Thallium pollution in sediments response to consecutive water seasons in Three Gorges Reservoir using geochemical baseline concentrations

Abstract

The Three Gorges Reservoir (TGR) is the world’s largest hydrological project. However, the response of thallium (Tl) in TGR sediments to reservoir operation remains highly uncertain. Here, we investigated the pollution characteristics of Tl in TGR sediments during five consecutive water seasons. Geochemical baseline concentrations (GBCs) were used to distinguish the anthropogenic input of Tl and to quantitatively estimate the Tl anthropogenic contribution in TGR sediments. The results showed that the average Tl concentration in TGR sediments was 0.72 ± 0.14 mg/kg (ranging from 0.26 to 1.07 mg/kg) during five consecutive water seasons. The average Tl content in tributary sediment was higher than that in mainstream sediment. In the mainstream, Tl in TGR sediments showed an accumulation tendency from the upstream to downstream. Additionally, the temporal variation revealed that the mean value of Tl in TGR sediments in the dry season was higher than that in the wet season during 2015–2016. Furthermore, the average GBCs of Tl (0.71 mg/kg) was higher than background values (BVs) in the Yangtze River sediments. Using the GBCs of different water seasons to calculate the anthropogenic input of Tl in the TGR, the average anthropogenic contribution was found to be approximately 6.43% (2.81%–9.12%). The assessment results by two different methods (geo-accumulation index and potential ecological risk index) used GBCs as the BVs, indicating that TGR sediments were at “uncontaminated” level and close to the “low ecological risk”, respectively. Comparing to the GBC of Tl, the assessment of Tl pollution in TGR using BVs in the Yangtze River sediments may be over-evaluated. This study is beneficial for assessing Tl pollution in TGR sediments response to consecutive water seasons and estimating the anthropogenic contribution of Tl.