Man, if there's septic system run-off... that's design, maintenance, or plumbing malpractice or most likely a combination of all three for older systems!
Guess some folks need to plant more carrots, shrubbery, and trees if they've got perimeter surface runoff issues... while deeper groundwater issues need much more geohydrologic study and moving to wells versus cores & lysimeters... its a big jump and will cost you, but a little knowledge goes a long way.