what kind of pines are they? loblolly, longleaf, pitch, virginia? from my experience, most coastal plane species have shallow roots and in IMHO a 6" tree wont have penetrated too deeply into the dam structure. i'm with bob and sosauty....cut them out confidently, and yer options would be to simply cut low to ground and leave the small stumps, or remove the main rootball just below surface and repack w/ clayey soil. i'd probably just leave the stumps.
