Dissolved and particulate partitioning of trace elements and their spatial-temporal distribution in the Changjiang River

Samples of raw water, 0.45 μm filtrated water, 0.20 μm filtrated water, and 0.45 μm filtrated suspended sediments were obtained and analyzed for trace elements (e.g., Se, As, Cd, Pb, Zn, and Cu) at 76 locations in the main channels and tributaries of the Changjiang River. The partitioning of the trace elements in the particulate and dissolved load, as well as their spatial and seasonal distributions, was investigated. We found that the concentrations of As, Se, Pb, and Zn in the dissolved phase (< 0.20 μm) were much higher than those of the world averages, and Cd, As, Pb, and Se were highly enriched in the suspended sediments. Metals and metalloids differ in their partitioning characteristics. For the metal elements (e.g., Cd, Pb, Cu, and Pb), the particulate phase is the dominant form during transportation. The spatial and seasonal variations of the total metal loads and their partitioning are controlled by suspended sediment concentrations (SSCs) as well as water pH, which controls the absorbance of the metals by the suspended particles. The particulate phase of the metals is much higher during the flood season, when there is high SSC, especially in the upstream of the Three Gorges Dam (TGD), but the dissolved phase increases gradually down to the lower stream with the decreased water pH. For the typical metalloid elements, e.g., As and Se, the dissolved phase is the dominant form of transportation. In the flood season, the total loads of As and Se are higher with a higher spatial variation than those in the dry season. The highest total As load occurs in the most upstream part of the river, and the highest Se load appears in the section between Luzhou and the TGD. The results also indicate that large amounts of the heavy metals have deposited on the bottom of the Three Gorges Reservoir. So, more work should have been done to evaluate their potential environmental hazards as they may release suddenly with water acidification or other disturbances.