I have worked as an engineer for a large gas company working in the Marcellus Shale in PA. I was fairly involved in Hydraulic Fracturing ("fracking") and can help you out.
Firstly... The actual process of pumping the water (and associated chemicals + sand) down into the well should not directly pollute groundwater (aside from rare accidents/spills at surface). The well is cased in thick steel pipe and cemented as it goes through depths where any fresh water may be present. The water is only injected very deep into rock or shale that contains extremely salty and gassy water that would be too deep and undesirable for any other use.
Man made water storage lakes and heavy road usage (trucking water) have some environmental impact, but the real environmental issues arise partly from the large amount of fresh water being used (wasted?), and mostly, the disposal of the dirty used frac water that flows back afterwards. There is also a small chance that the steel well casing my break - but this is a small risk in the big picture.
The flow back water returns mixed with gas, sand, oil-like gas condensates, salts, and trace heavy metals (about 50% of the water pumped down comes back up). Obviously this water must be cleaned and disposed of properly to prevent environmental damage. This is very expensive and may be something a slimy oil company may try to dodge. This is what all the fuss is about with respect to proper rules and enforcement. Also, try and find a good reference for the volumes of water being used and you'll be shocked - hundreds of thousands of gallons...
But no, fracking does not pump water into fresh water aquifers / water wells all willy-nilly. Yes, spills do happen. Yes, pipelines may occasionally break causing environmental damage, but this can happen with oil or gas pipelines just as easily - the risk has to be minimized, but can't be removed completely. The real debate is about the shit ton of fresh water being used and the proper disposal of the huge amounts of dirty flow back water.
