Fill hole in slab beneath tub to reduce radon

This is what I ended up doing after some research and opinion asking:

Chisel all the loose concrete edges so there is an undercut. This prevents the patch in the slab from rising. Backfill with sand. As you put the sand in moisten it and tamp it down. Make sure the piping is well supported and that the closet flange is level. Keep adding sand until it matches just below the bottom of the existing slab. Mix concrete and pour into the hole, backing it under the undercut and ensuring no voids exist. Keep filling until it is slightly higher than the hole and then screed it off.

So far it looks okay although I think my concrete could have been a little wetter.

First Radon is a pretty serious thing if you are working that much inside your basement. I have seen Radon levels in basement stay consistently at 4 times what is in the upper levels. Your 6.8 reading is high but not alarm bells until you get a few readings that high.

Initial steps:

• Open windows. Better than using fans is trying to create a convection pattern in your basement where the air is moving. Is your basement open enough to make this happen. Are your windows on sliders and easy to open. Get your basement aired out. Also don't close the doors by the steps. Let the basement air convect up to the upper level. Air flow is key.
• Seal any cracks that you have in your basement using caulk or epoxy sealants.
• Have your basement retested periodically over the next few weeks/months. I would say every 2-4 weeks.

If your future tests aren't dropping below the 4.0 pCi/L range than you need mitigation for sure. If they are falling below 4 then you need to figure out if you can keep up the good habits (airing the basement out continuously) even when the weather isn't great. So if your Radon levels drop with just improving airflow and sealing, then you need to test it every couple of months on its worst days. Radon levels can fluctuate greatly from day to day in a home (I had a house tested at 7.5 one day and two days later and me not touching airflow it was 4.3) so there is no way of knowing the "worst" day. But you can approximate during the winter "we haven't opened the windows in 3 days because it's cold as hell so let's test for Radon".

Now as far as mitigation, for most homes this will involve running a PVC pipe from your basement to the outside of your house - most of the time venting out your roof/attic. Sometimes the air cleaner is installed in the basement and sometimes in the attic. It probably involves opening a few walls. It is definitely a DIY type of job but that is a different question that would require a ton of details. I have installed Radon mitigation systems in basements in one day with just my son helping. A mitigation company will charge you anywhere between $800-2000 depending on the access to your basement floor (finished areas cost more), access to running the venting, how big your house is, if you have a circuit available to handle the electricity for the system, and what kind of system you want (you can get them at big box for $100-500 usually). DIY this could be a $200 job - it is just labor intensive - mainly patching drywall in your house.

How to get a good contractor? I would make sure they are certified to mitigate Radon. You can also call your local city hall and talk to the building inspector for local businesses that are certified or a suggestion. Radon mitigation is a lot of common sense (seal your basement and pump air out of it) so it doesn't take long talking to a contractor to tell if they understand how to do the job.