Abstract
Abstract. The Three Gorges Reservoir encompasses a riparian zone with a vertical height of 30 m and a total area of 349 km2 that has been subjected to alternate inundation and exposure due to regular impoundment. Sedimentation on the riparian landforms constitutes an important pathway for riverine contaminant redistribution. In an attempt to understand heavy metal enrichment since water inundation, riparian sediments and soils were sampled along five transects in a typical riparian zone composed of cultivated bench terraces in the middle reaches. Heavy metals (Cr, Ni, Cu, Zn, As, Cd and Pb) were determined to characterize the lateral distribution and vertical transfer ratio. The results indicated that all heavy metals were enriched to varying extents both in the riparian sediments and soils, compared with regional background contents in soils and the reference levels in sediments. However, heavy metal levels in the riparian sediments were generally higher than those in the riparian soils, while those in the upper riparian soils (0–5 cm) were overall slightly higher than those in the lower riparian soils (5–10 cm). There was a decreasing trend of heavy metal contents with increasing elevation. The elevated levels of heavy metals in the riparian sediments may be attributed to sediment yields from upstream anthropogenic sources, especially during major rainstorms in the wet season when large loads of contaminated sediment may be produced from diffuse source areas. Heavy metals can also be adsorbed to pure sediment in the course of mobilization or after deposition. Considering that the riparian soils are local weathering products without mobilization, the enrichment of heavy metals may principally be ascribed to chemical adsorption from dissolved fractions or vertical transfer from overlaid sediments. Heavy metal enrichment may further be affected by the specific type of hydrologic regime such that relatively long flooding duration caused by water impoundment and natural floods was responsible for the relatively higher levels of heavy metals in the lower portions of the riparian zone.
Highlights
The Three Gorges Dam intercepts the main course of the Yangtze River at the outlet of its upper basin and comprises a mountainous and hilly drainage basin of approximately 1 million km2 (Fig. 1)
All contents are significantly higher than the corresponding regional background levels of soil in the Three Gorges Reservoir Region (Tang et al, 2008) and the reference levels of sediment in the Yangtze River (Song et al, 2010), indicating the enrichment of heavy metals in the riparian sediment since the artificial flooding due to regular reservoir impoundment
The contribution of sediment production in the upstream catchments and local watershed may be predominant in sediment in the lower portions of the riparian zone due to substantial fluvial suspended sediment yields in the wet season when relatively low water levels were operated and significant sedimentation mainly occurred within this area
Summary
The Three Gorges Dam intercepts the main course of the Yangtze River at the outlet of its upper basin and comprises a mountainous and hilly drainage basin of approximately 1 million km (Fig. 1). The Three Gorges Reservoir was primarily impounded when the water level rose to 135 m in 2003. Full operation of the reservoir was implemented in 2009 when the water level rose to 172.4 m. Regular operation of the Three Gorges Reservoir with a strategy of “impounding clean water and draining turbid water” has caused inundation to the maximum level of 175 m during the dry season (October–April) for hydropower generation followed by a drop to 145 m during the wet season (May–September) for flood control (Zhang and Luo, 2011). Water level fluctuations between 145 and 175 m has formed a riparian zone with a vertical height of 30 m and a total land area of 349 km stretching 660 km from Yichang to Chongqing (Fig. 1). The riparian zone was transformed from the previous hillslopes with arable land, bench terraces, rural residential, urban area, wasteland, grassland and woodland to a unique aquatic-terrestrial landscape subjected to alternate flooding and exposure
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