Except for gravity, how could a water (e.g. treated wastewater) forced back into the aquifer?

Answer 1

Pump it using deep injection wells.

Answer 2

pump it into place. duh

Answer 3

The soil acts as a filter.  We don't want to inject water directly into the aquifer.

Answer 4

grab a bucket.

pumps that run on renewable energy.

something about a piston pump.. there is a water pump that uses momentum to pump water... piston, hydraulic, ....