A numerical simulation study of the position optimization of a pilot-scale permeable reactive barrier: a case study of the hexavalent chromium contaminated site

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.