They way I see it they have a few options of getting it done.

1. Quick and dirty - Get on a boat with a diver, licensed driller and trash pump. Flush the sediment out of the top of the casing to the level of the river bottom and cut it off. This would be cheap and very little time involved. This would be highly unlikely DOE would be OK with doing this. But if it's hurting people might be a possibility? Again very doubtful this would be acceptable.

2. The right way but far more expensive - bring a barge with a pump truck out there also with a diver. Cruise out there at high tide, wait for low tide. Flush the sediment out of the well, perforate the well and fill with bentonite grout then cut off at the bottom of the river.