Is it illegal for fracking companies to let water that has been used for fracking deep back into the ground?

It is illegal

Explanation:

Fracking involves drilling deep into the earth crust in which a water mixture with high pressure is directed at the rock with the purpose of releasing the gas inside. This process is common in the exploration of natural gas and other hydrocarbons.

It is however illegal because the water are mostly contaminated due to the hydrocarbons present and harmful to life. It is advisable and the best to remove the water and recycle them in treatment plants