I have an elevated radon level within my house. The house is 17 years old and recently I discovered our county has a risk of elevated radon levels. I confirmed the high level (4.6pCi/L in  our basement) several ways and hired a state certified local radon mitigation contractor. He installed a radon reduction system called an active subslab suction or subslab depressurization system. It was a challenging installation because our house sits on the lowest spot of our neighborhood in heavy clay and the soil underneath the slab was wet in all the locations he tested. We have a sump pump and external drain tile which I believe works appropriately, the sump runs when we have rain and is quiet when it is dry. We also have a french drain tile installed on the west and south side of our house in our yard to help divert water away form the house. We are in heavy clay soil. We have never had any problems with water or moisture in our basement. After the subslab depressurization was performed (the sump pump was sealed and a single suction point was installed and -this suction point was not in the sump pump)  the contractor rechecked the radon level and noted it had dropped to 2.8pCi/L. It was not as much as he had hoped for, (and not as much as he has seen with all of his previous installations), but now the level was with in the acceptable range. He encouraged me to recheck it myself independently of him and I did and got a level of 7.9pCi/L! He rechecked and got a level of 4.9pCi/L.We brainstormed more options and he is now suggesting a full perimeter installation of interior drain tile in our already finished basement. Our house sits on a 7% grade from west to east and has retaining walls and mature foundation plantings around the entire circumference. About 8 years ago I had a blower door test done and remedial sealing work done on the house because we had problems with ice dams and roof deck moisture and mold. These problems were solved.

Our contractor did go back to his Radon mitigation instructor and asked for more help. Here is the dialogue back and forth from the contractor and his teacher- I put it here because I think it has valuable information and I can’t understand it- maybe someone else will. One clarification is we only have 30′ of interior drain tile where a crack in the foundation is and it runs to the sump pump crock. The only thing I would disagree with is we don’t have a constant flow of water into the sump pump crock.  When it has not rained for a week here- the flow is gone.

Radon Contrator: Thanks for getting back to me.
It is a poured wall foundation with drain tile on the inside of the footing running into a sump.  They have a constant flow of water going into the sump and have a pump already installed.  

The constant flow of water tells me they are on a spring or something.

I have a mircomanometer and I have popped a few holes to test the field.  It travels a few feet from each pit.  That I think it would be useless to put a 3rd pit in.  

The material is silty sand and water under the concrete.  You could fill a glass of water in seconds from either pit I have put in.

I could try to do a horizontal pit in the wall as you suggest.  I guess that would at least be above the water level.  The wall is 8″ thick though.

Teacher:…let’s see. Wet soil and ponding pits. 50% reduction in warming weather. Finished basement. Size of footprint?  Internal footings? Forced air heating and cooling? Do you own a micromanometer? Can you do PFE tests? What is the material under the concrete floor? Where are you? What fan is doing the sucking? I assume airflow is very low. If you install a sump basin with pump dedicated to soil drying, that may serve the house and basement occupany the best.  Carpet on concrete? Sealing near suction points? HRVs can be a $1500 answer in under-ventilated rooms or basement zone. A blower door can help identify that. Reducing depressurization near entry points is more effective. Wet soil may require some direct low wall suction points plus the soil dewatering system.

The cost of interior drain tile for our house is over $15,000.00- we will do it if we must, but want other professionals to confirm this and /or weigh in on our difficult problem.

Thank you very much,

Lori