Pollution amplifies global water scarcity and hinders progress towards Sustainable Development Goal 6

Water scarcity is commonly assessed as a mismatch between water demand and physical supply, yet this framing can substantially underestimate scarcity when water quality is poor. This critical narrative review synthesizes evidence on how nutrients, pathogens, heavy metals, persistent organic pollutants, salinity, microplastics, pharmaceuticals, and other emerging contaminants reduce the volume of water that is safe and economically usable. It connects contaminant sources and environmental transformation with treatment burden, ecosystem degradation, climate extremes, socioeconomic impacts, and progress toward Sustainable Development Goal 6 (Clean Water and Sanitation). Anthropogenic pollution impacts almost a third of the world's water supplies and nearly 80% of wastewater is discharged without treatment. Recent global modelling indicates that 55% of the population is exposed to clean-water scarcity for at least one month each year, compared with 47% when only water quantity is considered. Projected exposure may rise to 56–66% by the end of the century under uncertain climate and socioeconomic pathways. Evidence from multiple regions also shows that droughts and heatwaves commonly concentrate pollutants and lower oxygen availability, whereas floods can mobilize contaminants and overwhelm sanitation systems. The review therefore proposes a quality-adjusted scarcity framework, critically compares source control, advanced oxidation, membranes, adsorption, nature-based systems, water reuse, and digital monitoring, and identifies limits to current evidence, including inconsistent mixture-risk assessment, limited full-scale data for emerging technologies, and unequal monitoring capacity. The synthesis indicates that pollution prevention, fit-for-purpose treatment, circular water use, ecosystem restoration, and accountable governance must be pursued together. Increasing supply alone cannot secure water where quality constraints remain unresolved.