Bad borewell water during rains in Purasaiwalkam, Chennai, Tamil Nadu

I live in Chennai (Purasawalkam). I have a borewell in my residence which is about 130 feet deep. This was comissioned in 2004. In the recent years (specifically after August 2006), the quality of water degrades immediately after rains and settles down after the rain stops. The water lathers with foul smell (visually clear however). The situation settles down after the rains have stopped. I am suspecting a drainage conduit about 3 feet away from the borewell. But if the sewage way is causing problem, then the problem should have been perenial, rather, it happens only during the rains.

Kindly request you to give me you expert view, so that I can take some corrective action.

Thanks in advance.