Address source(s) first The ...

Address source(s) first

The first step is to locate and remove the source(s) of the oil contamination.  Until you stop additional contamination from entering the groundwater your clean-up efforts will be fighting a losing battle.

The best in-situ treatment for petroleum products is biological - naturally-occurring soil microbes will break down the organic molecules as long as they have sufficient oxygen to do their job.  This will require adding dissolved air to the groundwater eventually because the microbes will use up available oxygen over time.  Ferric iron can also be used to feed the microbes, but generally dissolved oxygen will move through the aquifer more readily. Installing airlines with compressors in as many wells as possible is the way to do this.  This method can also be used to treat extracted groundwater using treatment vessels that accomplish the same thing - I'm not a vendor but there are many versions of this available.  This would be used for drinking water only because large volume treatment would be cost prohibitive in most cases.  There may also be a need for additional treatment for potable water.

Hope this helps and good luck.