What aspect of water usage might a hydrologist study?

A hydrologist primarily focuses on the distribution, movement, and properties of water in the Earth's atmosphere and on its surface, including how water interacts with geological formations. This encompasses studying water's flow through various geological layers, its interactions with soil and rock, and understanding groundwater systems.

It's important to note that while water recycling and its effects on air quality are significant areas of study within environmental science and ecology, they fall outside the core focus of hydrology. Hydrologists specifically specialize in understanding hydrological cycles, watershed management, and the physical characteristics of water sources. Thus, the study of how water moves through geological formations is a central aspect that aligns directly with the hydrologist's expertise.