Exposure assessment and disinfection management implications of trihalomethane and coliform levels in a Mexico City drinking water supply

A comprehensive study of trihalomethane (THM) and microbiological contamination was performed of one drinking water source for the Mexico City Metropolitan Area (MCMA). Surface water from the Cutzamala hydrologic basin to the west of the MCMA is collected, treated and transported 92 kilometers to the MCMA, where a portion is diverted to the southern region of the urban area and distributed to households.; Of 20 post-chlorination samples taken from the treatment plant to the delivery point, 35% exceeded the Mexican standard for total THMs (TTHMs) of 200 microgram per liter (μg/L). The raw water demonstrated the presence of total coliforms, fecal coliforms and/or fecal streptococci for all samples. The processed water at the treatment plant contained none of these organisms at any time. Current water treatment using chlorine is sufficient to destroy indicator microorganisms, but disinfection must be optimized to control THM formation as well.; TTHM concentrations measured in distribution tanks and in household tap water in the southern region of the MCMA ranged from 62 to 135 μg/L and from none detected to 142 μg/L, respectively. Microbiological analyses of the same samples found fecal coliforms confirmed in one tank and in tap water from one of the homes. While explanations exist for these outcomes, more effective localized pathogen control measures prior to water consumption are necessary while improving control of THM formation.; Household interviews were conducted to collect data useful for assessing population exposure. Exposure factors included tap water consumption for oral ingestion and showering time for inhalation and dermal contact exposures pathways. Inhalation contributed greatest to cancer risk, followed by dermal contact, with oral ingestion being the least important.; In considering water treatment alternatives for the Cutzamala system, various engineering options are available; however, sociopolitical factors may hinder implementing the optimal solution. Potential barriers include political, economic, technological, resource scarcity, and societal obstacles. Although this study focused on the MCMA, the THM/pathogen issues raised may be important for other megacities in less developed countries wrestling with similar challenges.