The actual practice of hydraulic fracturing (or “fracking”) is only a small part of the overall process of drilling, completing, and producing an oil and gas well.
Environmental issues that are specifically related to hydraulic fracturing include:
water availability
spills of chemicals at the surface
impacts of sand mining for use in the hydraulic fracturing process
surface water quality degradation from waste fluid disposal
groundwater quality degradation
induced seismicity from the injection of waste fluids into deep disposal wells
Any kind of oil and gas drilling can additionally cause:
reduced air quality
noise
night sky light pollution
landscape changes such as forest fragmentation
disruption to wildlife corridors and habitats
It is important to note that not all of these potential impacts occur at every site and many impacts can be avoided or mitigated with the proper practices.