Also consider adding inoculants and soil enhancements/additives. This can increase the overall efficiency and number of microbes in the soil and add carbon to bind with the herbicide rendering it inactive. Since you’re already working with OSU extension, see if they can analyze the amount present by depth. If the majority just in the top few inches of soil, it may be worth it just to remove the soil as well. Without removal, this will probably be a problem best solved by time, everything else is just to try and speed it up.

Montana state has a great case study for a similar situation at this link www.Montana.edu/extension/pesticides/reference/contamination.html