radon levels under slab

I've been using a radon detector (National Safety Products Pro 3) to check radon levels in our house. Levels are high, but within my expectations for our situation (25 in basement, 15 upstairs).
But, then I put a hole in the cellar floor, dug out a 1' deep hole, and put the detector down there. It's getting readings of 300+. I'm kind of surprised at this If anyone has any insight on 'in ground' radon measurements, I'd appreciate hearing.
(NB: It would be OK to spare me the 'OMG, evacuate the house and call a HazMat team" comments. Bandwidth is a precious resource.)
TIA, George
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George wrote:

Doesn't surprise me that readings are very high in-ground.
Some insight here: http://energy.cr.usgs.gov/radon/georadon/3.html
Google around. You may be able to find maps of Radon levels in your geographic area or state.
Jim
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I used to live in Lancaster, PA. We had a radon problem in that area of PA -- the "Reading Prong," I believe. The radon abatement mechanism was to drill a couple of large holes in the cellar floor and pump out the air under the slab. We got the reading down to somewhat less than 1 pC/L after a week, and could easily hold it there. They also caulked up any cracks in the basement wall/floor.
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George wrote:

In future, please abbreviate "bandwidth" as "bndwth," thereby saving precious bndwth.
Thnks
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Yu cn do bttr... abbrevte bndwth as bdwth 2b evn mor mizrly.
Joe
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see more reading: http://www.cdc.gov/nceh/radiation/brochure/profile_radon.htm
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