Are the terms mg/L and ppm interchangeable in water quality measurements?

The terms mg/L (milligrams per liter) and ppm (parts per million) are often considered interchangeable in the context of water quality measurements because they typically represent the same concentration in dilute aqueous solutions. Both units essentially describe the same concept of concentration, where mg/L indicates the mass of a substance in milligrams per one liter of water, while ppm indicates the mass of a substance relative to one million parts of solution.

In the case of water, where the density is close to 1 g/mL (or 1000 g/L), 1 mg/L is equivalent to 1 ppm. This equivalence holds true for a range of concentrations that are sufficiently low, allowing for the comparison between the two units without significant error.

While there are conditions under which mg/L and ppm may diverge (for instance, in more concentrated solutions or if the density of the solvent deviates significantly from that of water), in most routine water quality measurements, especially at the levels typically encountered in environmental assessments, they can be used interchangeably.

Understanding this equivalency is crucial for water operators who analyze water quality data to communicate effectively about pollutant concentrations and ensure compliance with environmental regulations.