Historic records on mineralogical and chemical compositions of a long sediment core from the Three Gorges Reservoir and implications for future studies

摘要

A comprehensive study on a 350-cm long sediment core revealed rich geochemical information in an interesting location of Xiangxi He (river) in TGR. The vertical profiles of trace metals (TMs) and nutrients displayed an irregular pattern, fluctuating around their background values, and no gradual increase was detectable. For TMs, average concentrations were Cd: 0.35 +/- 0.21, Co: 21.91 +/- 3.30, Cr: 27.06 +/- 5.03, Cu: 27.09 +/- 4.96, Ni: 34.08 +/- 5.23, Pb: 20.58 +/- 5.84 and Zn: 80.18 +/- 69.16, all in mu g/g (n = 61). For most of TMs, their bioavailable forms were only a minor fraction. Therefore, the conventional total metal concentrations will inevitably overestimate their potential risk to ecosystem. For nutrients, average concentrations were total phosphorus (TP): 0.81 +/- 0.14, total nitrogen (TN): 1.46 +/- 0.28, total sulfur (TS): 0.54 +/- 0.19, total carbon (TC): 32.02 +/- 8.35, total organic carbon (TOC): 20.90 +/- 7.5, all in g center dot kg(-1) (n = 61), and appeared somewhat higher than the "lowest effect level", which seems to explain the repeated observations of algal blooms in this relatively stagnant section of the river in TGR. The mineralogical composition identified by XRD agreed well with that obtained by chemical analyses. The study revealed that the Xiangxi sediment core (XX06) was remarkably heterogeneous in mineralogical composition, probably caused by natural processes and the dam management. The increased sedimentation rate within TGR should induce a mass dilution effect, thus decreasing the concentrations of pollutants. Large sampling scale and a universally accepted and uniformed analytical sampling and method, together with more vigorous analytical quality control, must be practiced and a solid database be built to facilitate the ecosystem assessment of TGR area.