Drilling the borewell deeper

Drilling the borewell deeper, which is already is good source of water.

I have a 80 feet deep 4.5" borewell that is now inside a large room with RCC slap on top a 10 feet. This borewell had been supplying 24 hours of water with a one inch pump. However, of late the water in the borewell have reduced a lot. The technician says that we need to drill the well 250 feet deep. I would like to know that since the old borewell was a very rich source of water at about 60 feet only, could drilling down to 250 feet make that water disappear all together? The technician has also suggested for casing the indoor of the borewell upto 80 feet inorder to drill down to 250 feet, should we use perforated pipes to ensure that the old water source is not blocked? Please advise.

Thank you
Khalid Hyderabad