Cows are a common source of E.coli.
Cows poo near a stream, the downstream water is used for irrigation or field produce cleaning.
E.coli is a group of bacteria that is in sewage/waste/animal poo.
Except for one strain, E. coli strain O157:H7, the bacterium is not pathogenic. E. coli are a subgroup of Coliform group. They are the most numerous coliform species, and most are considered non-pathogenic (normally not able to cause illness). E.coli can be found in the intestinal tract of warm-blooded animals.
The presence of E. coli in foods is considered to be an indication of fecal contamination.
The kind of E. coli that usually kills humans comes from cow fecal matter. That means if your lettuce, spinach, etc., was grown near a cattle farm that has shared a stream or river water, there may be a problem.
That is not to say that there are no dangers from human fecal matter; it is advised that professional cleanup always be performed.
E. coli, as well as Coliforms, are common environmental bacteria and may be found in soil, on hands, on equipment surfaces, in water and other environments. Coliform tests, as a group, are used as an overall indication of sanitation efficiency.
Most Coliforms are not harmful (pathogenic), but if a Coliform test indicates their presence, it is considered an indication of unsanitary conditions.
There are no standards for Coliforms for most foods. Many product specifications are written with a zero or low tolerance for Coliforms. They can be tested for in water samples, swab samples, and air samples. Water samples, if the water comes from a chlorinated system, must be treated with sodium thiosulfate. Swab samples must use liquid culture swabs to preserve the sampled bacteria. Testing can be performed for the presence/absence of these bacteria in the samples with or without enumeration.