What do parts per billion (ppb) and parts per trillion (ppt) concentrations in drinking water mean in simple terms?

Parts per billion (ppb) and parts per trillion (ppt) are the most commonly used terms to describe very small amounts or trace levels of contaminants in our drinking water.

One ppb is the equivalent of one drop of impurity in 500 barrels of water or 1 cent out of $10 million.

One ppt is the equivalent of one drop of impurity in 500,000 barrels of water -or- traveling 6 inches out of a 93 million-mile journey toward the sun.

Regulatory Framework Q&A’s can be found here RegulatoryFramework.pdf