Regardless of who put the weed killer on the lawn, the bank is responsible for the off-site migration of pollutants from their property. You can get a new lawn out of them.
If they balk, just ask them how much the Phase 2 site investigation is going to cost to clear it from having a chemical spill reported on it...
The "Bank" doesn't own the property. It is still in the name of the deadbeat cop. The bank won't own it until they foreclose on it. They apparently are in no hurry to do it.