Frequently asked questions (FAQ) on borewells: Part II - Maintenance of borewells and problems associated with existing borewell
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 in 2 parts, they also give links to relevant queries and provide 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:
Tools to check issues
Rules and regulations
Yield of borewell
- How to determine yield of an existing borewell ?
- Can an existing borewell be deepened to increase the yield ?
- Can an existing borewell be recharged for increasing its yield?
- What is well development and how is it carried out?
- The water yield from a 10 year old borewell has reduced considerably. Lowering the pump has not helped. What can be done?
- Our borewell has suddenly dropped in yield. There are lots of other borewells in the neighbourhood. What can we do?
Problems in dug borewell
Contamination in borewell
- One borewell is connected to another 6 ft away contaminated from sewage lines passing nearby. Any suggestions?
- Any treatment measures for improving quality of water in a borewell that is good in summer but degrades immediately after rains?
- Can a borewell located 15 ft from the double pits that collect sewage water from a house with no drainage be contaminated?
It is preferable to take up recharge measures such as recharge wells/borewells around the site of a borewell used for drinking purposes rather than putting rainwater directly into it. To maximise the benefit, it is always advisable that the entire neighbourhood is encouraged to take up recharge measures to benefit all the wells in the area.
In case the borewell itself is being used for recharging, proper filtering arrangements for removing silt particles and also proper piping system for allowing the only rainwater water from clean roof catchments inside the well without damaging the side walls is essential.
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The contamination will move towards the new borewell also as there is a direct linkage which was established while drilling itself. More over 6 ft is not enough distance to ensure clean water.
First try to hyper-chlorinate the old borewell, and then try to seal it off by pouring rock aggregate right up to 70 ft and then pour concrete in the remaining upper 70 ft to seal that borewell. Before using the new borewell, hyper-clorinate that well too, and then install a temporary test pump and use the borewell for at least 2 to 3 weeks before going in for a permanent pump.
For the next two to three years, during and after the rainy season use hyper-clorination to decontaminate the new borewell, as there is a chance that whatever contamination has leaked from the sewage has contaminated the surrounding areas in the subsurface and this may move towards the borewell due to high water table conditions during the the monsoon and post-monsoon periods.
Well water becoming bad with lather and foul smell in every rainy season but turning good in other seasons indicates poor construction quality of the bore well.
Groundwater in hard rock areas where the well is located, occurs both in top soil and in weathered rock under unconfined condition and in pipeline-like fractures within bottom hard rock under semi-artesian conditions. Bore wells are constructed in such areas using down-the-hole (DTH) hammer rigs with casing pipe to prevent top loose formations and top water from entering into the well, while no casing pipe is provided against bottom hard rock for free entry of semi-artesian groundwater.
Because of poor construction quality of your this well, the unconfined water at the top is finding entry into the bore well. Under the influence of the nearby Lake, water table in the vicinity of your bore well in rainy season rises so high as to get mixed up with nearby septic tank waters with E. coli, a strain of bacteria associated with human and animal faecal matter.
These waters contaminate the bore well by entering into it through the annular space between the casing pipe and the top formation. Soon after rains, the water table gets so much lowered as to get disconnected with the septic tank water with little or no scope for entry of top water into your bore well until the arrival of the rainy season once again.
The best course of action would be to construct a grout seal around the casing pipe for a depth of 10 feet or until the top loose formation gives way to hard rock (whichever is higher).
You may study the picture and description at http://www.wbadmorgan.co.uk/info_drilling.htm to know the precautions you have to take to prevent top contaminated water from entering into your well. There is however no need to fix any screen and gravel pack as shown in the picture.
Answers from our experts:
- Contamination depends on, how the bore well is constructed. If the bore well is provided with deep casing pipe and the static water level is much deeper and the gap between the bore well casing and annular space is grouted with clay or cement then the possibility is very very less or nil. If the annular space is not grouted, then slowly the water will percolate and reach the annular space and trickle down into the bore well. During rainy season the effect will be more.
Either your neighbour has to provide grouting in the well or you have to convert the second pit into a sump with 4 sides made of concrete and empty the water regularly with some local arrangement.
- This is a typical urban problem. Basically, the sump cannot get polluted by this if it is made of water tight construction in RCC or brickwork and in both cases duly plastered and is raised above the ground level. The borewell water in a bore of 350 feet deep cannot also get polluted by this water because the soil bacteria will purify the pollutants. The only objection will be that Bangalore being rocky, there can be crevices through which the polluted water can travel down, but then, here also the growth of bacteria in the crevices slowly grows further and becomes a "plug".
The most important issue is they can put the entire sewage into a septic tank and followed by an upflow filter and discharge the final effluent legally into the roadside drain. The only issue is whether their house basement level is high enough to permit the final effluent to flow by gravity into the roadside drain.
If not they have to pump it and store in an elevated tank from which the effluent has to go out at a mild uniform rate of flow through the day. Such an arrangement is easily made by inserting a hose inside the tank and typing its top end to a float and drilling a hole of some 15 mm in the flexible hose some 15 cm below the float.
The possible reasons for this could be
- Occurrence of a layer of boulders at a depth of around 60 feet damaging the casing during drilling,
- Failure to properly cement the casing pipes with one another while inserting into the bore well giving scope for loose material outside the casing pipe to enter into the bore well,
- Failure to install the casing pipe to snugly seat into the bottom hard rock leading to its movement during drilling and consequent damaged, and/or
- Physical damage to the casing pipe by the drill rod during drilling.
As keeping the defunct bore well intact without sealing can lead to pollution of any new bore wells nearby, it is essential to seal it. It is also not desirable to use the defunct bore well for artificial recharge of groundwater with rooftop rainwater. How such a usage had led to discharge of water with high variation in suspended solids was discussed at https://www.indiawaterportal.org/ask/5296. It desirable to make use of the same flexible rig used earlier to remove the PVC casing pipe installed in the defunct bore well. If that is not possible, drilling at the same site can be taken up to crush the casing pipe into small pieces for removal by compressed air. Mild steel (MS) casing pipe rather than PVC casing pipe may be used for the new bore well. The casing pipe should be driven as deep as possible to a depth of 110 feet or more to snugly seat into the bottom hard rock.
Our experts answer :
- Try to drop a small pebble (glass marble) , approximately 5 mm in diameter and measure the amount of time it takes to touch the water. The thumb rule is about 15 ft per second hence if water is at around 90 ft, it should take 6 seconds to strike water. The sound would be loud and audible on surface. Once that is confirmed pour some water, say about 1000 liters in the well and see if the pump is functioning properly. At a minimum, 60% of the water that you put in should be pumped out.
If no discharge is got from the pump after putting water in the bore well, get the pump and pipeline out.
At the same time check the level of water in the bore well by attaching a weight to a Nylon rope and lowering it into the bore well. If the Static water level in the bore well is around 90 ft the problem is with the pump and not the bore well. Check for leaks in the pipeline, and check the pump assembly, and try it out in a tank of water
If there is no water in your bore well, it is quite possible that the neighboring bore well which is at 344 ft is lowering the water table, which is affecting your bore well. The connection with it has already been confirmed as water started gushing from the neighbor's bore well while drilling
Air flushing is not needed. Just deepen the bore well. Hydro fracturing will not help if the water table is being lowered below the depth of your bore well. Moreover Hydro fracturing will cost as much or even more as compared to deepening of the existing borewell.
- It looks like a dipping conduit connection of the fracture between this borewell and the neighbour's bore well towards the neighbour's side. Further drilling would be possible with a higher capacity drilling rig. Once the conduit zone is crossed and if another spring is reached then the water head will rise above the conduit level and the pump can reach and push the water out. The deeper the drilling the higher will be the pressure head and it is likely to overcome the entry velocity while crossing the conduit zone and sufficient water may get pumped out.
Drilling a new borewell nearby runs the same risk of being connected with the neighbour's borewell. It is better to drill further in the same bore well.
Since there is some water at a great depth in the well, it is worth lowering the pump to 1-1.5 m above the well bottom and pumping out water.
Possible reasons are:
Loose fitting of casing pipe. The seepage water from the top loose formation while trickling down will carry the colloidal clay particle and enter into the bore well through the gap between seating and pipe end. The trickling would be a flow if you have met with good water yielding zone in the loose weathered part. This over the nonpumping period will fill the bore along with the source water from the other zone. If the static water level stands above the casing bottom then the flow will be less.
You might have got a sheared and fractured zone as your yielding zone for the bore well at any depth. The crushed and powdered sheared zone will dissolve the loose clay like soil when the ground water enters into bore well and this powdered particle will come out as mud while you pump out the water.
If you use a pump with higher capacity than the yielding rate or close to the yielding rate of the bore well, then the pump will suck more water from the storage zone. Due to high upward sucking rate, the mud settled at the bottom of the bore well will come out at the end of each cycle when the water level comes close to your pump. Normally some mud flow occurs during early minutes immediately after the start of the pump and seizes, when the contribution from the aquifer starts.
If you have obtained a soft weathered zone after the hard rock zone and if the water yield zone falls in the soft weathered zone at the bottom then also there is possibility of the inherent clay colloidal entering into the bore well. If it is in such a zone then, the solution is constructing a tube well with tiny pebbles and slots.
Our experts answer:
- To enhance yield in the bore well, you can opt for multi hydro-fracturing methodology, as it looks like your bore well is surrounded by fractured zones at deeper levels. May be some of the main fractured conduits might not have been feeding into your bore well fractures or some other bore wells are sharing your fractured conduits and they might be sucking water from your bore well owing to their locational advantage.
Alternatively if you have enough space, you can trace the main fractured conduits and drill new deep bore wells. This is an alternate solution. When hydro-fracturing technique was not known or was not being adopted in the Trichy and Coimbatore belt, many bore wells recommended close to such kind of low yield bore wells have given surprisingly higher yield at depth levels of 250 feet during late 80s in Trichy belt and depth ranges below 950 feet in early 2000s in Tiruppur and Coimbatore belt. They were hardly 5 to 8 feet away from the old bore wells.
Your terrain has soft formation at shallow depth and sometimes at intermediate depth the bore well may collapse. If it is hard strata then a new bore well is better. I would recommend hydro-fraturing technique which would be available at Bangalore.
To sustain the yield rainwater harvesting with appropriate designs have to be carried out not only by you but also by your neighbours. Please consult a good rainwater harvesting planner at your place and follow the latest design and technological advents.
- There could be a couple of reasons for something like this to happen.
- Some one else may have drilled a new well that is affecting the yield. You may have struck a huge pocket/cavity which is connected to the main aquifer but the connectivity is not very good hence initially you got the excess water and then it reduced.
- The pipeline may have developed a leak, probably where the pipe is joined to the pump and due to this the pump is unable to develop the required back pressure to deliver adequate water to the surface.
- If you have a three phase motor, you may have a phase reversal and the motor is rotating in the reverse direction.
You may do the following:
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- Check if some one else has drilled a new well recently in the area.
- Check if the amount of current drawn by the motor (In Amperes) goes up and the motor shuts off due to overload if the pump is switched on keeping the delivery tap closed. If it does not shut off then there is a leak in the pipeline. This needs to be done only if you have a good quality control panel with overload safety shut-off.
- You can always pull out the pump and pipeline for inspection.
The static water level in the borewell has gone down considerably. The low H.P submersible pump is unable to lift the water from present deeper depth into your over head tank. As such please first ascertain the static water level and get details of draw down (water level after carrying out pumping for an hour or two, in your case).
In reality draw down means stabilized water depth after pumping. Submersible pump needs to be lowered couple of meters below draw down level to avoid dry pumping. There is a possibility that the submersible pump needs overhauling. Higher horse power pump may not help in this case as 1 H.P pump itself from original depth has not yielded good results.
Since you cannot drill to deeper depths there is every possibility that you may have to live with reduced supply of water. Both the depth details can be found from mirror technique or from plumb line technique (Measuring the water level with a thread or tape attached to a small rock).
Well development is the process by which a borewell is cleaned and the permeability of the borewell is increased by removing fine materials like sand, clay deposits and rock cuttings accumulated in borewells and gravel packing provided around well screens.
It is also carried out to borewells which are facing reduction in water yield over a period of time to increase its yield. The reduction in yield can be due to clogging of pores spaces by silt and mineral deposits.
Usually it is done by:
- Flushing and Over Pumping in the borewell or flushing the borewell with adequate air pressure. In hard rock areas, flushing is done after the drilling process for at least 2-3 hours by using compressed air before drilling is stopped. Most of the drillers avoid this as it is an additional task. This is an important process that must be finally performed before completion of the drilling process at any given site. Flushing using air pressure or over pumping the borewell are the methods normally employed to improve the yield of such borewells.
- Bore Blasting is a technique which is used for opening up of fracture zones of borewells located in hard rock areas. Around 14 to 230kg explosives are used depending upon the depth at which the blast is to be carried out. It is usually carried out when a drilled borewell is dry and there is a potential to obtain water by opening up fissures in the rock at the specific site. This method could also affect the very existence of the borewell therefore taking professional help is must for choosing this method.
- Hydro-fracturing
is applying water under high pressure for the creation, propagation and cleaning of fractures and fissures deep in the rocky layer of the earth. Creation or extension of the fractures is done using very high pressure water pumped into the bore well with pressures reaching as high as 3000 PSI (pounds per square inch). It is one way of cleaning the blockage in the fissure and releasing water back into the bore wells. It can also open up the bore well to new fissures in the immediate vicinity not previously tapped by the bore well. A borewell camera is used to identify the fracture zones inside the borewell prior to the process. Hydro-fracturing is an expensive process which is normally adopted by Government Water Supply Departments. Recently, private agencies are also involved in providing such services.
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Deepening an existing bore well to a greater depth is undoubtedly cheaper than constructing a new bore well as there will be saving on the installation of casing pipe and lesser depth of drilling. However, drilling companies may be reluctant to deepen existing bore wells and therefore it is necessary to identify the one willing to take up such a work. Exact saving that can be achieved in developing an existing well over drilling a new well can be known by obtaining quotations from the drilling company for the two works. However, it is better to consult a Geologist to know the feasibility of finding adequate quantity of groundwater in the location deciding upon deepening the borewell.
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Yield of a borewell can be determined by conducting a step drawdown test, in which water is pumped at different rates (that is, so many litres per hour) by keeping the submersible pump at different depths to know the actual quantum of water that could be safely pumped and the resulting lowering of water level or drawdown. The test helps to know whether the borewell can meet the water requirements.
Usually, reputed pump dealers can conduct such tests at a fee before deciding to purchase a pump. Such tests are necessary for large settlements like apartments to arrive at very suitable pumps which can provide huge quantities of water needed.
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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.
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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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Please refer to this Xcel sheet which has a table that facilitates self-diagnosis. It also lists various factors that can affect borewell performance and parameters that can be monitored.
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.