Is 'Fracking' safe for local drinking water resources?

This is a quite complex process and perhaps not simple question and answer. "Fracking" is the process to break up the rock and release hydrocarbons. This process is the same as a "conventional" oil and gas well except for the ~week-long process to "stimulate" the well to make more oil flow using water that is pressurized. To encourage the well staying open, they add a fine sand (called a proppant) plus a mixture of chemicals. There is not "thousands" of chemicals as some claim, usually a mixture might contain 8-20 different additives that are are different for each well.

Most often, the drilling is a mile or more below and under many layers of rock, in these cases, and with new drilling techniques and well management, "fracking" has very low potential to contaminate groundwater for drinking water supplies. What happens after the well starts to produce is that a mix of water and oil and gas comes back, is separated and the oil/gas recovered. The remaining oil contaminated waste water has high salinity (salt), high oils plus a lot of metals and what ever is left from the chemicals put down there. In most places, this water is re-injected back down in another area (called a disposal well) and they inject it back 1-3 miles below ground. However, in a small but significant number of cases, something goes wrong such as a well casing cracking or a surface spills of the wastewater and the wastewater escapes where it can get into drinking water (either a river, lake or shallow groundwater). In some jurisdictions in US for example, they allow "fracking"wastewater to be applied to roads while in others, they allow it to be released to rivers, something I think needs to stop.

So the short answer is yes, it can be bad for drinking water, but normally/usually is not. If the oil and gas reserves are shallow, the potential for these chemicals to contaminate the groundwater increases. However, if the companies are using the more advanced water management techniques, more recent well design and monitoring techniques, and following proper disposal of wastes, "fracking" is as safe as most other industrial processes we allow (manufacturing releases, pesticides etc) and should not be a huge concern relative to the other risks we face.