Part 1 focused on Dug and Bored Wells. Here, we dive into the most common well type we find in modern construction, drilled wells.
Drilled wells are typically 4-6 inches in diameter and are commonly 50-900 feet deep. There are wells that are even deeper! One advantage of drilled wells is that they are less likely to be contaminated because the water is drawn from a much deeper source. The wall casings are typically steel, although they can also be plastic, brass, copper, or fiberglass. Where the water is acidic, plastic casings may be better than metal.
The wells are smaller diameter but must be large enough to handle the pump systems. Submersible pumps are lowered into the well. Drilled wells usually have a casing that goes down from the top of the well into the bedrock. Once the well is into the bedrock, the casing stops. We’re not worried about contamination of the casing below this point. The fissures in the bedrock allow water to flow into the well below the casing. The casing itself is watertight so no water should flow into the well below the casing. In some areas, only the first 20 feet or so of the well requires a casing. This will depend largely on the soil type and the depth of the aquifers.
When the well is drilled and the casing is inserted, the space between the outside of the casing and the earth is typically filled with a slurry designed to seal this space. We don’t want surface water to run down the outside of the well casing and get into the well below the casing. The slurries can be cement grout or concrete, bentonite or other clay slurries or well cuttings or, in some cases, surface materials. Very often the casing protection will be a combination of these. Below the depth at which you are worried about the contamination of the casing, the casing may be stabilized with sand, gravel or surface materials dropped in around the casing to stabilize it.
In some areas, as the performance of a well deteriorated, it was common to drop a stick of dynamite down into the well in hopes that either new fissures would be created or existing fissures would be widened to improve the flow into the well. There was the risk of collapsing the well, of course, but if the performance was so bad that it had to be abandoned in any case, this was often a risk worth taking. For some other obviously reasons, this practice is now frowned on in most areas.
Some well casings do not come right up to grade level. They may not stop in a pit below grade, which can make it difficult to locate. A below grade well pit can also be a source of water accumulation and a possible source of contamination if the pit is not watertight and self-draining.
Drilled wells are supposed to be vented. The vents for wells are sometimes completely enclosed in the well pits. Sometimes the vent line will run alongside the water supply line into the house and a vent pipe will be just visible inside the home where the water pipe enters the house.
If a well is drilled 200 feet deep, a submersible pump is not put right at the bottom of the well. Similarly, an intake for a jet pump does not go into the bottom of the well for fear of drawing mud and other debris into the water supply. The intakes are usually several feet above the bottom of the well.
We hope this discussion on the types of wells and information about them has been helpful. If you are on a well and need your water tested, please do not hesitate to give us a call.