What unit is commonly used to express chemical concentration in aquatic environments?

The correct choice is PPMV, which stands for parts per million by volume. This unit is commonly used to express chemical concentration in aquatic environments because it provides a clear way to communicate extremely low concentrations of substances in water, which is essential for assessing water quality and the potential impact of pollutants on aquatic life.

In aquatic systems, the concentrations of many chemicals, such as nutrients, pollutants, or toxins, are often very low, making PPMV an ideal measurement. It allows for precise quantification in a way that is easily understood and used in scientific studies, regulatory assessments, and environmental monitoring.

While other units like gallons, liters, or pints could be used to describe the volume of water or the amount of a substance, they are not as effective for denoting the very small concentrations typical in aquatic settings. Therefore, PPMV is the standard unit in this context.