Because of Presumed Responsibility, the gas company is responsible for any water contamination that occurs within 2,500 feet of their gas well.  They are "guilty until proven innocent" unless they can prove:

• The pollution was already there (so they test to be sure)
• You turned them away when they came to test (please let them in)
• Your well is outside 2,500 feet (only within that small radius)
• The pollution occurred more than 12 months after drilling stopped (don't wait too long to make a claim)
• Something else caused the pollution (It wasn't us...)

See it in the actual law: Oil & Gas Act

Keep in mind that the law does not prevent a landowner who claims pollution or reduction in water supply from seeking any other legal remedy.