Trends of global concerns on groundwater contamination and future directions

Groundwater is a vital freshwater resource but is increasingly threatened by both conventional pollutants and contaminants of emerging concern (CECs). This review synthesizes the evolution of groundwater contamination research using a bibliometric analysis of 38,759 publications from 1991 to 2024. Scientific production rose sharply after 2004, driven by China and India, while Africa and parts of South Asia remain underrepresented despite facing serious groundwater challenges. Thematic mapping shows a transition from conventional contaminants (e.g., MTBE, TCE) to CECs (e.g., PPCPs, PFAS, microplastics). Regulatory frameworks are well established for conventional contaminants, but most CECs remain unregulated due to analytical limitations, scarce toxicological data, and slow policy processes. Remediation still relies on pump & treat, in situ chemical oxidation, permeable reactive barriers, and bioremediation, although heterogeneity and rebound effects limit success. Advanced methods (e.g., high-performance adsorbents, advanced oxidation processes, membrane filtration) show promise but face challenges of by-product formation, cost, and limited validation. Treatment-train strategies, such as combining membrane filtration with advanced oxidation, offer pathways to overcome current limitations. Future progress will require shifting from rigid concentration-based standards toward performance-based metrics such as mass discharge reduction coupled with SMART goals, providing more realistic benchmarks for persistent contaminants. Sustainable progress will also depend on integrating renewable energy, while low-cost designs are essential to improve applicability in developing regions. Expanding data sharing and supporting underrepresented regions remain critical to strengthen global equity. Overall, this review synthesizes past and present research and highlights directions for advancing monitoring, regulation, and remediation under growing environmental pressures.