2,4-D has a relatively short half-life and is rather immobile in the soil.

There are some statistics at the link above.

Are you sure your are dealing with 2,4-D? I some sod that will only grow grass and hollyhocks after six years.

Yikes!

don't know what you're trying to kill, but I've had good luck with pickling strength (9% or 10%) vinegar.

Here is a link that might be useful: toxicity of 2,4-D