Marin County public health leaders said this week they are puzzled by a COVID-19 data spike linked to a San Rafael sewage treatment plant. The University of California at Berkeley’s Nelson Lab has been monitoring 14 wastewater plants across the state since the summer of 2020, including nine in four Bay Area counties: Marin, San Francisco, Contra Costa and Alameda. The amount of virus found in the wastewater at most of these plants has diminished over recent weeks as the omicron surge subsides.