Borewell- Frequently Asked
Questions
How to identify a suitable site for borewell?
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.
For
more:
Is there a best
suited time for drilling a borewell?
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.
For more,
What are the common
methods adopted for drilling borewells?
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;
·
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.
What is the standard diameter of borewells
drilled for domestic purposes?
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.
How charges for drilling a borewell are usually calculated?
Charges for drilling a specified size of borewell include
(i)
drilling cost per
feet,
(ii)
cost of casing pipe
per feet,
(iii)
cost of drilling
and installation charge for casing pipe per feet,
(iv)
flushing charges per hour for flushing the
borewell after drilling and
(v)
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.
For more, Discussion 3
What are the types of casing pipes used in borewells?
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.
For more,
What is well development?
·
Flushing and
Over Pumping
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. Usually it is
done by over pumping water 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.
Well development 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. Flushing using air pressure or over pumping the borewell
are the methods normally employed to improve the yield of such borewells.
·
Bore Blasting
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
www.indiawaterportal.org
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 very
existence of the borewell therefore taking professional help is must for
choosing this method.
·
Hydro-fracturing
As the term suggests, it 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).
Hydrofracturing is one way of cleaning the blockage in the fissure and
releasing water back into the bore wells. Hydro-fracturing 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, in
Bangalore a
private
agency is involved in providing such services. They can be
contacted at;
Mr. P.Ganesh,
Managing
Director, Fractech Equipments { p} Ltd,
2, Parvatha Industrial complex, Ganganna circle,
Jalahalli, Bangalore -560 013.
Ph
28384732, Subramaniam- 9443150582
E-mail- fractech@vsnl.net, trivenimachine@yahoo.co.in
For more,
How to determine yield of a borewell while
drilling?
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 wont 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 www.indiawaterportal.org
be always correct. Yield of a borewell is usually
referred in inches (i.e. depth of water flowing over the notch). Refer the
website http://www.lmnoeng.com/Weirs/vweir.htm to
calculate water yield for various types of
weirs and the depths of water measured over them (your computer should have
java to support the program).
How yield of an existing borewell can be determined?
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.
For more, Discussion 7 Discussion 8
How to measure depth of water and total depth of a borewell?
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 www.indiawaterportal.org
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, Discussion 9
What should be the water quality in the drinking
water borewells?
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. India Water Portal has a section which provides a comprehensive
list of water quality labs in India as well.
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 details, go through the links given in reference).
For more, Discussion 10
Discussion 11
Which types of pumps are suitable for borewells?
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.
How to choose a right type and capacity of pump for a borewell?
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 manufactured by various companies, the right HP of the pump
suitable can be selected.
For more, Discussion 12
Discussion 13
Can borewells be drilled inside an existing well?
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.
For more, Discussion 14
What is the safe distance between a borewell and
septic tank/soak pit?
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.
For more, Discussion 15
Discussion 16 Discussion 17
Can borewells be drilled in locations with narrow
roads in urban areas or houses where the rigs cant reach the site?
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, Discussion 18
Discussion 19
Can an existing borewell be deepened to increase the yield?
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 www.indiawaterportal.org
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.
For more, Discussion 20
Discussion 21
Is it safe to drill a borewell very close to the house?
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, Discussion 22
Is it possible to remove pump/pipes struck in a borewell?
Submersible pump or pipe assembly struck 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.
For more, Discussion 23
Discussion 24 Discussion 25
If pump or pipe struck in a borewell, should it
be abandoned from use?
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, Discussion 26
Is it necessary to obtain legal permission for
drilling a new borewell?
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. For more,
How to seal unused or defunct borewells?
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.
For more, Discussion 30
Can an existing borewell be recharged for increasing its yield?
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.
For more, Discussion 31
Can the treated sewage/grey water for recharging
ground water through recharge borewells?
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.
For more, Discussion 32
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 , Government of India
·
Water Well Manual