One of the chief worries that people have about fracking concerns what happens to fracking fluid left underground. It's not the problem you think it is.
This concern has been blown up out of all proportion by opponents of fracking in the UK. Arguments range from "it will pollute groundwater" to "it will migrate to who knows where" - arguments often made by the same people (exactly: how can they argue with certainty that it will somehow pollute groundwater but then also admit to not having a clue what happens to it?)
Here's what we've managed to piece together from the facts.
The fracking fluid is made up mostly of water, sand and a very small amount of chemical additives that must be non-hazardous to groundwater - so, the first thing to note is that, regardless of what happens to it, it can't pollute groundwater in a way that causes any harm. The shale is also encountered in a thick band from about 1,300 metres to 3,000 metres below ground, and so the second thing to note is that fracking fluid does its job a long way below the groundwater aquifer.
It is injected at a pressure just big enough to overpower the pressure of the rock and cause tiny, hairline cracks to form. These connect the minuscule pore spaces in which gas is trapped and provide a conduit back to the wellbore. The grains of sand hold open these cracks to allow the gas to escape once pressure is relieved at the surface and the well is "flowed back". The average particle size of the sand used to prop open the fractures is about the same diameter as 8 strands of human hair, so you can see how small they are.
Somewhere between 20 and 40% of the fracking fluid returns to the surface at this point, with the rest remaining in the shale rock, where it has essentially displaced a broadly equivalent volume of trapped gas.
Gradually, over time, the pressure of the surrounding rock eventually forces the fractures to close again, isolating the pores from one another. Without the connectivity these fractures once created, the fluid that remains underground is essentially immobilised and has nowhere to go. It can't flow through the rock because it is all but impermeable without the fracture network.
It's a bit like filling an empty glass bottle with water and then fastening it closed. The water displaces an equivalent volume of gas (air) and once the lid is on, remains contained indefinitely.
We can see why people might fear the fate of fracking fluid that's left behind, but when you examine it dispassionately like this, it's clear that it's not the problem that opponents of shale gas extraction would like us to believe it is.