Wikipedia has a good summary of the available remediation techniques:

https://en.wikipedia.org/wiki/Groundwater_remediation

As suggested earlier by Manner, a very good summary of known methods in this field is given in Wikipedia.

Please check also the cross-references cited therein.

There exist many ways how to remediate a contaminated aquifer. The methods are selected according to pollutant(s) present, according geology and hydropgeology of a contaminated site etc. Because in your case the pollutants are refined petroleum products, enhanced bioremediation could be suitable treatment methods.

Best regards

Vit