Hydrodynamic dispersion in an unsaturated dune sand
Hydrodynamic dispersion in an unsaturated dune sand
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DOI：
10.2136/sssaj2003.0703
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发表时间：
2003-05-01
影响因子：
2.9
通讯作者：
Leij, FJ
中科院分区：
文献类型：
--
作者：
Toride, N;Inoue, M;Leij, FJ
由于蜿蜒穿过（部分）饱和的孔隙复合体，溶质在土壤中水流过程中相对于平均位移位置扩散。扩散的特征是对流扩散方程 (CDE) 中的流体动力学扩散系数。该系数已针对饱和土壤进行了广泛研究。在这项研究中。非聚集沙丘沙的水动力分散系数被确定为体积水含量θ的函数，范围从饱和度到5厘米直径的柱中的0.08厘米(3)厘米(-3)。长度为 25 至 40 厘米。进行单位梯度流实验，使用四电极盐度探针在多个柱深度测量溶质突破曲线 (BTC)。 CDE 和移动-固定模型 (MINI) 的传输参数是通过优化观察到的 BTC 的分析解来确定的。在 theta = 0.13 时发现最大分散性 lambda 或 0.97 cm，而对于饱和流，无论孔隙水速度如何，其范围为 208 至 5878 cm d(-1)， lambda 近似为 0.1 cm。对于 MIM，流动水分数，theta(m)/theta，从饱和时几乎一致逐渐下降到 θ = 0.15 时的最小值 0.85，随后随着进一步去饱和而略有增加。当 θ > 0.15 时，流动相和固定相之间的交换时间 1/α 为 0.1 至 0.2 天，这可能是因为对流溶质混合的流动相对均匀。对于较低的θ，交换变得更慢，因为流动主要发生在包围沙粒的水膜中。由于 nu 较小且水膜较薄，分子扩散的佩克莱特数 (P-e) 在较低 θ 下随着横向扩散作用的增加而减小，而流动相和固定相之间的溶质交换阻力则增加。在非聚集沙丘沙的情况下，这些综合效应导致中等水含量下的最大分散性值。
Solutes spread relative to the mean displacement position during water flow in soils as a result of meandering through the (partially) saturated pore complex. Spreading is characterized by the hydrodynamic dispersion coefficient in the convection-dispersion equation (CDE). This coefficient has been extensively studied for saturated soils. In this study. hydrodynamic dispersion coefficients for nonaggregated dune sand were determined as a function of volumetric water contents, theta, ranging from saturation to 0.08 cm(3)cm(-3) in columns of 5-cm diam. and 25- to 40-cm length. Unit-gradient flow experiments were conducted to measure solute breakthrough curves (BTCs) using four-electrode salinity probes at several column depths. Transport parameters for the CDE and the mobile-immobile model (MINI) were determined by optimizing analytical solutions to observed BTCs. A maximum dispersivity, lambda, or 0.97 cm was found at theta = 0.13, whereas for saturated flow lambda approximate to 0.1 cm irrespective of pore-water velocity ranging from 208 to 5878 cm d(-1). For the MIM, the mobile water fraction, theta(m)/theta, gradually deceased from almost unity at saturation to a minimum of 0.85 at theta = 0.15 followed by a slight increase with further desaturation. The exchange time between the mobile and immobile phases, 1/alpha, was 0.1 to 0.2 d for theta > 0.15 presumably because of the relativelv homogeneous flow with convective solute mixing. For lower theta, the ex change became much slower since flow predominantly occurs in water films enveloping sand particles. The Peclet number for molecular diffusion, P-e, decreases as the role of transverse diffusion increases at lower theta because of smaller nu and thinner water films while the resistance increases for solute exchange between mobile and immobile phases. These combined effects lead to a maximum dispersivity value at intermediate water contents in the case of the nonaggregated dune sand.