Sweetwater Authority, which supplies drinking water to roughly 200,000 customers in Chula Vista and surrounding communities, learned last month that its main reservoir contains levels of a toxic industrial chemical that could require expensive treatment or necessitate decommissioning the reservoir entirely.  The elevated chemical finding came as part of a year-long testing process required by the Environmental Protection Agency, or EPA. An earlier sample taken late last year found PFAS levels high enough to require public notification. Samples taken last month found even higher levels of one chemical, PFOA, that the EPA has designated a “hazardous substance” posing “a substantial danger to the public health.”