Water source samples were obtained from ten wells randomly
selected from the total of 60 wells that form part of
the Mexico City water supply system. Samples were takendirectly from the wells prior to chlorine disinfection. For
each season and year, 1200 L of water was filtered through
a 1 MDS electropositive filtering cartridge at each well
(CUNO, Meriden, CO). Within six hours of sampling, the
cartridges were transported cold (4°C) to the laboratory.
For bacterial analyses, 1 L samples were taken in sterile
polypropylene containers. At each sampling point, pH,
temperature and conductivity were measured using a portable
YSI 3500 pH-conductivity meter (Yellow Spring, OH)
and dissolved oxygen measured with an YSI 51B oxygen
meter (Yellow Spring, OH). In the laboratory, the 80
water samples (10 samples from irrigation water and 10
water source samples, both taken each season for two
years) were analyzed for the following enteric viruses:
enterovirus (EV); rotavirus (RV); and astrovirus (AST);
and for indicator organisms including total coliform (TC),
faecal coliform (FC), and enterococci (FE), as described
below.