Computational modelling to predict the longevity of a permeable reactive barrier in an acidic floodplain

This study introduces a novel computational approach that couples conventional geohydraulics with time-dependent changes of geochemical and biological parameters applied to a permeable reactive barrier (PRB) installed to treat acidified groundwater. The key objective of this PRB is to reduce excess acidity (low pH values), as well as to remove Al3+ and total Fe (i.e. Fe2+, Fe3+) from the acidic groundwater that flows through the PRB prior to being discharged as treated (neutralised) effluent to the environment. The governing equations characterising the geochemical reactions between the acidic influent and the alkaline PRB medium are incorporated into two finite-difference numerical codes, namely, MODFLOW and RT3D. In addition, biological clogging leading to reduced porosity of the PRB material over time is represented by explicit mathematical equations that are integrated with these geochemical numerical codes. The predictions from this coupled model made along the centreline of the PRB (i.e. one-dimensional flow) agree with the field data while demonstrating that the optimal treatment occurs predominantly in the proximity of the PRB inlet. The model also confirmed the potential benefit of using calcitic limestone (97% CaCO3) in the PRB, where a lifespan of about 16 years can be expected.