From Legacy to Emerging Groundwater Contaminants: Combining Advanced Monitoring Tools to Assess Sources and In Situ (Bio)Transformation

Aquifers worldwide are under increasing pressure from the widespread use of synthetic chemicals, many of which, alone or as a mixture, are persistent, mobile, and toxic. Legacy and emerging organic contaminants pose major challenges for groundwater remediation, often involving substantial cleanup costs and requiring efficient monitoring strategies to trace their sources and transformation mechanisms. Groundwater bacteria have demonstrated remarkable capabilities to degrade diverse organic contaminants, particularly petroleum hydrocarbons and chlorinated solvents. However, the biotransformation of emerging contaminants, for example, pesticides and per- and polyfluoroalkyl substances (PFAS), remains only partially understood. This review explores the potential of integrating conventional hydrogeochemical monitoring with multielement compound-specific isotope analysis (ME-CSIA), biomolecular tools, and reactive transport modeling to evaluate sources and in situ contaminant transport and transformation. This integrative approach can provide decisive information on contaminant removal processes and enhance the selection of remedial actions and the evaluation of remediation efficiency. Furthermore, the synergy observed in multidisciplinary case studies has significantly advanced our ability to elucidate the sources and transformation of legacy contaminants in groundwater. This foundation is now essential for transferring knowledge to address the complex challenges posed by the pervasiveness of emerging contaminants in groundwater and develop suitable remediation and monitoring strategies.