Borewell failure provided by builder.

I bought a 2BHK Flat in Guduvanchery, Tamilnadu in June, 2015. The Apartment constructed with 2 Blocks in area around 4000 Sq.ft. Each block has 4 Kitchen so total 8 flats are there. But only four flats have been sold out, the remaining 4 flats are still vacant. There are Two separate RCC tank provided with support of 2 motors(2), one is for A-Block (4 family) and another one is for B-Block (4 family). One of the Borwell got failed in the flood of 2015, December. We taken up the issue to Builder notice. He asked us to pay around Rs.7500 to drill another Borewell for 150 feet. He estimated the cost of Rs.58k and divided into eight for each flats(8) of the Apartment. And told with occupants to pay the individual share amount Rs.7250/- in advance and mentioned the remaining amount for Vacant flats, he will put approximately Rs.29000/- We are telling the Borwell was not done properly, in adjacent(just 2ft gap) there is a Rain Water harvesting placed for 10 feet. Because of this, the Borewell got failed. He is not agreed, arguing with us. He estimating some additional work for connecting the existing Borwell and planed Borwell. Please advise.. Regards, Balaji G