Trihalomethane Formation in Zanzibar Groundwater: Environmental Drivers, Hydrochemical Controls, and Disinfection Practices

Abstract

Trihalomethanes (THMs) are regulated disinfection by-products generated when chlorine reacts with natural organic matter and other reactive precursors in water. Their formation and speciation depend on hydrochemical conditions, halide availability, climate, and operational disinfection practices. This review examines the interacting factors that may control THM formation in Zanzibar groundwater, a tropical coastal aquifer system affected by seawater intrusion, sanitation-related pollution, agricultural activities, high temperatures, and seasonal variability. Peer-reviewed studies published from 2010 to 2025 were synthesized thematically, with emphasis on seawater intrusion, organic matter and nutrient pollution, climate variability, water hardness and inorganic chemistry, and local chlorination practices. The evidence indicates that bromide enrichment from saline mixing can shift THM speciation toward more toxic brominated compounds, while natural and algal organic matter provide reactive carbon precursors. Warm conditions accelerate halogenation reactions, and carbonate alkalinity and hardness stabilize the chemical environment in which chlorine reacts with organic matter. Inconsistent chlorine dosing, limited monitoring, and inadequate precursor removal may further increase disinfection by-product risks. The review identifies major local gaps, particularly the limited routine measurement of bromide, THMs, and organic-matter reactivity. It concludes that Zanzibar requires hydrochemically informed monitoring, improved source protection, precursor-removal measures, and adaptive disinfection strategies that account for spatial and seasonal water-quality variability while maintaining effective microbial control.