I think that is a very interesting question.It seems to me that a tree that tall would need extremely deep roots to anchor it. A car at my old house fell into a sinkhole when the street collapsed.That was at least ten feet and there were tree roots exposed so twenty feet seems very possible in a tree that tall.

Pine roots can go that deep IF the soil is very free-draining, but only a very few, small roots will actually get that deep. So you can ignore the risk. Typically, 99.9% of the root system is in the top 50cm or so of soil.

Resin

I wasn't worrying about the well, wondering just how far they can root down and tap the water table. The soil is "fine sandy loam" so it's pretty tight, probably not many roots could make it that far.

Had some down time this morning due to rare snow fall in Tn. Did some research. This article does not address pine roots specifically but measured several types of trees.Looks like the average is 7 to 9 feet however they found some in excess of 20ft. Heres the link : http://www.deeproot.com/blog/blog-entries/how-deep-do-tree-roots-grow