What property does a pumping test primarily measure?

A pumping test primarily measures water level changes during aquifer pumping. This is a critical aspect because monitoring how the water level in a well changes over time provides insight into the hydraulic properties of the aquifer, such as its permeability and storage capacity. When water is extracted from an aquifer during the test, the resulting drop in water level reflects how quickly the surrounding water can move to replace the withdrawn water. This information is essential in groundwater studies for understanding the sustainable yield of an aquifer, evaluating the impact of pumping on water levels, and determining how different aquifer characteristics affect water flow.

The other choices, while related to water management or aquifers, do not directly measure the key component of a pumping test. The thickness of soil layers pertains to geological investigations rather than dynamic aquifer behavior. The flow rate of surface water is relevant to hydrology but does not assess subsurface conditions during a pumping test. Lastly, the chemical composition of groundwater is important for water quality analysis, yet it is not part of what a pumping test aims to measure; instead, it focuses on the physical response of the aquifer to pumping activities.