A numerical simulation study of the position optimization of a pilot-scale permeable reactive barrier: a case study of the hexavalent chromium contaminated site
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摘要: 可渗透反应墙（PRB）是一种高效的地下水污染原位修复技术。不同水文地质条件下，污染场地墙体位置布设合理性影响其修复效果，而利用地下水数值模拟可实现墙体位置优化。文章以某Cr6+污染地下水场地为例，基于Visual Modflow建立了研究区平面二维稳定流数值模型，并通过模型检验。根据墙体的设计尺寸（长20 m×宽2 m×深12 m）及填充材料的渗透系数（80 m/d），利用所建模型分别计算了4种布设方案（墙体尺寸大小和填充材料渗透系数相同，布设位置不同）下墙体的捕获区宽度、粒子滞留时间和通过墙体的Cr6+通量。结果表明：4种布设方案模拟的滞留时间和捕获区宽度取值差异性不大，变异系数小于2%；Cr6+通量差别较大，变异系数高达76.32%，主要由地下水中Cr6+浓度空间分布不均引起。对比分析4种方案的各评价指标，方案2求得的捕获区宽度为21.9 m，粒子滞留时间为4.1 d，Cr6+去除量可达127.7 mg/d，可作为最佳布设方案。本研究建立的地下水流数值模型符合场地实际情况，可有效评估PRB截获污染羽的范围和去除目标污染物的能力，为铬渣类污染场地PRB原位修复工程设计与实施提供技术支撑和参考依据。Abstract: Permeable reactive barrier is an efficient in-situ remediation technology for groundwater pollution. The remediation effect is affected by the barrier position under different hydrogeological conditions of contaminated sites. However, the wall location can be optimized through groundwater numerical simulation. A 2D steady state numerical model for a hexavalent chromium contaminated site is established and identified by using Visual Modflow. Based on a designed barrier size (length of 20 m, width of 2 m and depth 12 m) and hydraulic conductivity (80 m/d) of the filled material, three indexes, the hydraulic capture zone width, residence time and hexavalent chromium flux of four schemes (the same barrier size and hydraulic conductivity of the reaction material, but different locations) are estimated via numerical simulation, respectively. The results show that there is no significant difference between the residence time and the hydraulic capture zone width of the four schemes, in which the coefficient of variation is less than 2%. However, the coefficient of variation of the hexavalent chromium fluxes is as high as 76.32%, which is mainly caused by the uneven spatial distribution of hexavalent chromium concentration in groundwater. By analyzing the indexes of four plans, scheme 2 is selected as the best design, in which the capture zone width is 21.9 m, the residence time is 4.1 days and the hexavalent chromium flux is 127.7 mg/d. The established model is suitable to the actual site situation, which can evaluate the width of PRB to intercept the pollution plume and the ability to effectively remove the target pollutant, and can also provide technical support and reference for the design and implementation of PRB in-situ remediation of chromium contaminated sites.
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