Assessment of Mitigation Measures against Benzene Breakthrough into Subsurface Concrete Pipe

Work in this paper investigates the efficacy of mitigation measures to minimize the contaminant ingress into subsurface concrete pipes. The 3D study model of the concrete pipe and the soil domain are implemented in Visual MODFLOW version 6.1. The site parameters are developed from data for a site in Jacksonville, North Carolina, at which subsurface contamination occurred in the presence of subsurface concrete drainage pipe. Modeling results indicate that after 20 years, the installation of a clay barrier reduces the concentration of benzene within the pipe by 22% compared to no clay barrier case. In comparison, the use of flowable fill leads to a 99.9% reduction in the concentration of benzene breaking through the pipe, and the use of an antiseep collar leads to a 60% reduction in the concentration of benzene. The natural hydraulic gradient of the site affects the level of contamination breaking through the pipe with a smaller breakthrough mass with lower site hydraulic gradients. Additional research is needed to obtain data on the efficacy of the studied mitigation measures through field implementation and performance monitoring with time. Such field implementation and monitoring studies are needed on concrete pipes and mitigation measures installed in saturated and unsaturated subsurface soil profiles.