The contamination will move towards the new borewell also as there is a direct linkage which was established while drilling itself. More over 6 ft is not enough distance to ensure clean water.
First try to hyper-chlorinate the old borewell, and then try to seal it off by pouring rock aggregate right up to 70 ft and then pour concrete in the remaining upper 70 ft to seal that borewell. Before using the new borewell, hyper-clorinate that well too, and then install a temporary test pump and use the borewell for at least 2 to 3 weeks before going in for a permanent pump.
For the next two to three years, during and after the rainy season use hyper-clorination to decontaminate the new borewell, as there is a chance that whatever contamination has leaked from the sewage has contaminated the surrounding areas in the subsurface and this may move towards the borewell due to high water table conditions during the the monsoon and post-monsoon periods.
Anil Lalwani (http://www.indiawaterportal.org/users/wellwaterworks)