Arsenic mobilization and recirculation dynamics. Groundwater circulation wells for enhanced decontamination in complex coastal aquifer environments

Arsenic (As) contamination poses serious risks to groundwater quality and human health, with millions globally exposed to unsafe levels, making remediation a critical challenge. Coastal aquifers are especially vulnerable due to heterogeneous lithology, saline intrusion, and complex hydrogeochemical dynamics, which limit the effectiveness of conventional pump-and-treat (P&T) systems and call for innovative remediation solutions. This study evaluates groundwater circulation wells (GCWs) as a technology to mobilize and remove As, focusing on the development of the GCW-induced recirculation cell. The research was conducted at a coastal industrial site with longstanding As pollution, representative of complex hydrogeology. A relational geodatabase integrated data from 76 boreholes, 41 piezometers, 2 multi-level sampling wells (MLSWs), and 1 GCW. A 3D stratigraphic model supported the design of a 23 m GCW with screened sections at 6÷8, 12÷15, and 19÷22 m. Groundwater was extracted, treated above ground, and reinjected to establish a recirculation cell. Hydrochemical and electrical conductivity (EC) monitoring over 550 days captured the spatial and temporal dynamics of As mobilization, removal, and salinity mixing. The GCW-induced recirculation cell mobilized As up to ~9,000 µg/L—around 900 times the World Health Organization (WHO) guideline of 10 µg/L—before stabilizing at ~6,500 µg/L. The system achieved 66% removal efficiency, extracting ~80 kg of As while promoting groundwater mixing. EC data from MLSWs provided unprecedented insights into the spatio-temporal evolution of the recirculation process. These findings demonstrate that GCWs provide a viable and sustainable alternative to conventional remediation strategies for addressing persistent As contamination in complex coastal aquifers.