234Th activity profiles in Puget Sound have been studied using a model that incorporates reversible exchanges between dissolved, fine particulate, and macroaggregate Th reservoirs. Macroaggregate settling is made responsible for the downward flux of Th and the vertical gradients of activity in measured profiles. Least squares fits of model to data yield rates/time scales for the exchange processes involved. Fine-particle scavenging by macroaggregates is found to occur with a time scale of 4--6 days over a large range of macroaggregate settling speeds, ws. Macroaggregate-disaggregation time scales are 1--4 days when ws is 100 m/d. Rates of sorption and remobilization characterizing the exchange between dissolved and fine-particulate forms of the isotope cannot be individually identified from these data, but acceptable model values include those measured in the laboratory. Rates of sorption that depend on particulate concentrations which increase to the seafloor result in profiles of dissolved Th having above-bottom maxima. Based on inferred exchange rates, the residence time for fine particles introduced at the surface of this deep (~200 m) estuary is estimated to be 11--16 days when ws=100 m/d. ¿ American Geophysical Union 1991