Particle Dynamics in a Managed Navigation Channel Under Different Tidal Conditions as Determined Using Multiple Radionuclide Tracers

Particle dynamics play significant roles in sedimentological and geomorphological processes as well as biogeochemical cycles in estuaries. We collected a suite of water samples from the North Passage of the Changjiang (Yangtze) Estuary during different tidal conditions in summer 2009 to observe variations in the particulate ²³⁴Th, ⁷Be, ²¹⁰Pb, and ¹³⁷Cs activities as well as in particle transport, deposition, and resuspension. The mean activities of ⁷Be and ²¹⁰Pb_ex in surface water during low tide were lower than those during high tide. The residence times of ⁷Be (1.1–3.4 d), ²³⁴Th (4.7–28 d), and ²¹⁰Pb (7.9–34 d) in the North Passage increased with the increase of salinity and suspended particulate matter. The estimated results, based on a two-dimensional model of ⁷Be, ²³⁴Th, and ²¹⁰Pb, showed that sedimentary particles in the North Passage experienced net deposition during neap tides whereas those in more seaward areas of the North Passage exhibited net resuspension during spring tides. The ⁷Be/²¹⁰Pb_ex activity ratio in seaward area of the North Passage was lower during spring tide than during neap tide but both were close to values in surface sediments beyond the estuary. Using ⁷Be/²¹⁰Pb_ex and ¹³⁷Cs/²¹⁰Pb_ex activity ratios as tracers, we deduced that most of particles in the North Passage were not derived from the direct deposition of Changjiang input particles but were transported from sediment deposited offshore. These results emphasized the important contributions of offshore inputs to sediments in this channel, which should be considered when conducting channel dredging of other estuarine systems.