How Your Well Works
Part I - The Well
When most of us who live in the city think of wells, it usually conjures images of a large hole in the ground, perhaps lined with stone and a bucket that brings the water to the surface. While this approach worked well for hundreds of years, it would be extremely difficult to use today. For example, if the bucket held 1 gallon of water and it took 5 minutes to crank it up from the bottom, then you would only be able to get 1 gallon every 5 minutes. Considering your average toilet uses around 3-5 gallons per flush, that means it would take almost a half hour to get enough water for just 1 flush!
Luckily, we don't use stone wells or buckets today. When you look at a well today, all you're likely to see is a 6" or 8" piece of steel or PVC pipe sticking out of the ground with a cap bolted to the top. This casing provides two important functions. First the casing provides structural support to the first few feet of the borehole which are primarily composed of unincorporated sand or earth. This soft material without the help of the casing will usually collapse in on itself. Secondly, the casing provides protection against groundwater contamination from surface contaminants. The length of the steel or PVC casing depends on where the first solid rock material is encountered. If the well is shallow (typically anything less than 75' is considered shallow) then the casing will most likely run the entire length of the well as there will be no solid rocky material to support the borehole. Starting a few feet from the end of the casing, large slits are cut into the material creating a kind of screen. This screen allows groundwater to flow in from the sides of the casing while also keeping large objects such as loose rock from entering the well and potentially damaging the pump.
Once the borehole is drilled and the casing is in place, a special grout mixture (usually bentonite) is poured in the space between the well casing and the ground which helps to prevent surface water or contaminants from running down the side of the casing into the ground water.
As for how the water actually enters the well, this depends greatly on the location where the well is drilled. If the well is drilled near a large body of surface water such as a lake or a stream, the water will most likely come from the soft and super saturated ground in the first few feet of the well. If it is a deep well in a rocky area, the water will most likely come from water bearing fractures in the rock material. No matter how deep your well is, the area at which water is first encountered is called the static water level. This is the level at which the static pressure in the well (the pressure of gravity pushing the water down to the bottom of the well) is equal to the static pressure of the surrounding water. Think of a plastic jug with a small hole in the bottom of it. If you were to press that down as hard as you could in a large body of water, you would notice that the jug will only fill to the same level as the water outside of it. In order to ensure you have plenty of water in your well, the driller usually drills several feet below this level.
In some rare cases where the water table is actually above where the borehole is drilled, the static pressure outside the well is actually lower than the static pressure at the bottom. (imagine an underground river with water flowing rapidly from the highest point to the lowest). In this case the water will actually flow out the top of the well, even without a pump! These wells are called artesian wells.