Assessing the heavy metal contamination of soils in the water-level fluctuation zone upstream and downstream of the Manwan Dam, Lancang River

Abstract

The intersections of the terrestrial and aquatic systems (water-level fluctuation zones, WLFZs) suffer from the increasing pressure of hydroelectric construction and operation. Taking WLFZs upstream and downstream of Manwan Dam (first dam completed in Lancang River) as an example, the dam operation impacts on the heavy metals in soils in WLFZs were assessed. In December 2011, 117 soil samples were collected in the WLFZs and infralittoral reference zones (IRZs) with respect to heavy metal (Cd, Cr, Cu, Ni, Pb and Zn) and metalloid (As) concentrations. Their concentrations were determined with the application of inductively coupled plasma atomic absorption spectrometry, ICP-AES. Enrichment factor (EF), ecological risk index (RI) and principal component analysis (PCA) were employed for element contamination and ecological risk assessment and source identification, respectively. The multivariable statistic approaches, an analysis of variance (ANOVA) and a paired-samples T test were performed in the data analysis. Parts of element concentrations in the upstream were significantly higher than those in the downstream and significant differences existed between the WLFZs and IRZs. No significant difference was detected between the different water elevations in the WLFZs. Compared to the soil quality guidelines, As and Cd exceeded the standard in both the WLFZs and IRZs. In addition, the water-level fluctuation during the submergence increased the enrichment degree and risk level in the WLFZs. The sources of the studied elements could be divided into natural factors and anthropogenic inputs. In addition, the submergence changed the major contaminants in the upstream WLFZs. The results verified that dam operation had an effect on the heavy metals in the soils in WLFZs. In addition, this study proposed a useful tool (modified enrichment factor) to assess the enrichment degree caused by the water-level fluctuation. Assessing the heavy metal contamination of soils in WLFZs could be an important first step to the rational management of the dam operation.