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Long term Reliability of water yield and Suitable Pump- Bangalore South

Hi Experts, We got a bore well dug at Bsk 6th stage, Bangalore few days ago. We took the opinion of two experts viz. one from Geological Survey in Bangalore and another from a freelance hydro-geologist. They both suggested same points.

The possible water bearing zones were

1. 60-90 ft

2. 200-250 ft

3. 360-390 ft

4. 510-530 ft

They suggested that we go max till 600 ft and not beyond. The first yield was at approx 60 ft. Beyond that, the rock was found. Hence casing pipe of 60ft was put. Beyond 60 ft, it was full rocky(white powder was coming out). At around 300ft, we were able to observe damp powder and drillers said there was 1-1.5 inches of water.Beyond 320-350, there was again the dusty powder(rock). At about 505-510 feet, they hit the water again and it was forceful. They managed to go another 5 ft after which they stopped. The water started to throw up in the air to about 30-32 ft. The driller told me that there is minimum of 4 inches of water.  There are hardly 2-3 houses near my place.

Now my question is

1. How accurate is the driller's opinion about min 4 inches of water?

2. Is there any chance of outflow, which might reduce the yield, say over 4-5 years?

3. What type of pump and how much hp of pump should be used?

4. Any precautions to be taken?

Regards.

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Comments

1. Dear Member, 1. The water

Dear Member,


1. The water spring gushed at 60 feet is nonsustaitainable over the period. It very much depends on the seasonal general water table fluctuation that in turn to a larger extent vary with on rail fall of the previous years.


When more and more bore wells or ground water abstracting structures come up it will contribute less to this bore unless it is situated in a Geohydrologically ideal most place.


2. The water struck below 500 feet is reliable if some body else could not scull out and find the same fracture conduit and drill a bore. In that case you may have to share the source. It may happen if the area is studded with numerous deep and ultra deep bore wells. It is only a hypothetical case.


3. The driller reported yield may be true if really beyond 5 feet more if he could not negotiate drilling. This implies that the hydrostatc pressure potential is more than the compressor air pressure. Normally the rig capable of negotiating 600 feet in hard rock area will possess high pressure air releasing compressor either Hydraulic or pneumatic type. More over they would not like to forgo more cash return. If the reported yield is less he could have drilled deeper. I hope the stated discharge would be reliable.


4. The ground water system is dynamic and always moves from higher towards lower potential gradient. But it is controlled by not only the aquifer characteristics but also by the terrain and basin characteristics.


The sustainability mainly depends on the recharge veins and conduits that supply source to your fracture conduit / micro pipe like structure. Which rely on the ground water recharge that gets effected by the rainfall intensity and pattern. As long as the rain fall type and nature for the preceding years is the same as that took place in the past years say over a period of 5 or 10 years then no question of out flow fear should come to your thought.


The main controlling factor is recharge. Since your source is comparatively deep, the better would be to recharge directly your same bore well during rainy season. By letting all the roof water collected directly into this bore and if possible letting outside run off also into the bore. You can refer to this portal for knowing and adopting the suitable methodology and gadgets etc., this portal has many of such articles.


5.  Regarding pump - the most ideal is submersible pump. The h.p etc depends on how much water you want to pump out or how many litres of water you need to suffice your domestic requirement.


If you are planning for a 1000 litres tank then 1. 5 H. P single phase motor with one quarter inch delivery pipe installed at about 200 feet may be alright. Please opt for ISI make and choose threaded 10 feet length PVC pipe. If you are planning for an apartment the design changes according to the requirement. I hope your bore may yield more than 15000 litres per hour, non stop, if the reported yield is correct.


With best wishes,


A. Rajamohamed Ambalam
Geophysicist
Coastal Energy
Chennai

2. Long term Reliability of water yield


Dear Member,


As per what you have described


You have struck water at 60 ft - in unconsolidated formation, which was later cased and hence not available.


The sencond one was at aproximately 300ft, - about 1.5 inch  which is equivalent to 1500 lph,


Last one aroudn 520 ft, which the driller say is 4 inch - nearly 19000 lph.


Now to your questions:


1. How accurate is the driller's opinion about min 4 inches of water?


Ans: If the driller says 4 inches and is nto drilling further, usually the yield is near about that much.


2. Is there any chance of outflow, which might reduce the yield, say over 4-5 years?


Ans: about the chance of outflow, there is always a chance when the others also drill, whether the yield will decrese, cannot say, it all depends upon the quantum of recharge and the quantum of abstrtction. Both in your caase are unknow, only time will tell.


It coudl also be that you have struck a pocket of water, and the connection to the aqufer is not so well developed, so once the pocket is used up the refillign may not be so fast.


Secondly, if there is some change int he recharge area which woudl definately be quite far from where youa re located, this too could affect the yield of the borewell.- but as you have mentioned that you have encountered 2 zones and the yileld is much more that what one requires for domestic purpose I dont think there is much to worry about, unless there are multiple failure of monsoons.


Just so that there is direct recharge availabe to the system, you could go ahead and implement rooftop rainwater harvesting system and divert the water falling onto your roof to percolate down in to your borewell. Please get proper technical advice for this, as there is a chance that you will contaminate the groudnwater system and render it unusable in the near future.


3. What type of pump and how much hp of pump should be used?


Ans: You have not mentioned daily water requirement, and the static water level in the borewell after drilling.Assuming you have a small requirement of maximum 5000 liters per day, and your water level in the borewell is at about 60 ft below groundlevel, you could install a 1.25hp 17stage pump or a 1.5 hp 18 stage pump and lower the pipes down to atleast 350ft,  That way you are sure that you can fully utilize the aquifer atpped at 300ft. this could be good enough to meet your requirements.


4. Any precautions to be taken?


 


Regards.


Anil Lalwani


www.wellwaterworks.com

3. Dear Member, 1. One can

Dear Member,


1. One can masure the dischrge by V-notch, or otherwise volumetric.
2. There is chance of reduction in yield of well in long run.
3. You may use submersible pump. The HP of the pump is based on yield of well and daily requirement. Low HP is always recommended.
4. Operate the pump in phases and restrict the withdrawal to match the need only.


D. Chakraborty
Scientist
Central Ground Water Authority (CGWA)
New Delhi

4. Thank you Experts for your

Thank you Experts for your valuable insights.

In addition to the information already posted, here are few more inputs.

1. The water is used for domestic purposes only(around 1000 litres a day)

2. The water is flowing from 1st to the 2nd point and as per Geologists, 2nd point is downstream with respect to 1st and hence bore was dug at 2nd point.

3. As per my knowledge and based on info from people staying there, rains occur copiously in and around that area. So rainfall intensity has been good.

4. Other people who have stuck bore in closeby areas claim to have struck water at 200-300ft range(their places are located at a lesser gradient/altitude than ours by approx 50-60 ft)

Now my questions are

1. Will the "reduction in yield" still hold good?

2. Rain water recharging will be done but is it required to be done directly to the borewell?

3. Which pumpset(submersible) is better in terms of maintenance? I was suggested KSP, Texma and Crompton Greaves

 

Thanks in Advance

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