Pump depth of a borewell In Bangalore, Karnataka increased to 280ft depth but good water comes only for 10 mins - Need a solution

I run a hostel in Bangalore at Kengeri ,our bore depth is 350ft, we had let the 2hp 25stage .

We had water from 2004to 2010 ,this year 2011 as summer is approaching we are facing problems.

There is a lot of silt probably due to the city drinage which flows 300ft away.

i have got the bore washed and increased the pump depth from 250ft to 280ft.

I still get good water for 10 mins only,if we switch off the bore well for 2hours we get another 10mins of water.

please suggest or guide as whats o be done.