There are legitimate concerns about ensuring that drilling and hydraulic fracturing do not contaminate ground and surface water, and that is why the industry is heavily regulated under the CWA, Safe Drinking Water Act, and Spill Prevention, Control and Countermeasures Rules.
Wells are carefully constructed to ensure that any fluids pumped into them, as well as the oil or natural gas coming out, do not leave the wellbore and contaminate underground sources of drinking water. Two to three alternating layers of steel pipes and cement are constructed in the well bore to keep fluids from migrating out into the rock formations and to keep water from drinking water aquifers from flowing into the well.
Well construction is heavily regulated to protect drinking water during the drilling, fracking and production phases of the well. Drinking water aquifers are also protected from contamination by the thousands of feet of rock between them and the formations that contain oil and natural gas.