Sedimentary geochemistry mediated by a specific hydrological regime in the water level fluctuation zone of the Three Gorges Reservoir, China

The water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) acts as an important sink for inflowing suspended sediment loads over the inundation periods following regular dam operations. This study depicts the sedimentary geochemical dynamics along a sedimentary profile based on the determined chronology and explores its links to the specific hydrological regime created by dam flow regulation and riverine seasonal suspended sediment dynamics. A compact 345-cm-long sediment core was extracted near the base water level (145.3 m) from the WLFZ of the TGR and sectioned at 5-cm intervals. Extracted sediment subsamples were analyzed for grain size composition, organic matter (OM), total nitrogen (TN), and geochemical elements (Na, K, Ca, Mg, Pb, Zn, Ni, Co, Mn, Cr, Fe, and Cu). The sediment core chronology was determined using ¹³⁷Cs elemental analysis. Sedimentary geochemistry and grain size properties of extracted sediment core exhibited greater variations during initial submergence years till the first complete impoundment of the TGR (2006-2010). Afterward (2011-2013), although upstream inflowing suspended sediments and reservoir water level were comparable, sediment deposition and concentrations of sedimentary geochemical constituents showed considerably fewer variations. Seasonal variations in sediment deposition and geochemical composition were also observed during the rainy (October-April) and dry (May-September) seasons, in addition to annual variations. Grain size, OM, and other sediment geochemical constituents all had significant correlations with each other and with sediment core depth. The concentrations of geochemical elements in various sediment stratigraphic layers exhibited staggering associations with each other and were dependent on each other in several ways. The arrangement of geochemical elements in various stratigraphic layers of the extracted core illustrated amalgamation with inputs from upstream seasonal suspended sediment dynamics and reservoir water levels. During shortened submergence periods and higher input sediment loads, geochemical elements demonstrated impulsive distributions. Alternatively, during longer submergence periods, elemental distributions were relatively uniform attributed to higher settling time to deposit according to grain size and geochemical affinities. Higher suspended sediment loads in association with seasonal floods also resulted in rough sediment deposition patterns, imparting variations in the distributions of geochemical elements. Interim mediations in geochemical element concentrations are associated with seasonal distal flash floods and local terrace bank collapses, which generate significant amounts of distal sediment loads that are quickly deposited and are not sorted hydrodynamically. Overall, although a specific mechanism was devised to circumvent the siltation process, a considerable amount of sediment is trapped at pre-dam sites. In addition, siltation caused nutrients and geochemical elements' enrichment.

Soil aggregate disintegration effects on soil erodibility in the water level fluctuation zone of the Three Gorges Reservoir, China

Spatial hydrological alterations can affect soil structural stability. Over time, forces induced by water weaken soil aggregates and this has a negative implication to soil health. The Three Gorges Reservoir (TGR) in particular, experienced a long-term hydrological condition and repetitive seasonal water level fluctuations that could affect soil health. The present study was conducted to investigate the effects of different water levels on soil aggregate disintegration rate over time and its relation to soil erosion susceptibility in water reservoirs. Samples from different elevations (155 m, 160 m, 163 m, 166 m, 172 m, and 180 m) in the water level fluctuation zone (WLFZ) were exposed to continuous wet-shaking for 3, 9, 27, 54, and 81 min resulted to different WLF intensity accordingly. The results showed a comparative difference between aggregates size before and after the experiment where micro-aggregates (<0.25 mm) increased with respect to elevations increase. The exponential prediction proved that aggregate stability decreased with the increase of WLF intensity, insisting the effects of continuous hydrological stress to aggregate break-down. A couple of factors definitely confirmed that soil erodibility (k) is primarily determined by disintegration of soil aggregates for the surface soil of the TGR. Despite the fact that Disintegration rate (Dr) and k showed a positive relationship, R² = 0.73 (p < 0.05), the results showed that the soil properties decreasing Dr also decreases soil erodibility in the study area. Non-effective role of soil organic matter (SOM) for stabilizing soil aggregates was primarily related to water level fluctuations inhibiting decomposition. Relying on the present findings, environmental problems mostly soil erosion in the TGR could be therefore linked to excessive destabilization of soil aggregates. Therefore, the results of this study should play a major role in determining the factors primarily inducing soil erosion in river reservoirs.