Substantially excess uranium in the waters of an intertidal salt marsh: A case study of eastern Chongming Beach

Abstract

Knowledge of the geochemical behavior of uranium is key to an accurate tracing of the decay chain of the uranium series nuclides for material transportation processes on different temporal scales. However, direct observations for understanding dissolved uranium input in tidal water are very limited. Therefore, dissolved uranium was measured in tidal creek water, overlying water, and pore water collected from the central cross section of eastern Chongming Beach, Changjiang Estuary, from November 2012 to October 2013, to observe the spatiotemporal distribution of uranium. The results indicated that the behavior of dissolved uranium is conservative in the tidal creek waters during the flooding process, and an excess of dissolved uranium is observed during the ebbing process; excess uranium phenomena are dominant in the overlying and pore waters throughout the year, particularly at the middle and high tidal flats. The element concentration profiles in pore water showed that uranium release follows Mn oxide reduction in the subsurface suboxic zone. Hence, the exchange process between tidal water and pore water, coupled with ebb tide, represents a potential path for uranium release into the estuary. The three-end-member (Changjiang River, seawater, and pore water) model estimates that 35% of the amount of uranium in ebbing water is derived from pore water. The exchange rate between tidal water and pore water is 196.4 nmol/(cm2·d), with an export flux of 2.34 × 104 mol/a. We suggest that the release of uranium from intertidal salt marshes is an important uranium source in estuarine and coastal regions.