Water Chemistry, Pollution and Treatment

Water is simultaneously the most essential resource for life and one of the most extensively perturbed by industrial civilisation. This chapter provides a comprehensive treatment of water chemistry, pollution, and treatment — from the molecular equilibria that govern natural water quality through the chemical and biological processes by which human activity degrades aquatic systems, to the engineering chemistry of the treatment technologies that attempt to restore water to drinking and ecological standards. The chapter opens with the hydrological cycle and global water distribution, then develops the chemistry of natural water systems: carbonate equilibria, hardness, redox chemistry, and the dissolved oxygen dynamics that underpin aquatic ecosystem health. It examines in detail the principal pollutant classes: heavy metals (lead, mercury, arsenic, cadmium, chromium) and their speciation-dependent chemistry; nitrogen and phosphorus loading and the full eutrophication cascade from nutrient enrichment through algal blooms and cyanotoxin production to hypoxia and coastal dead zones; persistent organic pollutants; per- and polyfluoroalkyl substances (PFAS) and their resistance to conventional treatment; pharmaceutical pollution and antimicrobial resistance selection; and microplastics. The treatment section surveys the full spectrum from conventional coagulation-flocculation and chlorination through advanced oxidation processes, membrane filtration, and biological nutrient removal, with specific attention to the challenge of removing recalcitrant micropollutants that conventional treatment cannot address. Three case studies — the Flint lead crisis, the Lake Taihu cyanobacteria crisis, and PFAS contamination at Australian military bases — illustrate the convergence of chemistry, governance, and environmental justice in real-world water quality failures.