What unit is equal to a milligram per liter in water quality measurements?

• A. Parts per million
• B. Percent
• C. Parts per billion
• D. Micrograms per liter

Answer: (A)

In water quality work, milligrams per liter is essentially the same as parts per million for dilute aqueous solutions. That’s because water’s density is about 1 kilogram per liter, so 1 milligram in 1 liter corresponds to 1 part per million by weight. So mg/L and ppm express the same concentration when measuring dissolved substances in water.

Other options don’t match this scale: micrograms per liter is a smaller unit that equals parts per billion (1 µg/L ≈ 1 ppb), not mg/L. Percent is a ratio to 100 and would reflect a much larger concentration, not the same as one mg/L in a liter of water.