Bacterial and viral indicators of fecal pollution in Mexico City's southern aquifer

Mexico City with a population of about 18 million people relies on groundwa ter to supply about 70% of its water needs. In order to understand the exte nt of microbial pathogen contamination of these reserves, a 10 month long m onitoring study of the southern aquifer was undertaken. Groundwater samples were collected from five different locations and analyzed (100 mL) for tot al coliforms, fecal coliforms, and fecal streptococci. Larger volume sample s (5 L) were collected and concentrated for quantitative and qualitative (p resence/absence) determination of microorganisms including bacteriophages. Gene amplification (PCR) approaches were employed to screen for Escherichia coli/Shigella specific (uid) sequences. Laboratory microcosms were conduct ed to evaluate the potential survival of pathogenic viruses in the groundwa ter using MS-2 and PRD-1 as model viruses. Coliphage as a single indicator, or in conjunction with fecal coliforms and fecal streptococci were found t o have value as an indicator of fecal pollution in this geographical region . The results indicate that the southern aquifer underlying metropolitan Me xico City can pose a significant risk to public health when water is distri buted and used without adequate disinfection. The pumping wells located in the transition and mountain areas indicated the presence of extensive micro bial pathogen contamination. There was surprisingly, no difference between the dry and rainy seasons in terms of the presence of fecal pollution micro bial indicators.