If the well is not properly drilled, cased and cemented, then there is a small potential for groundwater to become contaminated. However, it is unlikley as the gas will most likely rise to the ground surface and NOT go into groundwater aquifers. The gas will want to rise upwards to where the pressure is lowest - almost always the atmosphere at the surface. Why would it push/force its way into a underground aquifer when it could more easily float upwards??