There is a lot of propaganda from both sides of the fracking issue. By it's very nature, fracking breaks down shale deposits releasing natural gas trapped within the shale. Some of that gas can easily leach into ground water supplies and cause "flaming" water. Whether or not the burning gases are toxic to humans depends on the types of gasses. If it's methane then it won't be toxic, just very explosive and stink like shit. If it's natural gas then it could be very toxic.
The area I live in northern California was once the egg capital of the world. We had tens of millions of chicken in this county from the late 1800's through the mid 20th century. Consequently our ground water is very high in nitrates from nearly 100 years of chicken shit leaching into the ground, and they don't expect those nitrates to go away any time soon. It doesn't take a rocket scientist to figure out that chemicals pumped deep into the ground for fracking could eventually work their way up into the ground water. Depending on what those chemicals are is the big question, and what can really destroy the environment long after the fracking has stopped in one area. Almost daily here in California we hear on the news about a several square mile area with contaminated ground water where the navy dumped chemicals during WWII, or where a semi-conductor company manufactured computer chips during the 1980's.
I'm not against fracking, especially if it means we're less dependent on arab oil, but I would like to know what the long-term implications are after the drilling company packs their bags and has moved on to another area. At the rate we're going, our grand kids are going to need a new planet to live on.