A brief description

An overview of what a borehole is and how it works.

What is a borehole?

The simple terms a borehole is a hole drilled into the ground down to the water table in order to extract water. It may have to pass through several different layers of substrate before it meets the water table, the depth of the water table is variable for each location.

 

Borehole construction: –

For general domestic applications the borehole will be a 6” diameter hole, which is lined with a casing. The casing is a tube which is solid for most of its length, but then slotted or perforated at the lower level. The casing is grouted into position.

At the top of the casing is the well head. This is capped. The well head should be constructed such that there is no surface water ingress possible. This may be a brick construction with a watertight lid.

At the bottom of the borehole and linked to the surface by a pipe (the riser tube) is the borehole pump. This is a submersible pump that is suspended below the water level.

The pump will have a safety cord attached to it to prevent it being lost in the borehole should the riser ever give way. The riser tube passes through the cap on the well head and terminates in a fitting suitable for connecting to the water supply to the property. Often the pump will have a dry run cut off that shuts the pump down should the water supply drop below the pump inlet. The pump will burn out very quickly if allowed to run dry.

 

How Deep is a borehole?

Simply put it will be at deep as the water table. With the advice of a good hydro-geologist the borehole will only be as deep as necessary, often this is between 40 to 70 meters.

 

What is the cost of a borehole?

The cost is a great variable as it will depend on the depth of the borehole and the type of strata that has to be drilled through. The cost is often based on a time and depth calculation. A correctly constructed borehole will rarely cost less than several thousand pounds. You can expect to pay anything from £8k to £14k.

 

Problems encountered with a borehole: –

Boreholes rarely give problems. Most problems reported are about the water that is drawn from them. These are a few of the issues that we see: –

  • The borehole may run dry or the yield from the borehole may become reduced. This may be natural or it may be caused by third party disruption of the land which diverts the incoming water supply to the aquifer.

  • The quality of water from the borehole may naturally change from time to time.

  • Pump failure.

  • Casing collapse (usually very old boreholes).

  • Mineral build up.

  • Poor borehole maintenance.

Drinking Water from a borehole: –

It is not often that we find a borehole supply that complies with statutory drinking water regulations straight from the ground. A new borehole should be run for as long as practically possible in order to flush out any drilling debris and to allow the aquifer to settle back to a state of equilibrium.

At this point the water should be tested to determine what deviations there are from the prescribed standards. For peace of mind, an accredited laboratory should be used both to determine the condition of the raw water and then the treated water after the filtration equipment.

In West Yorkshire we will often find that the water will contain higher levels of Iron and Manganese than permitted and usually the pH will be on the acidic side of neutral.

Treating Iron and Manganese is usually quite simple. Acidic water is treated using a pH Correction media.

Posted by Kirsten Scott.