The report addresses that question. The likely source was the cattle crossing the river at one of the numerous cattle crossings noted in the report and leached into the underground water & into the ungrouted irrigation wells.
Except they looked for pig tracks and didn’t find any.
After the fact. No pig tracks on a dried up patch of dirt where the spinach had been planted, grown, & harvested. Not a big surprise since there wasn't anything to eat in the field. But, both pig & cattle manure in the adjacent pasture did test positive for that specific sub-strain/ fingerprint of the 0157:H7 E coli bacteria identified in the outbreak. And those cattle had access to the river & crossed it at numerous points.
I notice you didn’t mention on other possible reason for the absentee workers — they were illegals and afraid the investigation might look too closely into the workers’ backgrounds.
You're referring to an event that was weeks before the investigators were on the scene. Of course the workers, legal, illegal, otherwise were absent--the field had been harvested.
“The report addresses that question.”
Show me where in the report they considered it may have been workers defacating in the fields.