Determination of transport rates in the Yellow River-Bohai Sea mixing zone via natural geochemical tracers

In light of the current problems facing the Yellow River and surrounding areas (e.g., periods of zero river discharge, increasing nitrate concentrations of the Bohai Sea), we examined the coastal mixing dynamics around the mouth of the Yellow River. Naturally occurring radium isotopes (Ra-223, Ra-224, Ra-226, and Ra-228) and other geochemical tracers (Ba, Si, and salinity) were employed to determine river plume transport scales and rates. Barium and radium exhibit elevated concentrations within the salinity gradient where they are desorbed from particles via ion-exchange. Once they are added to the system, they decrease offshore from dilution with lower concentration Bohai Sea water, and in the case of Ra-224 and Ra-223, by radioactive decay. Using radium ages to assess the dissolved material transport scales and rates proved to be a useful tool in this environment. The ages based on the Ra-224/Ra-228 activity ratio increased gradually until salinities reached similar to 25 when they rapidly increased due to decreased mixing at higher salinities. Integrated net transport rates through the salinity front ranged from 1.4 to 1.6 cm/s and did not vary significantly with river discharge. Thus, tidal mixing appears to dominate in this system, at least over the range of discharges investigated (80-600 m(3)/s). Determining the temporal scale of flow across the coastal zone in this region is a valuable first step toward examining whether the Yellow River is contributing to the increasing inorganic nitrogen concentrations in the central Bohai Sea. (C) 2008 Elsevier Ltd. All rights reserved.