Abstract Micro-scale properties of peat pore space and their influence on hydraulic and transport properties of peat soils have been given little attention so far. Characterizing the variation of these properties in a peat profile can increase our knowledge on the processes controlling contaminant transport through peatlands. As opposed to the common macro-scale (or bulk) representation of groundwater flow and transport processes, a pore network model (PNM) simulates flow and transport processes within individual pores. Here, a pore network modeling code capable of simulating advective and diffusive transport processes through a 3D unstructured pore network was developed; its predictive performance was evaluated by comparing its results to empirical values and to the results of computational fluid dynamics (CFD) simulations. This is the first time that peat pore networks have been extracted from X-ray micro-computed tomography (µCT) images of peat deposits and peat pore characteristics evaluated in a 3D approach. Water flow and solute transport were modeled in the unstructured pore networks mapped directly from µCT images. The modeling results were processed to determine the bulk properties of peat deposits. Results portray the commonly observed decrease in hydraulic conductivity with depth, which was attributed to the reduction of pore radius and increase in pore tortuosity. The increase in pore tortuosity with depth was associated with more decomposed peat soil and decreasing pore coordination number with depth, which extended the flow path of fluid particles. Results also revealed that hydraulic conductivity is isotropic locally, but becomes anisotropic after upscaling to core-scale; this suggests the anisotropy of peat hydraulic conductivity observed in core-scale and field-scale is due to the strong heterogeneity in the vertical dimension that is imposed by the layered structure of peat soils. Transport simulations revealed that for a given solute, the effective diffusion coefficient decreases with depth due to the corresponding increase of diffusional tortuosity. Longitudinal dispersivity of peat also was computed by analyzing advective-dominant transport simulations that showed peat dispersivity is similar to the empirical values reported in the same peat soil; it is not sensitive to soil depth and does not vary much along the soil profile.

中文翻译：

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使用孔隙网络建模和 X 射线显微计算机断层扫描评估泥炭土的水力和输运特性

摘要 泥炭孔隙空间的微尺度特性及其对泥炭土水力和输运特性的影响迄今鲜有关注。表征泥炭剖面中这些特性的变化可以增加我们对控制污染物通过泥炭地传输的过程的了解。与地下水流动和输送过程的常见宏观（或整体）表示相反，孔隙网络模型 (PNM) 模拟单个孔隙内的流动和输送过程。在这里，开发了一种能够通过 3D 非结构化孔隙网络模拟对流和扩散传输过程的孔隙网络建模代码；通过将其结果与经验值和计算流体动力学 (CFD) 模拟结果进行比较来评估其预测性能。这是首次从泥炭沉积物的 X 射线显微计算机断层扫描 (μCT) 图像中提取泥炭孔隙网络，并以 3D 方法评估泥炭孔隙特征。水流和溶质传输在直接从 μCT 图像映射的非结构化孔隙网络中建模。对建模结果进行处理以确定泥炭沉积物的整体特性。结果描绘了普遍观察到的水力传导率随深度的降低，这归因于孔隙半径的减小和孔隙弯曲度的增加。孔隙曲折度随深度的增加与泥炭土的分解程度增加以及孔隙配位数随深度的减少有关，从而延长了流体颗粒的流动路径。结果还表明，水力传导率在局部是各向同性的，但在放大到核心尺度后变得各向异性；这表明在核心尺度和田间尺度观察到的泥炭导水率的各向异性是由于泥炭土层状结构在垂直维度上存在强烈的异质性。传输模拟表明，对于给定的溶质，由于扩散曲折度的相应增加，有效扩散系数随着深度的增加而减小。泥炭的纵向弥散率也是通过分析对流主导运输模拟计算得出的，该模拟表明泥炭弥散率与相同泥炭土壤中报告的经验值相似；它对土壤深度不敏感，沿土壤剖面变化不大。这表明在核心尺度和田间尺度观察到的泥炭导水率的各向异性是由于泥炭土层状结构在垂直维度上存在强烈的异质性。传输模拟表明，对于给定的溶质，由于扩散曲折度的相应增加，有效扩散系数随着深度的增加而减小。泥炭的纵向弥散率也是通过分析对流主导运输模拟计算得出的，该模拟表明泥炭弥散率与相同泥炭土壤中报告的经验值相似；它对土壤深度不敏感，沿土壤剖面变化不大。这表明在核心尺度和田间尺度观察到的泥炭导水率的各向异性是由于泥炭土层状结构在垂直维度上存在强烈的异质性。传输模拟表明，对于给定的溶质，由于扩散曲折度的相应增加，有效扩散系数随着深度的增加而减小。泥炭的纵向弥散率也是通过分析对流主导运输模拟计算得出的，该模拟表明泥炭弥散率与相同泥炭土壤中报告的经验值相似；它对土壤深度不敏感，沿土壤剖面变化不大。由于扩散曲折度相应增加，有效扩散系数随着深度的增加而减小。泥炭的纵向弥散率也是通过分析对流主导运输模拟计算得出的，该模拟表明泥炭弥散率与相同泥炭土壤中报告的经验值相似；它对土壤深度不敏感，沿土壤剖面变化不大。由于扩散曲折度相应增加，有效扩散系数随着深度的增加而减小。泥炭的纵向弥散率也通过分析对流主导运输模拟计算得出，该模拟表明泥炭弥散率与相同泥炭土壤中报告的经验值相似；它对土壤深度不敏感，沿土壤剖面变化不大。