三峡库区消落带土壤胶体颗粒的释放及其辅助镉迁移机理的研究

The water-level fluctuation zone of Three Gorges Reservoir (TGR) is potentially affected by rain events during summer after its long flooding due to the anti-seasonal operation mode of TGR. Soil colloids generated during rainfall infiltrations have been demonstrated to be the transport carriers for various contaminants. However, colloid release mechanisms and its further impact on heavy metal transport is still unclear. In this study, the flooding time intervals on soil physicochemical and hydraulic properties in the water-level fluctuation zone of TGR is studied for the potential evaluation on in-situ release of soil dispersible colloids. In-situ colloid release mechanisms during simulated rainfall events are studied through the undisturbed soil column experiments. The impacts of rainfall characteristics, pre-event soil moisture conditions and flooding intervals on colloid release and transport are valued. Cadmium transport in the water-level fluctuation zone of TGR in both soluble and colloid-associated forms are studied using batch equilibrium and undisturbed column experiments. This study potentially provides the scientific evidences for evaluating the effects of non-point source pollutions induced by soil structure degradation in the water-level fluctuation zone of TGR for the post Three Gorges Era. It also promotes the ecological safety and water environmental protection in this area.

长江三峡水库反季节的运行方式，导致三峡消落带在经历长期淹水后的出露阶段受到雨季集中降雨的作用。降雨释放的土壤胶体是多种污染物迁移的载体，但胶体释放及其对重金属运移行为的影响还缺乏研究。本项目以三峡库区消落带不同淹水时长土壤为对象，对比研究消落带不同高程土壤理化性质和水力学性质的差异，评价其对土壤可分散性胶体库的影响。通过非扰动土柱模拟降雨实验，揭示淹水时长、降雨特性（水化学条件、降雨雨型）以及土壤前期干旱程度对土壤内源胶体释放及迁移过程的影响机制。通过批量平衡实验和原状土柱实验，揭示干旱与田间持水量两种前期土壤水分条件下镉在消落带土壤中的迁移形态（水溶态与胶体结合态）及其动态变化特征。本研究可为“后三峡”时代消落带土壤因结构性退化对库区面源污染机理及负荷的影响评价提供科学依据，对库区消落带生态安全和水环境保护有重要的现实意义。

三峡库区消落带不同高程差异性淹水-排干交替作用可能造成消落带土壤基本理化性质和水力学性质发生变化，从而对重金属分布、土壤胶体颗粒的释放与迁移以及胶体辅助重金属迁移的潜力产生影响。本研究选择三峡库区腹地两个农业消落带不同淹水高度（160 m和151 m）土壤并对照非消落带（177 m）土壤，综合使用土壤质量可视化评价、土壤水分特征曲线以及三维μ-CT成像技术研究消落带土壤结构和孔隙分布特征；探索镉（Cd）总量和形态的分布特征及其与土壤理化性质变化的关联；揭示不同前期含水量条件（饱和、田间持水、前期干旱）下土壤胶体颗粒的释放及其辅助Cd迁移的动态。主要研究结果如下：（1）177 m、160 m和151 m高程土壤的主要孔隙域分别为土壤结构性孔隙、土壤基质性孔隙和非排水性微孔，淹水导致消落带土壤中非排水性孔隙比例增加，土壤团聚体稳定性降低，土壤结构和水力学性质总体随淹水时长增加而恶化。（2）土壤有机质（36.2%）和孔隙度（87.3%）分别是影响Cd在消落带旱地土壤和水稻土中空间分布的主要因素。（3）水力剪切、气液界面裹携（非饱和）及水化学性质（pH、EC、Ca2+、Mg2+、DOC）变化是土壤胶体颗粒原位释放的主要机理，胶体释放与迁移通量与土壤前期排干时间正相关。（4）消落带土壤胶体对Cd具有显著的辅助运移能力，辅助运移效果与前期含水量正相关，与高前期含水量（低氧化还原电位）下铁锰氧化物还原溶解而释放其束缚的Cd2+有关。因此，“后三峡”时代消落带管理过程中应重点关注消落带下部土壤中胶体颗粒释放以及胶体结合态Cd的迁移动态，对于库区水环境和生态安全有重要意义。