What is the unit commonly used to express herbicide concentration in water?

Parts per million (ppm) is the unit commonly used to express herbicide concentration in water because it provides a clear understanding of the amount of herbicide present relative to the total volume of water. This unit allows for consistent measurement and comparison of concentrations across different applications and studies. When dealing with aquatic pest management, precise herbicide concentration is critical to ensure effective treatment while minimizing potential environmental impact.

The other options are measurements relevant to different contexts; gallons per acre typically refer to application rates for spray products, acre-foot is a measure of volume typically used in water resource management, and liters per hectare is commonly used to express application rates or dosages rather than concentration.