Non-Fickian phenomenon of solute transport in hierarchical groundwater flow systems of homogeneous sandbox aquifer
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摘要:
探讨均质砂箱含水层多级水流系统排泄点及内部溶质运移的非费克现象与关键影响因素,明晰该模式下非费克现象的表现规律,为复杂嵌套式盆地地下水污染防控与治理提供实验和理论依据。基于室内砂箱实验与 COMSOL Multiphysics 数值模拟,通过改变降雨入渗强度,构建单一区域、局部 + 区域二级、局部+中间+区域3级3种水流系统模式,开展溶质运移动态监测与穿透曲线分析。溶质运移穿透曲线分析显示:同一水流模式中,非费克现象显著程度为区域水流系统>中间水流系统>局部水流系统;不同水流模式中,非费克现象显著程度为单一区域水流系统>局部+区域二级水流系统>局部+中间+区域三级水流系统;砂箱河谷排泄点与含水层内部监测点均呈现明显的早到、拖尾非费克运移特征。该非费克现象受地下水流速、溶质运移路径和降雨入渗强度共同影响,浅部受流速影响更大,深部受运移路径影响更显著;降雨入渗强度与深部非费克现象呈明显负相关,对浅部则无明显相关性。研究成果丰富了均质含水层多级水流系统溶质运移理论,为实际盆地地下水污染评价与修复提供了科学参考。
Abstract:ObjectiveGroundwater solute transport is a core research content in hydrogeology, and the non-Fickian phenomenon widely existing in the transport process is the key to revealing the intrinsic mechanisms of solute migration. Traditional studies have mostly focused on the non-Fickian phenomenon caused by the heterogeneity of aquifer media, while the research on solute transport in homogeneous aquifers under the hierarchical groundwater flow system model remains relatively limited. This study aims to investigate the non-Fickian phenomenon of solute transport at the discharge points and inside the homogeneous sandbox aquifer with hierarchical groundwater flow systems, as well as its key influencing factors. It is also intended to clarify the manifestation patterns and dominant controlling factors of the non-Fickian phenomenon under the hierarchical flow system model in homogeneous aquifers, and to provide experimental and theoretical basis for the prevention, control, and treatment of groundwater pollution in complex nested basins.
MethodsBased on the combination of laboratory sandbox physical experiments and COMSOL Multiphysics numerical simulation, three different groundwater flow patterns were constructed in the aquifer by adjusting the rainfall infiltration intensity: single regional flow system, local + regional two-level flow system, and local + intermediate + regional three-level flow system. The dynamic monitoring of solute transport at river valley discharge points and internal monitoring points of the aquifer was carried out, and the non-Fickian characteristics of solute transport were systematically analyzed by using the breakthrough curve of solute transport as the core analysis index.
ResultsThe analysis of solute transport breakthrough curves showed that in the same flow pattern, the significance degree of the non-Fickian phenomenon in different hierarchical flow systems followed the order of regional flow system > intermediate flow system > local flow system. Among different groundwater flow patterns, the significance degree of the non-Fickian phenomenon was ranked as single regional flow system > local + regional two-level flow system > local + intermediate + regional three-level flow system. Distinct non-Fickian transport characteristics of early arrival and tailing were observed at both the river valley discharge points of the physical sandbox and the internal monitoring points of the numerical sandbox, directly reflecting the non-Fickian phenomenon in the hierarchical flow system of the homogeneous aquifer.
ConclusionsThe non-Fickian phenomenon of solute transport in the hierarchical groundwater flow systems of the sandbox aquifer is jointly affected by groundwater flow velocity, solute transport path, and rainfall infiltration intensity, with notable differences in the dominant influencing factors of aquifers at different depths. The non-Fickian phenomenon in the shallow aquifer is more significantly affected by groundwater flow velocity, while that in the deep aquifer is mainly controlled by the solute transport path, with a longer path leading to a more obvious non-Fickian tailing phenomenon. There is a significant negative correlation between rainfall infiltration intensity and the non-Fickian phenomenon in the deep aquifer, meaning the smaller the rainfall intensity, the more prominent the non-Fickian tailing phenomenon in the deep part, while rainfall infiltration intensity shows no obvious correlation with the non-Fickian phenomenon in the shallow aquifer. This study enriches the theoretical system of solute transport in hierarchical groundwater flow systems of homogeneous aquifers, and provides important scientific reference for the practical evaluation and remediation of groundwater pollution in nested basins.
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表 1 砂箱溶质运移实验方案
Table 1. Experimental cases for solute transport in sandbox
方 案 水流模式 降雨强度/(m·d−1) 上游降雨盐水
浓度/(mg·L−1)上游 中游 下游 Case A(R) 单一区域水流系统 1.102 1.046 1.048 1704 Case B(L+R) 局部+区域2级
水流系统3.274 3.656 3.362 1699 Case C(L+I+R) 局部+中间+区域3级
水流系统8.323 4.353 8.384 1688 -
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