What does parts per billion (PPB) measure?

Parts per billion (PPB) is a unit of measurement that expresses the concentration of a substance in a solution or mixture. Specifically, it quantifies how many parts of a substance (in this case, an herbicide) are present in one billion parts of the total mixture. This is particularly useful in environmental science and water quality assessments, as it provides a means to measure trace amounts of contaminants, such as herbicides, in water or soil.

When measuring concentrations at such small scales, using PPB allows for a more precise understanding of the potential impact of these chemicals on aquatic ecosystems. It is crucial for regulatory and safety assessments, as many harmful substances can exist in very low concentrations while still posing risks to health and the environment.

The other options do not accurately represent the definition or context of PPB; for instance, the concept of concentration is not appropriately framed in terms of a million or a specific volume or total amount, which misrepresents the meaning and application of the measurement unit. Thus, the correct answer highlights the specific nature of PPB as it relates to parts in a billion, ensuring a clear and accurate understanding of substance concentration in various contexts.