The Effects of Timing of Inundation on Soil Physical Quality in the Water-Level Fluctuation Zone of the Three Gorges Reservoir Region, China

Abstract

Core Ideas The soil pore network was analyzed by micro-CT images of intact soil core samples. The VESS method using the mass fractal dimension was applied to evaluate soil structural quality. Inundation decreases the number of soil pores and total porosity. Drainable micropores shift to non-drainable micropores after inundation in the reservoir zone. With the completion of the Three Gorges Dam, soils between the elevations of 145 and 175 m are exposed or submerged seasonally. Soil hydrological and physical properties are changed. A detailed investigation was performed to examine the influence of the timing of inundation on soil physical quality. Samples were collected in the water level fluctuation (WLF) zone and non-WLF zone in Zhongxian County, Chongqing. Soil samples were taken from depths of 0 to 10, 10 to 20, 20 to 30, and 30 to 40 cm. Soil structural quality was assessed by the visual evaluation of soil structure (VESS) method in situ; mean weight diameter (MWD) and mass fractal dimension (Dm) were measured to represent soil aggregate stability; and the soil water retention curve and computed microtomography (micro-CT) images were used to show soil pore networks. Our findings show a deterioration of soil structure indicated by a high structural quality (Sq) score in VESS (the higher the Sq score, the worse the soil structural quality) and a decline in soil aggregate stability indicated by low MWD and low Dm in the WLF zone. The data from both the soil water retention curve and micro-CT images demonstrate a deterioration of soil physical quality through decreased porosity and a shift of drainable micropores (0.1 < radii < 125 μm) to non-drainable micropores (radii < 0.1 μm) in the WLF zone of the Three Gorges Reservoir. The potential of environmental hazards of soil in WLF zone was also investigated.