Rates of sediment bioturbation and advective exchange of water across the sediment-water interface in Narragansett Bay, Rhode Island were studied by radioactive tracer experiments. The 'biological pumping rate' of water across the interface (.7 + or - .3 cc/sq cm day) was calculated from a two box model in which the decrease in (22)Na concentration in water overlying a box core was ascribed to animal pumping of water across the sediment-water interface. The 'diffusion coefficient' of solid sediment was determined to be 3 + or - 10 to the -7th power sq cm/sec from the depth distribution at the end of the experiment of (141)Ce tagged microspheres and (59)Fe (which was quantitatively scavenged onto particles at the start of the experiment). The pore water profile determined for (54)Mn in the cores was similar to stable Mn profiles previously determined for the site. Of the radioisotopes added to the water overlying the box core, nearly 100% of the Na and 50% of Sr remained in solution, while only 10% of the Ba, Mn, and Hg and less than 1% of the Cd, Zn, and Fe remained in solution when the experiments were terminated. (Copyright (c) 1979 Academic Press Inc. (London) Ltd.)