Numerical modeling of clogging in a permeable reactive barrier and rejuvenation by alkaline fluid injection in the Shoalhaven Floodplain, Australia
A permeable reactive barrier (PRB) was utilized to remediate the acidic water generated from acid sulfate soil (ASS) in the Shoalhaven Floodplain approximately 100 km south of Sydney, Australia, where acidic groundwater is a severe environmental, social, and economic problem. After testing more than 20 alkaline materials, recycled concrete aggregates were chosen as a capable alkalinity source and utilized in this PRB. The current study aims to simulate the performance of the PRB by coupling geochemistry with geohydraulics. Secondary minerals precipitate during the remediation process, causing chemical clogging, which reduces the porosity and hydraulic conductivity of the reactive material. Laboratory column experiments were carried out to understand the remediation process taking place between acidic groundwater and recycled concrete particles. Commercially available finite different codes, MODFLOW and RT3D, were used to simulate the time-dependent modeling of the PRB. An original geochemical algorithm was developed capturing all the chemical reactions and fed into RT3D. The column experiments and field results are in good agreement with the model calculations, which proved that the reduction in hydraulic conductivity due to mineral precipitation occurs mostly at the entrance zone of the PRB and was insignificant in the middle and exit zones after 6 years of operation. A long-term column test was conducted to assess the reconditioning of armored recycled concrete with alkaline wastewater, with the aim to restore and enhance the acid neutralization capacity (ANC) and longevity of the PRB. Benefits of the alkaline wastewater injection included sharp but short enhancement of the concretes' ANC, as indicated by an increase in effluent pH (pH 3-7.7) and alkalinity (0-21.6 mM CaCO3), and a reduction in oxidation reduction potential (530-160 mV). Although results showed that the alkaline wastewater did not significantly reduce chemical armoring, it assisted in the release of mineral precipitates between the concrete aggregates, reducing the extent of chemical and physical clogging.