Frequently asked questions (FAQ) on borewells: Part I - Site selection & borewell commission; Part II - Maintenance of borewells and problems associated with existing borewells
The Borewell FAQ, Part I & II, are meant to provide a primer on the basics related to all aspects of borewells. A well compiled document, it also give links to relevant queries and provides a much needed starting point for all those contemplating a borewell !
- Borewell FAQ, Part I : Covers topics from the initial planning, selection of site to final commissioning.
- Borewell FAQ, Part II : Deals with maintenance of the structure and problems associated with existing borewells.
The most popular FAQs are listed below. Please click on a topic to view more detailed information:
Space and time
Rules and regulations
Methods of drilling
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- What are the common methods adopted for drilling borewells ?
- Can borewells be drilled in locations with space constraints where rigs can’t reach site?
- Can borewells be drilled inside an existing well?
- What is the standard diameter of borewells drilled for domestic purposes?
- What is videographing used for ?
Pipes & casings
Pumps
Costing
Yield of borewell
Recharging of borewells
Stuck pipes/ pumps
Problems while drilling a borewell
A dry bore well during drilling can be water bearing after some time. You may check whether this has happened to your dry bore well.
Any groundwater occurring in water-bearing fractures beyond 1000 feet will not be able to provide sustainable supply as nature takes very long time to fill back the empty fractures once again. It is therefore not advisable to go deeper in the same bore well.
Although electrical resistivity surveys are best suited to select favourable sites for drilling bore wells in open areas, there are limitations in their use in urban areas. For example, to test groundwater availability at a site at a depth of 100 metres, apparent resistivity has to be measured by fixing four electrodes along a straight line for a distance of 150 metres on either side of the site. Because of space constraints, it often becomes necessary for the geophysicist to guess conditions at such depths without actually taking the readings.
One normal approach followed by geologists is to select a new well site sufficiently far away from the existing wells so that there will be no well interference. Although this approach is good if groundwater occurs everywhere uniformly, it is better to select the well site by ensuring that it lies in a linear zone with successful bore wells on either side along the zone.
Because of serious limitations in the use of scientific methods in urban areas, water diviners claim their method to be superior. There are again geologists, who employ both divining and electrical resistivity surveys to locate well sites. It must be however known that no one can be 100% successful.
Most of us are not aware that groundwater is available in two discontinuous layers. The top layer, which can be thought of as the shallow aquifer extends up to hard rock and the other, the deep aquifer is what is available within the rocky layer. It is the shallow aquifer which can be sustained through artificial recharge during monsoon.
The rain centre in Chennai has started creating awareness among urban residents about the importance of “Conjunctive use of aquifers”. According to this newly discovered concept, we suggest to the residents to create a shallow groundwater source (if they do not have one) extending up to rocky strata and start exploiting it immediately after monsoon and switch over to the deep aquifer subsequently.
What is happening now in almost every big city in India is that the shallow aquifer, which gets replenished every year after the monsoon, is completely ignored. People having gone in for a deeper source are totally dependent on that, which over the years has depleted with very little chance of getting replenished through rainwater harvesting.
Please view the following suggestions:
- To find out the depth at which hard rock is encountered within the premises.
- Create a shallow tube well (if there is’nt one at present) up to that depth.
- Arrange for aquifer recharge by means of recharge wells.
- Exploit this shallow aquifer immediately after monsoon till it goes dry and switch over to the deep bore well.
- Try and find out from the bore well digger whether he has provided plain or slotted casing pipe up to the rocky strata. If he has provided only plain pipe get it replaced by a slotted pipe so that recharge will benefit the deep aquifer also.
The implications may be:
- The driller probably reached this sandy layer after pinching out a hard clay layer and sand was coming out below the clay roof. Then its better not to put the strainer and keep it open at the bottom of the well so that it would behave as a Cavity type Well. Cavity would be formed in the water bearing stratum at the bottom of clay roof during drawing of water from the bottom of the well and water from the aquifer will enter the well pipe through this cavity in a spherical pattern of flow. After continuous pumping, the spherical area of the cavity will increase outwards with the result that the velocity of flow will reduce and consequently, the sand particles will stop entering the well pipe. In the beginning sandy water will be obtained from this cavity type tube well but with the passage of time clear water will come out.
Development of cavity well
A centrifugal pump is usually better suited for developing a cavity well than an air compressor. The cavity should be developed slowly and with great care keeping low discharge. Otherwise, sand is likely to rush into the casing and choke it. When discharging water becomea clear, the depression may be increase slightly, which may result in further sand being drawn out. This process will be repeated till the sand-free discharge is obtained. The pumping may then be stopped for an hour and then resumed. The discharge after restarting may again contain sand. The pumping may be continued till the clear water is obtained. The procedure may be repeated till the well is developed fully.
- When the pink colored granite is fissured and highly crushed it changes into a formation similar to that of a river sand with pinkish and yellowish tinches, that are due to pyroxenite, feldspars and quartz minerals.
There is a need to drill the bore well and blow out the silica rich sand like particles. This will come out with moisture and sometimes with flow of water. Blow out a maximum of about a truck load of the particles or more and keep on blowing air until there is no more sand coming out. Keep 5 mm diameter or 10 mm diameter pebbles ready near the bore well.
When the nice sand like crushed particle have been pumped out from the bore, stop the drilling and blowing of air and pour the pebbles inside it while slowly the rig is rotating but the air is not coming out.
This can be done by many drillers who have drilled in Gujarat, U.P and Rajasthan. Replace double the amount or just near to the volume of the sand or silt you removed from the borewell.
The collapsing of bore will not take place and due to hydrostatic pressure the ground water will come to the static water level and the yield will be more than double the yield of the normal bore wells. If the surrounding bore wells are yielding for 5 H.P and your bore well will yield for 10 to 15 H.P.
Please do not do this if you get clay like silt or if very nice black or white colored powder is coming out from the bore well as water yield would be very low. If coarse grained sand gravel like material is coming out only then will this method work.
- Bangalore is a hard rock terain and there is no possibility of sand as such at depths of 250 ft. There is a possibility that the bore hole encountered soft formation at that depth and has a tendency to back fill the bore hole. It is quite natural to encounter such formations at these depths. The chances of bore hole collapsing depends on the rock formations above 250 ft. If the soft formation was met only at 250 ft and drilling stopped immediately then it is not likely to cave in.
However if it gets caved/ filled up to the extent of choking the water source, flush the well with air compressors, clean out the soft material, and lower a smaller diameter pipe with perforations into it (3" or 4" diameter appropriate for the bore well) and the well can be used again. Drilling rig crews are usually are aware of this method. But make sure that there is actually a case of soft material as has been reported.
- Highly fractured granite or brecciated granite yields sand like material even at greater depths. Measure the depth to the bottom of the bore well. Let us assume that the borewell got filled upto 200 feet and the water level is at a depth of, say, 100 feet below ground level. Fit a 7.5 HP submersible pump at around 175 feet and test the well for quantifying the yield or the driller may pump the well using the compressor and a V-notch. If the yield is sufficient for the given purpose, then start using the well. There is no use in drilling further after a collapsible formation has been encountered.
Video graph or camera test is useful only to ascertain the type of formation and sequences of the strata encountered. Video testing can be done in the water column to analyse and reassess the formation details. It is used in problematic wells to find out the cracks and fractures and its depth, so as to ascertain the packer installation depths for hydro fracturing or blasting. It is useful in solving some well construction problems where it will be difficult without the data of the strata structures.
The best course of action would be to find out a local pump supplier who can test your borewell and install a suitable pumpset that discharges enough water. If enough water does not come out, then you lose only the testing charges. If enough water comes out, then the pump supplier will be able to sell you a pump set besides collecting testing charges.
Vertical Electrical Sounding is one of the very common geophysical methods used for domestic borewell exploration. However, sufficient open area around the location is needed for conducting the survey. Geologists charge around Rs.1,500 to 5,000/- for identifying a borewell site.
Other methods of geophysical survey are used in special cases and they are quite expensive. Local well diviners who use wooden sticks or magnetic blocks are also to some extent successful in identifying borewell sites. But this method may not be always reliable, especially in areas where the geological formation is critical.
As such there is no prescribed season for drilling a borewell if the location of site has adequate ground water potential. However, identifying a suitable site and drilling the borewell in summer months would give the worst possible scenario in the location at the point of time.
Therefore, summer months are usually preferred by some experts as the best suited period for drilling a new borewell. Also, as most agricultural sites can be accessed by drilling rigs only during summer months, they are usually available in these periods.
Methods of drilling to be adopted depends on factors like suitability of a method for a particular type of geological formation (i.e. such as alluvial, bouldery and hard rock), cost factors, diameter and depth of borewell and the purpose intended.
Most commonly used types of drilling methods are;
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- Water Jetting - Shallow bores in alluvial formations
- Augur Drilling - Shallow bores in alluvial formations
- Calyx Drilling - Shallow borewells in both hard rock and alluvial formation
- Percussion Drilling – Deep bores in bouldery formation
- Rotary Drilling - Most common method used for drilling large and deep bores in alluvial formations.
- Down the Hole Hammering (DTH) Drilling – Most common method for drilling large and deep borewells in hard rock formations.
For domestic purposes, 4.5 and 6 inches dia borewells are usually drilled. While 4.5 inch is most commonly drilled borewells for domestic purposes, 6 inch borewells are drilled when higher yield is required for large apartments or buildings and also for agricultural purposes.
Initially larger diameter bits are used to place the casing pipes up to the hard rock zone prior to drilling specified size of the borewell.
Charges for drilling a specified size of borewell include
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- drilling cost per feet,
- cost of casing pipe per feet,
- cost of drilling and installation charge for casing pipe per feet,
- flushing charges per hour for flushing the borewell after drilling and
- transportation charges of the rig to the site from nearest town per km.
Drilling rates are sometimes charged in slabs for specified ranges especially when deep borewells are drilled. As drilling rates usually vary depending on the availability of rigs, local demand and site conditions, it is better to cross check with few drillers to get a competitive quote.
Currently, PVC pipes are very widely used as casing pipe. GI/Mild steel casing pipes which were used earlier are not being recommended due to the problem of corrosion. PVC casing pipes installed in borewells must confirm to IS 12818: 1992 that governs the specification for unplasticized PVC screen and casing pipes for borewell of the Bureau of Indian standard. This standard covers the requirements of ribbed screen, plain screen and plain casing pipes of nominal diameter 40 to 400 mm produced from unplasticized polyvinyl chloride for borewell for water supply.
Slotted PVC casing pipes are used if aquifer yields through sandy zones to protect collapse of borewell side walls and to prevent entry of fine sand into the borewell which might clog the borewell. This requires detailed study by Geologists to decide slot size and at what depths these have to be provided along with gravel packing around the casing pipe.
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If there was loose and friable material encountered at the depths mentioned where one struck small quantity of water then one should install the inner casing all the way down to the depth at which water was finally struck. It will prevent the fine sand etc. from entering into the well, filling up the borewell and jamming the pump. If there was no soft or friable material encountered, then there is no need to install this casing.
This Inner casing can be installed at a later date but before the pump is lowered in the borewell. If the material is to collapse and fill up the borewell it will happen when the pump is installed and the borewell is dewatered and water along with fines material starts entering the well from the surrounding area. Actually, a casing with proper screen opening and a gravel pack around this PVC casing should be done.
Usually V-Notches (a metal plate/wooden board with V shaped slot on top) are used by borewell drillers for determining yield of a borewell during the borewell drilling process. To measure the flow, water coming out of the borewell during drilling process is allowed to flow through an earthen barrier created around the site is fixed with a V-notch temporarily.
Drillers won’t be inclined to do so as this is an additional task they have to do. Based on their experience, they tell the rough yield which may not be always correct. Yield of a borewell is usually referred in inches (i.e. depth of water flowing over the notch).
You can calculate water yield for various types of weirs and the depths of water measured over them here.
We can find the water level in a borewell from the ground by time calculating the time taken for a very small stone (0.5 inch) to strike the water surface. All you need is a watch and a very small stone. Drop the stone and note the time taken for it to strike the water surface. Multiplying with 9.8 (i.e. an object free falling near the earth surface would travel 9m/sec due to gravity) to the time taken for the stone to strike the water surface measured in seconds, we can get the water level depth in meters (Ex: for a 10 seconds time, depth to water is – 9.8 x 10 = 98 meters) .
Repeat the exercise few times to get the correct depth.
A thin nylon rope longer than the depth of borewell firmly attached to a small sized stone (2 inches) or metal ring ( a small automobile bearing is ideal for this purpose) in one end is used for finding both the water level and total depth of the borewell. While leaving the stone/metal block attached to the rope into the borewell, when reduction in the weight of the stone is felt due to buoyancy of water after it strikes water surface, mark the point on the rope and remove the entire portion of rope let inside the borewell to measure for the water level from the ground.
Similarly, when you feel that the rope is not freely moving downwards from your hand once the stone strikes bottom of the borewell, mark the point and measure for the total depth of the borewell after removing the rope.
For more:
Water Quality is an important measure to be analysed in borewells. The IS Code 10500:1991 governs the water quality standards for drinking water. Chemical and bacteriological parameters of the water in a borewell can be analysed by taking the water samples to a water quality labs. Government water supply and geological departments have such labs in every district and city. Reputed private labs also conduct such tests at a fee.
Usually borewells constructed in hard rock areas are free from bacteriological contamination if due precautions like proper installation of casing pipes and avoiding stagnation of water around borewells is taken care off.
If any strong smell or heavy froth forms in the borewell water, it could be due to entry of sewage/contaminated water. Chemical parameters such as iron, fluoride, TDS, arsenic and nitrate are some very important parameters to be normally analysed as they occur in ground water depending upon the geological formations. Other parameters such as pesticides, cadmium, etc., can be assessed on specific cases where such contamination is suspected.
For more:
Jet, compressor and submersible pumps are the widely used pumps for domestic purposes in India. Jet pumps which are installed above the ground level over or near the borewell are suitable for depths upto 150 feet. If the ground water is at lower depths submersible or compressor pumps are used.
Compressor pumps are suitable for low yielding borewells and also they are easy to install and maintain as they are installed at ground level. However, they are noisy and may also require frequent repairs.
As submersible pumps of 4” dia which can work in single phase electricity connection are available in the market now, these have become a preferred choice for domestic borewells. They can be installed for meeting higher water requirements. However, submersible pumps will be prone to frequent repairs if they are installed in the borewells containing heavy silt particles.
Capacity of the pump to be chosen depends on the depth at which the pump/foot valve is to be installed and the desired discharge. Based on the total depth of the borewell, the depth at which the pump is to be installed (usually 10-25 feet above total depth of pumps) is decided.
The total head is determined using the depth of pump, over head tank level and friction loss expected in the suction and delivery pipes. Using the total head and the discharge desired, from the pump rating curves for each models of pump anufactured by various companies, the right HP of the pump suitable can be selected.
For more:
Dry wells can be brought to use by drilling borewells inside them. It is mostly done in large wells used for irrigation purposes. Flexible rigs which have separate tripod and drilling head, can be used for drilled a borewell inside an existing well if it is above 3m x 3m size to accommodate size of the rig and working space. These rigs are capable of drilling up to a depth of around 300 feet.
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Maintaining sufficient distance between a borewell and septic tank is always recommended as there are chances of contamination. If the situation warrants location of them in close proximity, it advisable to properly grout the borewell casing pipe with cement slurry till hard rock formation. In shallow aquifer zones, it is not advisable to locate borewells used for drinking purposes and the septic tank in close proximity. It is better to take professional assistance in such cases to take preventive steps around the borewell as well as the septic tank.
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Portable flexible rigs which function even when their compressor unit is parked at a distance can be used for drilling in such locations. These rigs are only capable of drilling up to a depth of around 300 feet which may be a limitation in sites where deeper borewells are to be drilled.
For more:
A Borewell can be drilled close to a house if the house is built upon normal foundations that are usually recommended for house construction. Shocks waves generated during drilling will be dissipated through soil and rock layers.
However, if you are planning to drill a borewell closer to a massive building which carries heavy loads, it would be better to check for the foundation stability.
For more:
Submersible pump or pipe assembly stuck in a borewell can be removed in most cases using various types of fishing tools. After determining the position of the objects struck in the well, water is poured into the borewell to loosen the earth covering the pump or pipe struck inside the borewell before using fishing tools. Borewell rehabilitation agencies or mechanics usually develop special fishing tools that can be attached to the grounded pumps or pipes in a borewell based on their experience. Usually, this may be a time consuming and expensive attempt which may not be always successful. The entire operation may some time be a catastrophe that can lead to complete damage of the borewell; therefore adequate caution must be taken before deciding upon such an exercise.
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If there is sufficient water depth and yield of water in remaining portion of the borewell above the location where the pump/pipe is stuck, the borewell can be used by installing new pump/pipe assembly. However, it is better to check that the incident has not caused damages to side walls of the borewell which might cause yield of muddy water. It is also disinfect the borewell before it is used for drinking purposes as the process would have led to falling of external contaminants into the borewell.
For more:
Some states have implemented groundwater legislation acts which prevent drilling of borewells without the permission of Government Bodies in water scarce regions. However, some states allow only drilling of drinking water borewells without the need for obtaining permission. Therefore, it is advisable to explore the details of it before drilling a borewell.
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Sealing defunct or unused borewells is essential to prevent contamination of ground water. If left open, unused borewells can lead to rapid contamination of the ground water as they allow contaminants directly into the aquifers.
Good quality clay materials can be used to seal the borewell using different methods after removing the casing pipe (details in the reference given at the end). In case it is not possible to take such measures, cover the borewell properly using a well cap or provide adequate earthen cover over a concrete or stone slab kept on the borewell.
Ensure not to allow entry of waste water or any external material inside an unused borewell. Alternatively, unused or defunct borewells can be used as rainwater recharge wells after providing proper filter arrangements and intake arrangements around the well.
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It is not advisable to use the treated water from STP or any other treatment process derived from sewage/grey water for recharging using recharge borewells or wells as it may still contain impurities. The impurities present in the treated water can easily enter the aquifer through recharge wells and contaminate the ground water.
Only rainwater caught from roof catchments and uncontaminated surfaces should be allowed for recharging the ground water through recharge wells.
However, it is advisable to use the treated water for gardening purposes. The excess water may be allowed to soak through the ground by making soak pits which are up to 10feet deep.
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Some rock from the uncased portion of the bore must have caused the obstruction. If not, loose material from the top must have seeped into the borewell through the annular space between the casing pipe and the surrounding formation. The first thing to do is to take the help of the driller to flush the bore well to remove the obstructions and test the yield once again. A suitable submersible pump can then be set up taking into account the pumping head and yield of the well for drawing water from the borewell.
Video clipping is not needed. Use a dummy tied with nylon rope and jerk the rope a few times to push down the filled in material. If this is not successful, then use the same drilling machine and flush / clean the bore well. If the block is due to collapsing nature then there is a need to insert the casing further down. A professional should be consulted for this.
Due to wear and tear or corrosion, the falling of pipe and motor is quite common in bore wells. The bore well companies and pump mechanics have special tools to fish out the same. Kindly contact a pump mechanic's shop, pump/motor selling shop or pump distributor. They will have the list of such specialist mechanics who can fish out the pipe and revive the bore. Please note that fishing out pipe/motor is a specialised job and it is important to engage the right mechanic for the same even if you have to pay a little more.
References:
- India Water Portal – Ask a Question (Borewells section)
- Hand Book on Borewell – BIS , GoI
- Guidelines For Sealing Groundwater Wells – WRMD, GNFL
- Water Quality Implementation Manual – Dept. of Drinking Water Supply , GoI
- Water Well Manual
Research and Compilation by
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- Ramesh Sakthivel
- Dr. R. Jagadiswara Rao
- A. Rajamohamed Ambalam
- Balachander
- S Halder
- C Udaya Shankar
- R Ramesh
- Sabita Kaushal
IWP requests users to view the same as a starting point in collating information on Borewells and to add more suggestions, information as responses in this thread.