Program

Estuaries are the main pathways where terrigenous materials transported from land via rivers reach the ocean. Studies in the 1970s indicated that dissolved U behaved conservatively in some estuaries. However, Maeda and Windom (1982) found that there was substantial U removal in the Ogeechee Estuary and Savannah Estuary located in the southeast United States. Studies since then have shown that the behavior of dissolved U in estuaries was not uniform. The different behaviors of U in various estuaries will result in different flux calculations of dissolved U transport from rivers to the ocean. We noticed that there were different behaviors of dissolved U in estuaries with different SPM concentrations. Therefore, we hypothesize that the SPM concentration plays an important role on the behavior of dissolved U in some estuaries, such as the Yellow River Estuary. Three simulation experiments were conducted to examine the behavior of dissolved uranium (U) in the Yellow River Estuary. Comparing our results with other field studies and simulation experiments, we found that dissolved U behaved differently depending upon the concentration of suspended particulate matter (SPM). Our data indicated that two main factors controlled the transfer of U between dissolved and particulate forms during estuarine mixing: (1) flocculation of iron and phosphate; and (2) desorption/dissolution of U from SPM. When SPM concentrations are high (>0.8 g/L), desorption/dissolution of U from SPM is predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are low (<0.3 g/L), flocculation becomes important and dissolved U behaves non-conservatively, showing removal during estuarine mixing. When removal and enrichment rates are equal, dissolved U will exhibit conservative-like behavior. We calculated the addition of dissolved U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (WSRS, Jun 30th –Jul 14th, 2014) and found it to be about 1100 kg, which accounted for 11.3% of the riverine flux of dissolved U during that same period.