FAQ’s

What is the advantage of ground water supply?

The drilling of a borehole gives a potential client access to safe and potable water through access to primary water supply or the augmenting existing municipal water supply. Water is a basic human need, important for health and quality of life. With the growing prevalence of water shortages throughout South Africa and the world, a reliable water supply is still taken for granted, with no real thought about its sustainability and quality. This attitude is most starkly in areas where there is a reliance on bulk water supply which is assumed, will keep on producing – at the same rate – continuously and forever. Groundwater is out of sight, and hence, largely out of mind, but it is one of the best sources of water that man has been able to utilize.

The principal re-plenishment source of groundwater is rainfall. A portion of rainwater will seep into the soil and the other portion will end up as run-off in streams and rivers.

A borehole drilled into the saturated ‘sponge’ will release water from storage. This water can be pumped out, and all being well, more water will enter the hole to replace that which has been withdrawn.

A borehole is not just a hole in the ground. It must be properly designed, professionally constructed and carefully drilled. Boreholes for extracting water consist of a hole, constructed with durable casing to prevent collapsing of the side walls. Perforated casing with a natural or artificial gravel pack forms part of the construction allowing clean water to enter the borehole. Drilling a borehole is an expensive process and boreholes require professional expertise for both their design and construction. Borehole water often requires no treatment and is less susceptible to decline in water level during periods of drought or limited rainfall.

Which drilling methods can be used?

Once a suitable site has been selected and the drilling decided on, the proper drilling method must be chosen. A few drilling methods can be utilised and includes, jetting, mud- and air rotary percussion. Rotary air percussion is the most common method used for the development of boreholes in South Africa.

 Jetting (well point)

 A well point can be established in sandy areas with shallow static water tables by means of jetting where high-pressure water is pumped through steel pipes which penetrates the underlaying sandy soil. Once the desired depth is reached, LDPE pipes with a filter is installed and the steel pipes is removed. A suction pump is then installed above ground level.

Well points are faster and more cost-effective to install, as shallower depths are needed to reach the water.

 Rotary air percussion

The most common drilling method used in South Africa is rotary air percussion which uses a combination of rotation and air percussion (reciprocating piston driven hammer and tungsten bit) to break hard rock into small chips which are removed from the borehole by high air pressure on the outside of the drill string. The compressed air is supplied by high-powered air compressors which provide up to 1100 cubic feet per minute at pressures up to 30 bar.

Mud rotary drilling

In South Africa direct-mud rotary drilling is used to drill through unconsolidated (sand and gravels) to depths of 250m. With this method the borehole is advanced by rapid rotation of a drill bit mounted on the end of the drill rods. The bit cuts and breaks the material at the bottom of the hole into small pieces (cuttings). The cuttings are removed by pumping drilling fluid (water or water mixed with a fluid enhancer, such as guar gum) down through the drill rods and bit and up the annulus between the borehole and the drill rods. The drilling fluid also serves to cool the drill bit and stabilize the borehole wall, to prevent the flow of fluids between the borehole and surrounding earth materials, and to reduce cross-contamination between aquifers.

How deep should I drill?

The question, then, is: When to stop drilling? This decision will have to be taken by the consultant or client in conjunction with contractor. The decision will be taken if the required need is satisfied i.e. a hand-pump, for instance, does not demand a large supply (0.5 litre/sec is more than enough), whereas a submersible or motor pump supplying a storage tank for a dwelling, village, or a facility such as a school requires a significantly greater yield.

What is borehole reaming?

Contractors uses different approaches when they are drilling domestic boreholes. Some contractors will drill for instance a 165mm borehole and install either steel or uPVC casing with a smaller diameter which will fit into the completed borehole. This method does not allow for the installation of bigger casing should the amount of water intersected be much more than anticipated. In this instance the borehole should be reamed with a special drill bit (say from 165mm to 219mm or 254mm) depending in the size of casing that needs to be installed.  The depth of casing installation will be determined by the stability of the formation as well as the water bearing fractures.

How does the contractor decide on the type of casing and where to place solid and perforated casing?

When a borehole has been completed, if not pre-determined by the consultant, the contractor and client will decide on the type of casing to be installed. They can either install steel or uPVC as required. Steel casing and UPVC are produced in different sizes and wall thickness. Steel casing is usually manufactured in 6m lengths and the individual lengths era welded on site as they are installed. uPVC casing is screwed and socketed and comes in 2,9m or 5,8m lengths.

The placing of solid and perforated will be done after examining the drill samples. The casing alongside non-water bearing zones must be solid casing and along a water-bearing zone needs to be perforated to allow water to enter as efficiently as possible while holding back loose material from the formation. These perforated sections are known as borehole or well screens; they come in sizes and joints like solid casing, so they can be interconnected with suitable plain casing in any combination.

What is formation sampling?

Formation samples needs to be obtained as drilling proceeds: the usual sampling interval is one metre but can be up to six meters. All geological samples and water strikes should be logged by the drillers and the supervisor, as this important information will be required for designing the borehole and the equipment to be installed.

The main attributes of a borehole logs are accuracy and consistency; a good set of logs can be a useful resource when planning casing placement and future drilling programmes.

What is formation stabilizer or a gravel pack?

The drilling of a borehole gives a potential client access to safe and potable water through access to primary water supply or the augmenting existing municipal water supply. Water is a basic human need, important for health and quality of life. With the growing prevalence of water shortages throughout South Africa and the world, a reliable water supply is still taken for granted, with no real thought about its sustainability and quality. This attitude is most starkly in areas where there is a reliance on bulk water supply which is assumed, will keep on producing – at the same rate – continuously and forever. Groundwater is out of sight, and hence, largely out of mind, but it is one of the best sources of water that man has been able to utilize.

The principal re-plenishment source of groundwater is rainfall. A portion of rainwater will seep into the soil and the other portion will end up as run-off in streams and rivers.

A borehole drilled into the saturated ‘sponge’ will release water from storage. This water can be pumped out, and all being well, more water will enter the hole to replace that which has been withdrawn.

A borehole is not just a hole in the ground. It must be properly designed, professionally constructed and carefully drilled. Boreholes for extracting water consist of a hole, constructed with durable casing to prevent collapsing of the side walls. Perforated casing with a natural or artificial gravel pack forms part of the construction allowing clean water to enter the borehole. Drilling a borehole is an expensive process and boreholes require professional expertise for both their design and construction. Borehole water often requires no treatment and is less susceptible to decline in water level during periods of drought or limited rainfall.

What is borehole development?

After the installation of the permanent casing, screens, and gravel packs (if any), dirty water, mud, crushed rock, oil (from the drilling machinery), and perhaps other debris will be left in the hole. Drilling tools smear crushed rock and clay all over the walls of the borehole, and the drilling process forces dirty water and clay into the rock matrix around the hole, sealing off many water bearing zones from the aquifer. The development process requires compressed air down the hole to settle the gravel filter pack, eliminating voids, which may necessitate topping up the gravel pack if necessary. The process should continue until the water being discharged from the borehole is, in the judgement of the contractor or supervisor, as clean as possible. Small particles of sand might occasionally, but cloudiness (turbidity) of the water should have disappeared before development is stopped.

What is a blow yield?

Air is blown down and into the borehole and its pressure adjusted so that the outflow of water is equal to the inflow: this is known as the ‘blow-yield’. Measurement of this flow gives an indication of the performance of the borehole and helps to design a subsequent pumping test.  Blow-yield can be easily measured if the drillers arrange that all the water (in practice, most of the water) being ejected from the casing can be led along a shallow channel or pipe in the ground into a measuring device, such as a bucket in a pit or a V-notch weir. Filling of a bucket of known volume can be timed to give the discharge, which, in the opinion of this writer, is a much simpler and more reliable method, less prone to error.

When does a borehole needs to be rehabilitated?

Rehabilitation is the action taken to repair a borehole whose productivity has declined or that has failed through lack of monitoring and maintenance of the pump and/or borehole construction. The basic rehabilitation process should consist of the breaking-up of clogging deposits and incrustations.  It is usually difficult – if not impossible – to remove old casings or screens to clean or replace them, so other methods often need to be used. Air jetting at high pressures can be a particularly effective means of de-clogging and cleaning the internal surfaces of boreholes.

 Why do I need borehole testing?

After drilling has been completed, the client will have to decide whether a test pump is required. It has the following objectives:

To measure the performance of the borehole.

To determine the efficiency of the borehole, or variation of its performance under different rates of discharge.

To quantify aquifer characteristics.

Blow-yield test during development will indicate to the client or supervisor that a borehole has the potential for good production. Low-yield boreholes (less than 0.5 litre/second) do not need to be tested, and time should not be wasted doing this.

In such situations, it is possible to design a simple test-pumping programme using the blowing yield as the assumed maximum safe extraction rate. Basic single-well pumping tests involve pumping at a variable and/or constant rate and measuring changes (‘drawdown’) in water level during pumping and recovery. Recovery is the rate at which the water level in the borehole rises (‘recovers’) back to static water level once the pump has been switched off.

More information can be obtained if measured variable discharge rate (or ‘step drawdown’) and constant discharge rate tests are conducted, as the one test complements the other.

Both step and constant discharge tests evaluate borehole performance, but the constant discharge test can provide information on long-term borehole performance, aquifer characteristics, and even aquifer geometry. When the only information required is whether a particular pump can produce from a particular borehole on a sustained basis, a simple test – running that pump (preferably at a rate below that of the blowing yield) in the well – is all that is needed; but beware of over-pumping. This ‘test’ will not provide any information about the borehole or the aquifer if pumping drawdown levels are not recorded.

 Water Quality

Laboratory testing of various chemical, microbiological and physiological parameters of borehole and potable water sources (drinking water) is imperative to household users of a borehole water source. Results of analyses from water testing laboratories provide third-party verification of the quality of drinking water sources and the suitability of water sources for human consumption.

Reports needed from the Contractor?

On completion of a borehole, the contractor needs to issue the following reports:

Drilling log

Penetration log

Construction log

Borehole test data and graph

Management recommendation

Completion certificate

What is the cost of a borehole?

The demands of commercial competition mean that drilling contractors are often reluctant to quote prices unless one can provide specific details of geological conditions and anticipated borehole depths at potential sites. They may, for instance, request that a hydro geological site survey be carried out beforehand. Drilling contractors charge on a system of units of rates and materials used. A fixed rate for borehole drilling might be offered only if the contractor is very familiar with the location in question and knows exactly what to expect. Quoted prices will normally include drilling, casing, reaming and gravel pack.