is surrounded by less permeable earth, such that water will flow in but not out, like a lake. Side note, once the aquifer is full, the more permeable soil is saturated, and thus because just as solid and impermeable as the layer below it, meaning water just flows over the top and keeps going like a pool with too much water.
A well is a device which digs through the earth below the “surface” of the aquifer. The surface is depth where water can not escape once it’s in the bowl. This is the “top” I was referring to earlier. In its simplest form, the well makes a tube of almost completely impermeable material to keep water from flow through the sides. The process of water flowing through the sides of a hole would cause erosion, that’s why wells need walls. Because the bottom of the well is below the surface of the aquifer, the water pressure at the bottom of the well is less than the air above it, so water is forced into the well. The well continues to fill up until the pressure from the weight of the water in the well matches that in the aquifer. Importantly, because the earth that the water is flowing through disperses some of the load, the water level in the well is not perfectly even with the level of the aquifer.
Because the water is seeping up from the bottom of the well, it doesn’t bring any soil with it. The result is nice crystal clear clean ground water. It’s even better than surface water because the earth that it flowed through acted like a filter. The only dirt it could have brought with, is the dirt at the bottom of the well, which was left behind. Some wells will even line their bottoms to make them less permeable. This slows the refill rate of the well, but improves its clarity, especially when the aquifer is low.
Importantly, aquifers can easily be destroyed. They rely on water flowing in faster than it can leave. Mining, earthquakes, erosion, construction even water contaminants have a chance to destroy this. If a tunnel bores into the aquifer, and pierces that outer layer, now water can flow out more easily. On the scale of big aquifers a single tunnel is like a pinhole in a hot air balloon, but for smaller water systems, or with enough holes you can do serious damage.
Also, when water does leave an aquifer, it can erode the soil it’s flowing through, especially if the soil underneath the impermeable layer let’s water flow away quickly. Rhe result is a void space, where pressure from the withheld water will squish the boundary into the shape of the empty space the water wants to flow into. Importantly, it’s not empty as it’s just air. It’s empty in that the dirt on the other side is not full of water. So, the water is constantly wearing on the boundary. Change in lower lying ground water effects upstream aquifers by reducing the saturation of their surroundings soils, and increasing the pressure on the protective seam of impermeable soil. Eventually it can leak or weaken, draining the upstream aquifer, in the same way that improperly baked pottery will change shape as it heats and cools and tear itself apart if it’s stretched too far at once or becomes too brittle too quickly.
Chemicals dissolved in water can also make it deeper into the ground than other contaminants. These chemicals react with the rocks and soil at deeper levels which are not usually exposed to weathering from rain and surface water. Clay, which is the most common impervious material for an aquifer, can be weakened and eroded much more quickly by acidic ground water for example.
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