Injection wells are an underground well where water or other liquids are injected into the well instead of being pumped out.
Injection wells are mainly used for waste water dumping, where waste water are injected into the impermeable layer of rocks to avoid polluting fresh water supplies. And when the injection well is full, it would be cemented to seal it to avoid leakage from the injection point.
Theoretically, injection well is a safe way to dump waste waters as wastes are injected into layer of rocks where water can't go through and building an injection well is strictly controlled, however injection wells are known to be prone to leaking because of earthquakes and fractures in the rock structure.