Numerical simulation of three-dimensional soil-groundwater coupled chromium contamination based on FEFLOW
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摘要:
`土壤-地下水耦合数值模拟是定量刻画水流和溶质运移的主要手段。现有大范围场地尺度的研究受到数据采集难度及模拟计算量的限制，多是将土壤和地下水分成两个系统，这种方式不利于模型之间的计算反馈，易出现计算误差，因此将土壤和地下水作为整体系统研究具有重要意义。为精确刻画实际场地土壤-地下水系统中污染物迁移规律，揭示变饱和反应溶质迁移模型的参数敏感性，以某铬污染场地为研究对象，基于现场试验及前人研究所获数据，采用Galerkin有限元法建立三维土壤-地下水模型，定量描述六价铬在土壤-地下水中的迁移规律。在此基础上，通过改变补给条件，研究潜水面在土壤-地下水系统中的波动。并讨论阻滞系数和反应常数对溶质运移的影响。结果表明：在土壤中，污染物最大水平迁移距离为场地东南侧300 m；地下水中污染晕最大分布面积约为1.632 km2；垂向上土壤中的六价铬仅需15.6 h即可下渗至潜水面，第6天贯穿含水层。当潜水面随着补给量变化而波动时，地下水中六价铬会随水流进入土壤，影响土壤中污染分布。对溶质运移参数的讨论显示，当反应常数由0增大至10−6 s-1时，迁移出场区边界时地下水中污染物浓度约减少2000 mg/L，较难迁移至涟水河。基于FEFLOW的数值模型，能够解决各系统之间交互性差的问题，提供较为精确的模拟结果。
Abstract:Soil-groundwater coupled numerical simulation is the main method to quantitatively describe the flow and solute transport in a groundwater system. The existing researches on a large-scale site are limited by the difficulty of data acquisition and the amount of simulation calculation. Most of them divide the soil and groundwater into two systems, and it is of great significance to study the soil and groundwater as a whole system. In order to accurately depict the migration of contaminants in the soil-groundwater system of the actual site and reveal the parameter sensitivity of the variable saturation reaction solute transport model, in this paper, a 3D soil-groundwater model is established by using the Galerkin finite element method to quantitatively describe the migration of hexavalent chromium in soil-groundwater based on the data obtained from field tests and previous studies. The fluctuation of phreatic surface in the soil-groundwater system is studied by changing the recharge conditions. The effects of retardation coefficient and reaction constant on solute transport are discussed. The results show that in the soil, the maximum horizontal migration distance of contaminants is 300 m to the southeast of the site; the maximum distribution area of contamination halo in groundwater is about 1.632 km2; the vertical hexavalent chromium in soil only needs 15.6 h to infiltrate into the phreatic surface, and penetrates through the aquifer in the sixth day. When the groundwater level fluctuates with the change of recharge, hexavalent chromium in groundwater will enter the soil with the water flow, affecting the distribution of contamination in the soil. The discussion of solute transport parameters shows that when the reaction constant increases from 0 to 10-6s-1, the concentration of the contaminants in groundwater decreases by about 2000 mg/L at the boundary of the migration site, which makes it difficult to migrate to the Lianshui River. The numerical model based on FEFLOW can solve the problem of poor interaction between systems and provide more accurate simulation results.
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Key words:
- soil /
- groundwater /
- coupling simulation /
- contaminants transport /
- FEFLOW
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表 1 六价铬检测结果
Table 1. Statistics of soil test results
层位 标准限值/(mg·L−1) 分析总数/个 超标
个数/个超标率/% 最大值/(mg·L−1) 最小值/(mg·L−1) 平均值/(mg·L−1) 相对偏差/% 上层土壤 30 203 29 14.3 3410 <1.0 283 733 下层土壤 30 94 17 18.1 3430 <1.0 414 876 地下水 0.1 45 23 51.1 109 <0.01 21.2 30.6 
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表 2 研究区水文地质参数取值表
Table 2. Values of hydrogeological parameters in the study area
参数 非饱和区 饱和区 第一亚层 第二亚层 第二层 第三层 第四层 第五层 Kxx/（m·d−1） 0.0864 0.0864 0.1 1 33 4 Kyy/（m·d−1） 0.0864 0.0864 0.1 1 33 4 Kzz/（m·d−1） 0.0864 0.0864 0.01 0.1 3.3 0.4 孔隙度 0.5 0.1 0.05 0.1 0.3 0.1 最大饱和度 1 1 1 / / / 剩余饱和度 0.12 0.12 0.12 / / / 
/m−11.2 1.2 1.2 / / / n 3 3 3 / / /
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表 3 溶质运移模型参数取值表
Table 3. Parameter values of the solute transport model
参数 非饱和区 饱和区 第一亚层 第二亚层 第二层 第三层 第四层 第五层 弥散系数/
（10−9m2·s−1）200 200 200 2300 2300 2300 纵向弥散度/m 1 1 1 100 100 100 横向弥散度/m 0.2 0.2 0.2 20 20 20 阻滞系数 0.1 0.1 0.1 0.01 0.01 0.01 反应系数/
（10−4·s−1）0.0016 0.0016 0.0016 0.0016 0.0016 0.0016
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表 4 污染晕面积表
Table 4. Contaminant halo area
时间/d 面积/km2 时间/d 面积/km2 100 0.831 1100 1.416 200 1.030 1200 1.350 300 1.246 1300 1.255 400 1.321 1400 1.055 500 1.413 1500 0.876 600 1.478 1600 0.601 700 1.569 1700 0.376 800 1.596 1800 0.241 917 1.632 1900 0.0018 1000 1.465 2000 0.0014
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