Gday Mike
The chemicals you mentioned have the potential to persist in the soil for quite a long time. Dual is probably the chemical "metolachlor". You can find out more about it and "Atrazine" by doing a search on the web under those names.
These chemicals will break down over time, but this can vary from 2 months to a couple of years depending on conditions. Both of these chemicals are mobile in water and will likely have been flushed out to a large extent, but they can also bind to clay type soils for a long time.
The breakdown of these chemicals is dependant on microbial action, so the best thing you can do is turn the soil over a few times. This will aerate the soil and help the microbes to digest the chemical. The deeper you can furrow, the better, as atrazine and metalochlor is particularly persistant in undisturbed sub-surface soils.
As these chemicals are mobile in water, there is a risk to aquatic plant life if you release large amounts of it by turning the soil. So if you want to avoid putting your local creek at risk, you are probably better to wait for the warmer months before you disturb the ground. Atrazine and Metalochlor both break down quickly in warm moist conditions.
Its also worth finding out how long ago and how often your patch has been sprayed with chemicals. Quiz your farmer friend and find out if there is a history of chemical use on your land. While the most recent dose may have worn off, you dont know what is lurking beneath the soil.
There is no need to be paranoid about the effects, neither of these chemicals is particularly toxic to humans, but it doesnt hurt to be informed and to take action accordingly.
Good luck
Mike Clarke
Centre for Green Chemistry
Monash Uni, Melbourne Australia.