– Non-Detects (ND) – laboratory analysis indicates that the constituent is not found.

– Parts per million (ppm) or Milligrams per liter (mg/l) – one part per million corresponds to one minute in two years, or a single penny in $10,000.

– Parts per billion (ppb) or Micrograms per liter – one part per billion corresponds to one minute in 2,000 years, or a single penny in $10,000,000.

– Parts per trillion (ppt) or Nanograms per liter (nanograms/l) – one part per trillion corresponds to one minute in 2,000,000 years, or a single penny in $10,000,000,000.

– Parts per  quadrillion (ppq) or Picograms per liter (picograms/l) – one part per quadrillion corresponds to one  minute in 2,000,000,000 years or one penny in $10,000,000,000,000.