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High water level in new borewell - Reasons?

We had recently had three borewells drilled in our rather large apartment situated in the Mahadevapura area of Bangalore (former CMC). Of this one borewell yielded about 1 1/4" of water at 400 feet and we went down to 600 feet.  A second borewell yielded again 1 1/4" of water at various levels and we went down to about 800 feet before stopping.  A third borewell yielded less than an inch of water which was again terminated at 800 feet as recommended by the geologist report and data from other borewell drilledin our area. We also have an existing borewell with decent yield in use, pump at 350 ft depth. Builder had it drilled and we do not have much info about that borewell.

I wish to ask a question about the third borewell (1 inch yield, 800 foot deep).  Hard rock was hit at 26 feet.  Driller (supposedly well known in Bangalore) lowered 6.5 inch heavy gauge MS casing to this depth as well as a 10 inch PVC pipe.  Water was struck at around 450 feet and 600 feet and then we went down to 807 feet, cleaned and capped the borewell.  There was an enormous amount of rain in Bangalore during and after the drilling.  Yesterday (April 24), we found that the water level in this borewell was just 10-20 feet below the surface (I could see it from above!).  Is this normal? What could be the explanation? Could water be leaking into the borewell from somewhere? Could the casing pipe be cracked - say at weld? I am worried about any surface run off getting injected into the borehole and contaminating the ground water.  Open to any suggestions on what to look for and fix.

We did not see this phenomenon in the other two borewell that had been also drilled at around the same time.

Thanks,

-Sanjay

Bangalore

PS1:  We have an issue of sub soil saturation in our area.  Rock level is shallow.  In parts of out apartment, making a hole 20 foot deep will yield water also.

 PS2:  Until a year back, our apartment was receiving good supply of city (Cauvery water), but has now massively dropped and tanker water quality, upon testing was horrible.  Hence the decision to go for these new borewells.  Also trying other cobnservation, harvesting and recycle measures.

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Comments

1. Reasons for High water level.

Dear Sanjay,

As you have mentioned towards the end that one encounters water if land is excavated to about 20 ft in your area, well it is possible that some water had been struck just after the casing has been installed after striking hard rock, but was not noticed as the quantity was very low, there could have been a small gap where no dust was being generated while drilling. this is also indicative of an aquifer being encountered, but was not recorded.

Any way, this could be one reason, or as you say that,there has been lot of rains recently, it is quite possible the said borewell has encounted a fracture which is in hydraulic continuity whith the shallow system in the recharge area, and hence the recent rains have managed to charge this borewell and the water has risen to about  10-20 ft.

Just to check if there is no leak in the casing installed, my advice is to lower a suction pipe of any small domestic self priming monoblock pump down to 40 ft in the borewell and dewater the borewell to that depth, once  that is done you will be able to observe any inflow in to the well from the sides ( if any) if no water inflow is observed into the well then you could be sure that it has got nothing to do with surface runoff being injected in to the borewell.

Why this is not observed in the other borewells I cannot comment upon whith the limited information, but probably in a day or two the water level in the othe borewells would also rise to the same level.

Regards

Dr. Anil Lalwani

www.wellwaterworks.com

2. Dear Sanjay, The rise in the

Dear Sanjay,


The rise in the borewell No. 3 to 10-20 ft. is due to improper construction of the well. The water level should drop down after some days. The other wells do not show such phenomena. This means that the borewell No. 3 is not going to last long.



With regards,
D. Chakraborty
CGWB, Raipur

3. Conduct Aquifer Performance Test

The confined water when encountered by drilling always show a rise in the water level. For sustained yield from this tubewell it is recommended to conduct Aquifer Performance test and Step drawdown Test in the tubewell and note the rate of recovery.

Professor S. Mukherjee PhD Commonwealth Fellow (UK)

Professor of Geology & Remote sensing

School of Environmental Sciences

Jawaharlal Nehru University

New Delhi-110067

4. Check for any surface signs of Leakage

Dear Sanjay,


Yours is a classic case. As all of the previous experts suggested  - it possibly could be due to faulty construction. Two things to be checked right now. Excavate a pit around the third bore PVC Casing of additional radius of 15 inch and depth of approximately 10 inch. Pack the casing boundary with the local formation to as much watertight as possible(judge  by visual inspection and your logical perception). Plug this pit with RCC against the PVC casing and let it cure. Also seal the gap between the PVC & the MS casing completely(you can again use a wet cement slurry for this). This would take atleast 7 days for curing. Meanwhile you can check the water level of all the three bore wells as well as any dug wells in the nearby area and compare. If the static water level in all the bore wells are more or less equal - The chance of leakage would be minimal. However if the water level in the dug well and that of the bore well are identical - a possibility of leakage remains. But cement sealing should take care of that if the casing has been set properly.


Sometimes it happens that just beneath the casing, there exists a shallow fracture - connected to the phreatic  / saprolite zone( dug well zone in some cases)  which sometimes remains undetected because of local hydrogeologic conditions. Once rainfall occurs - these fractures starts contibuting water to your well and may cause a marginal increase in yield seasonally in post rainfall period. Just check for the presence of the reported hard rock formation in your third bore from the well cuttings if any and make yourself sure that it is indeed the hard rock and not the iron stained saprolitic material.


Hope this takes care of your needs for now


regards,


A. Choudhury


CGWB, Bhubaneswar

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