what the essentially did in the 90s was to dig giant trenches around entire city block, way down to the impermeable clay layer ... then fill the trenches with a

what the essentially did in the 90s was to dig giant trenches around entire city block, way down to the impermeable clay layer ... then fill the trenches with a similar impermeable clay creating a giant block sized container which they are still now pumping water through to slowly clean out the polutants