Chemical or Biological would be much better for area denial than radiological (or at least more realistic).
Chemical and Biological agents can stay in the environment for very long times (possibly tens of decades or more), as in the biological case they are self repairing and can stay dormant for long times, and for the chemical case they can be very stable chemicals which do not decay naturally.
Radiological denial systems will only last for about 20-40 months as a serious threat, then will cause minor illness for a few years. This is as the most lethal isotopes have half lives on the order to 10s or 100s of days, and will quickly drop below externally lethal levels. The minor illness cases will last a bit longer, and are mainly a threat if ingested.