From geochemical pathways to microplastic vectors: a systematic review of groundwater degradation and adaptive remediation

Abstract Groundwater pollution poses significant global health and socio-economic burdens, evidenced by arsenic crises in Bangladesh and €1.0 billion annual nitrate-related costs in the EU. This PRISMA-guided review evaluates geochemical processes, anthropogenic stressors, and remediation strategies. Natural processes mobilize toxic metal(loids) through redox and water-rock interactions, fundamentally dictated by the host aquifer lithology. In Bangladesh, reductive dissolution elevated arsenic to 19-fold the WHO limit, while in the US, nitrate-induced oxidation elevated uranium levels above the EPA safety limit (30 μg/L). However, anthropogenic influences like agricultural runoff and over-pumping severely outweigh natural factors and climate change. While sea-level rise induces gradual seawater intrusion (4.5–10 km over decades), over-pumping rapidly pushes salinity up to 50 km inland, impacting 150–1200 km 2 of coastal aquifers. Concurrently, over-pumping has increased the salinity of Egypt's Quaternary aquifer by 25%. Furthermore, microplastics (MPs) act as emerging toxic vectors, mobilizing pollutants through surface adsorption and biofilm formation. To mitigate these effects, localized remediation shows high efficiency. For example, modified biochar and bioremediation achieve superior removal of Cd (29.9 mg/g) and 90 Sr (up to 97%) from groundwater. Similarly, advanced filtration, air flotation, and electrocoagulation remove 97–100% of MPs. Hybrid hydraulic systems also reduce seawater intrusion rates from 80 to 35 m/year. However, field-scale transition remains constrained by a realism gap. To bridge this, we introduce an integrated geochemistry-remediation framework, an aquifer-type comparative matrix, and a strategic decision-making framework, calling for adaptive management and field validation to ensure sustainable groundwater protection.