Excess 222Rn concentrations decrease downstream in the fresh water section of the Pee Dee River. Ground water is the primary source of the excess 222Rn to the River. Using the radon concentration gradients determined during four sampling periods, gas exchange rates based on the stagnant film model are calculated. Stagnant film thicknesses range from 19 &mgr;m to 48 &mgr;m and mass transfer coefficients range from 2.1 m/d to 4.1 m/d.