Natural Hazards: The source of the spinach E.coli contamination might be the cows next door:

The strain of pathogenic E. coli O157:H7 was found in three cattle fecal samples collected at one of four ranches under investigation, the officials said. The ranch is within a mile of produce fields.

....But Reilly said investigators don't have a "smoking cow" and cannot say for certain whether the cattle tested were indeed the source of the contamination.

Cows do produce a prodigious amount of manure. And with rain and run-off, it isn't hard to imagine the E. coli strain getting into the irrigation water.