Soil colloid release and transport in the water level fluctuation zone of the Three Gorges Reservoir

Soil colloids are generally defined as fine particles with diameters ranging between the nanoscale (down to 10 nm) and microscale (2 μm). As colloids are characterized by large surface areas and active functional groups, they exhibit strong affinities to hydrophobic contaminants such as phosphors, heavy metals and pharmaceuticals. In addition, natural colloids in the vadose zone are negatively charged, which potentially decreases the possibilities of colloid straining and/or retention by soil matrix. As such, a great potential of environmental risk from natural colloids is posed to the shallow groundwater. In the Three Gorges Reservoir (TGR), the Water-Level Fluctuation Zone (WLFZ) actsas the final barrier before the entrance of terrestrial contaminants into the reservoir water. High intensity and periodic wet-dry cycles in the WLFZ potentially affect soil physicochemical properties, internal structure and erodibility, which further influences the release and transport of soil colloids. However, the systematic investigation is still lacking regarding the release dynamics and transport potentials of soil colloids from the WLFZ or the riparian soil subject to periodic wet-dry cycles. The investigation of colloid release and transport is also highly demanding for the evaluation of colloid-facilitated contaminant transport into the reservoir water. In this study, the release and transport dynamics of soil colloids were explored in the intact soil columns from the WLFZ at an altitude of 160 m and the non-WLFZ at an altitude of 177 m within the TGR. Column-scale leaching experiments were carried out in the saturated flows, where the conservative tracer (Br-) was used as an indicator of the degree of preferential flow in the columns. Correlation and regression analysis, as well as hierarchical partitioning were applied to identify the effects of critical factors of water chemistries on the release and transport dynamics of soil colloids. The results showed that colloid concentration of the leachate from the WLFZ generally showed a rapid decrease from 184.58 to 28.04 mg/L within 0-3 pore volumes of injection, followed by a slow increase from 21.18 to 97.58 mg/L within 3-6 pore volumes. A large temporal variation of colloid release from the WLFZ was observed with a variation coefficient of 0.46. The accumulated amount of colloid release from the WLFZ column was 714.43 mg within 6 pore volumes of leaching, which was 34.4% higher than the released colloid from the non-WLFZ column. The peak and median size of the leached particles from the WLFZ column were 13.25-19.90 μm and 14.98-22.90 μm, respectively, both of which were much smaller than those from the non-WLFZ column. These results indicated that the periodic alternations of impoundment and exposure could contribute to the release and transport potential of colloid and fine particles from the soil in the WLFZ. Dissolved Organic Carbon (DOC) was identified to be the critical influential factor for the release of soil colloid from the WLFZ, showing a high explanation rate of 42.3% to the dynamic of colloid concentration. In contrast, water chemistries including EC, Ca2+ and Mg2+, showed a stronger effect on colloid release and transport in the soil of the non-WLFZ. The reduction of DOC loss from the WLFZ soil can be prioritized to alleviate the released and transport potential of soil colloid from the WLFZ. A strong suggestion was proposed to explore the potential sources of DOC in the soil of the WLFZ, as well as the potential transport of various contaminants such as agricultural wastes facilitated by DOC-colloid associates into the reservoir water in the future.