First off...the oil/gas is trapped in a SHALE which is basically impermeable.
2. The shale layer is down several miles deep....
3. Most of the "chemicals" (water, sand, lubricants, glass beads etc) are pumped out and recycled.
So the chances of surface contamination are zero.
"Chance" is like flipping a coin and having it land on its edge
Here's our Grate Reader in decision making process to show what that looks like:
with the Senior Advisers (old balding white geezers).....
Heads......Ragu Beef
Tails.......Lobster
and my chance of getting re-elected if the coin lands on its edge....OK guys....