There several parameters ...

There several parameters that I need to know before I can fully answer your question.  But with the information you provided the following mitigation techniques are ones that I have used successfully in an very similar situation

1. Create a trench around the contaminated site (ensuring that the trench depth makes contact with a low hydraulic conductive clay) and fill the trench with activated bentonite (usually in a slurry form).  Assuming that the bentonite being used is of low hydraulic conductivity (at least