What measurement unit is often used to express the concentration of pollutants in water?

The measurement unit commonly used to express the concentration of pollutants in water is parts per million (ppm). This unit provides a way to quantify the amount of a pollutant present in a given volume of water, allowing for consistent comparisons and regulatory compliance.

Using parts per million is especially important in environmental science and water treatment because pollutants are often present in very low concentrations. For example, if a water sample has a concentration of 5 ppm of a specific pollutant, it means that there are 5 parts of pollutant in one million parts of water. This makes ppm a practical and effective way to assess water quality and ensure that it meets safety standards for human consumption and environmental impact.

Milliliters would refer to a volume measurement, which is not applicable when discussing concentration directly. Gallons per hour is a flow rate measurement, indicating how much water moves in a certain timeframe, and is not related to concentration. Degrees Celsius is a temperature measurement, which has no direct relation to the concentration of pollutants in water.

Thus, parts per million is the appropriate choice, as it accurately reflects the concentrations of substances in water, which is crucial for both health assessments and compliance with environmental regulations.