But he added that since PFOS was detected at the relatively high level of 90 parts per trillion in Mitchell plant water during the October quarter of 2014, continued testing there has consistently found PFOS below the 40 parts-per-trillion threshold at which water departments are supposed to report the finding to federal and state authorities.

Last year, the EPA established a “health advisory” level of 70 parts per trillion for PFOS in drinking water. The agency believes PFOs below that level won’t harm people over a lifetime, based on the rate of water consumed by women who are breastfeeding. Lactating women were chosen as the standard because they tend to drink more water and because they can pass along contaminants to nursing infants.

Three other rounds of Mitchell plant testing during 2014 detected no PFOS in the January quarter, 44 parts per trillion in the April quarter and 64 parts per trillion in the July quarter, Drew said.

By contrast, the last time Greensboro tested its drinking water for PFOS two months ago, samples from the Mitchell and Townsend plants registered at similar levels — 30 and 28 parts per trillion, respectively.

Treated water from the Mitchell plant was never unsafe to drink, Drew said.