The presence of rotavirus, enterovirus and astrovirus, as
well as the abundance of indicator bacteria in the water
source and in water used for irrigation was determined
from samples obtained during the cold-dry (November to
February) and warm-rainy (May to October) seasons in
2001 and 2002. These seasonal categories were defined
according to two meteorological parameters: temperature
and rainfall [21,22]. Samples from water used for irrigation
were obtained from ten sampling points, randomly
selected from a regular grid of 250 observation points covering
the Xochimilco canal network, which had been set
up for previous studies in the area [23]. For viral detection,
a 20 L volume was collected at each sampling point
for each season per year. Samples for bacteriological analyses
were collected at a depth of 40 cm in 1 L sterile polypropylene
flasks.