We did what should have been done when the house was built.
Our neighbor first told us about radon issues in their house. We were surprised! We're built on at least 18 feet of sand! However, research with the State and the EPA told us that most of Colorado is a Zone 1 (high incidence) of radon. We tested and we were at nearly 10 pCi/L on a long-term test over the winter.
We have a French drain system under the basement floor that feeds into a sump basin in the basement. The cover of that basin is loose-fitting, so we concluded that to be the major source of our radon issues.
With Val's guidance (and supplies that we bought mostly from him at very competitive prices), we installed a tight-sealing sump basin cover and then built a full ASD (active soil depressurization) system. We also bought the electronic monitor.
We just finished the installation and saw an immediate reduction to a very safe 1.1 pCi/L. That has since reduced to 1.0 pCi/L. Obviously, we are very relieved at the outcome of our labors.
As for the neighbor, they paid a contractor a pretty hefty sum for their system, but the contractor never even addressed the loose-fitting sump basin cover. They merely drilled the floor about 10 feet away, put in a pipe and fan, and collected their money. Is the radon level at the neighbors' house now down? Likely not!
We encourage all to do your research, do the work, save some money, and get an awesome result.
Check out Val's website for really good advice, and the supplies you'll need at very fair prices. He clearly knows his stuff!