"Parts per billion" means the number of units of the chemical, in its pure form, found in every billion units of water.
"Parts per trillion" means the number of units of the chemical, in its pure form, found in every trillion units of water.
From a general perspective, the City of Woodbury pumped approximately 2.6 billion gallons of water in 2020. That means a PFAS detected at a level of 0.025 ppb (parts per billion) would equal about an 8-ounce cup (or 0.06 gallons) of the substance in 2.6 billion (2,600,000,000) gallons of water.