I just had laminated flooring installed in my entire basement. They used 6
mil plastic as well to help block the radon. Everything is pretty nicely
sealed. But they were only able to partially do my crawlspace. (obviously
because of the water pump , water heater in the way) Plus the area
underneath my stairs inside the crawlspace is exposed. (poured concrete)

I'd say theres about 40 to 50 square feet of crawl space thats expose.

Before I get my radon levels tested again, I'd like to sheet off the
crawlspace with 6 mm plastic. Im just not sure how to keep it a tight seal
down there. Taping the plastic to the sheetrock should be no problem, but
there are sections where i'd have to tape it down on the concrete as well,
and im sure that wont stick for long. any ideas ?