Right, but there's ground and there's ground.

Geological strata vs shallow landfill sitting above aquifers and subject to near-term erosion.

Disposing of this stuff in deep mines seems like it'd be fine, unfortunately we haven't yet, at a society/economy level, found the discipline to do so. Presumably after a few mya of heat and pressure it'll be indistinguishable from other petrochemicals (which aren't particularly nice to begin with).