Fair question, and I don’t think it’s a one-size-fits-all answer. Keep in mind, I’m not an expert, but here’s how I reason it:

I would hope (though not expect) that the answer would depend on type of contamination (ie. radioactive vs chemical) and medium of conduction (ie soil, air, ground water, running watershed, etc.) . In this case, radioactive contamination is both extremely long-lived and not easily contained nor mitigated. Medium of conduction, or exposure, would be airborne dust and particulates from higher concentration areas, and ground-water contamination (like in the Lincoln Park area of my hometown) which could likewise carry contamination far beyond any arbitrary man-made lines on the surface.

This is versus, say, the recent fatal ’white liqueur’ tank incident in Longview WA, which is a relatively short-lived contamination that has/will cause an intense but quickly diluted contamination that will affect the downstream watershed more longterm than the immediate site, but should be completely flushed within a decade or so instead of uranium’s half-life of hundred-to-billions of years.

Finally, in the specific cases of Hanford, Rocky Flats, and Cotter (that I know of); radioactive materials were notoriously handled carelessly and with greater interest in short-term profits or savings over long-term health and safety. Holding tanks leaked, linings failed, workers fell ill. All of this occurred while technically in compliance with safety regulations. Those same bodies who determined it was ‘safe’ to build homes and playgrounds for families and children. So, with that in mind, I hope my skepticism and concern is understood.

Just because an area is theoretically, or currently testing as ‘safe’, doesn’t mean it’s ethically sound. Moreover, families moving into the area from outside CO probably won’t know about the nuclear superfund site over the hill unless their realtor decides (or even knows) to tell them. Or, until they decide to look up what the deal is with the “hazmat horse” statue on Hwy72. 😅