Dang, to me, not understanding the science mind you, it seems that the Fukushima area will be radioactive for a generation or more.
I pray for the folks there.
The answer to your questions, (at least to the extent of my limited knowledge and training), is indeterminate at this time.
All radioactives have a “half-life”. It can, depending upon the element, span from nano-seconds to thousands of years. They can also be relatively inert, or easily taken up by plants or migrate to other locales as dust. Right now you can walk out to “ground zero” for the Trinity, NM test site without harm/risk, but I suspect digging in that area would involved increased dose rate.
Since this “accident” did not involve any “fusion explosion” I suspect the contaminants are mostly light elements carried by smoke/steam having shorter half-lives. Its what I’ve heard described as “tramp”, because it moved about so easily. (FWIW, the “housecleanliness” of a well-run nuke would make the fussiest housekeeper blush as dust control is exposure control too.)
Depending upon the element(s) and contaminants dispersed, they will likely find their way into the biosphere. But the key fact to keep in mind is the dose rate and type of exposure to individuals. Clothing/skin exposure is minimally threatening. Internal exposure via ingestion or inhalation may be more serious. Indeed there’s a school of thought contending radiation exposure has beneficial health effects as it stimulates the immune system. Its known many of us receive much higher “doses” and “dose rates” than others depending upon where we live, our occupations, even the construction/materials in our homes. I’d opine our ancestors received far more “dose”, due to extended periods outdoors in sunlight, than many do today. >PS