The level of contamination was highest immediately around the mannequin, but evidence was also found up to 3.5m away, the study, published in the Building and Environment journal, found.
"The level of contamination suggests that Comigel was not following the appropriate production process for our Tesco product, and we will not take food from their facility again, " said Tim Smith, a Tesco executive.
They test nearby water wells to determine the level of naturally-occurring methane and other contamination before and after drilling in order to ensure that their drilling and completion activities have not caused any issues.