One problem with fracking

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In conventional drilling,
  - The gas has migrated from an oily shale strata
  - to a porous sandstone reservoir.
  - A hole into the reservoir accesses the pressurized gas.
In fracking,
  - The gas has not migrated from the oily shale.
  - Sandy fluid is forced in to make channels for the gas
  - Some of the fluid comes back out and is a disposal problem.

Fracking injects sandy water plus additives into the well, to make cracks in the shale, and jam the cracks open. A third of the fluid that goes in comes out again, but now mixed with hydrocarbons and metals from the oily shale. The first step in disposing of these produced fluids is to allow the fluid to evaporate.
These vapors are one problem with fracking.