The Novel Application of Electrical Resistivity Tomography for Spatiotemporal Monitoring of Urban Stormwater Bioretention Cells

ABSTRACT: Bioretention cells are an increasingly popular low impact development (LID) stormwater management system for attenuating and delaying stormwater and improving its water quality. Bioretention cells comprise an engineered soil mixture that removes pollutants using physical and chemical mechanisms that would otherwise enter municipal storm sewer systems. However, bioretention cells do not always work as intended with some cells found to store, and later release, high levels of pollutants such as phosphorus. Effective characterization and monitoring strategies are necessary to understand pollutant behaviour and mobility within bioretention cells. Previous studies have relied on discrete moisture sensors and water quality samples that were sparsely located within the cells, thereby providing limited spatial and temporal information. Electrical resistivity tomography (ERT) is a geophysical technique that provides non-invasive, continuous imaging of the variation in electrical properties within the subsurface. Despite its strong potential for mapping changes related to moisture content and water chemistry, ERT has never been applied to bioretention cells. The objective of this study is to evaluate the effectiveness of ERT for characterizing and monitoring both moisture and pollutant dynamics in bioretention cells. Two in-service bioretention cells, located in London, Ontario, were used for this study. Six ERT lines, each containing 48 electrodes with an inline and interline spacing of 0.5 m, were permanently installed along the surface of the cells. Soil moisture sensors and pore water samplers (for measuring electrical conductivity, chloride and phosphorous) were also installed to validate the ERT. During storm events, ERT surveys were performed repeatedly to monitor the evolving moisture and pollutant behaviour within the cells. Results demonstrated a strong correlation between ERT, soil moisture and pore water quality. ERT was effective for monitoring the 3D spatiotemporal distribution of moisture and pollutants during storm events, thereby providing a better understanding of bioretention cell performance for stormwater management.