Final answer:
The rate of groundwater flow is primarily controlled by the rock's permeability, which is dependent on the interconnectivity and size of the pores within the rock.
Step-by-step explanation:
The primary control on the rate of groundwater flow in a rock, as modeled in images, is permeability. Permeability refers to a rock's ability to transmit fluids through its pores and fissures.
Even though a material like rock may be porous and have void spaces, if these spaces aren't interconnected to allow the movement of water, then it is not permeable. The rate of groundwater flow is greatly influenced by the size of these spaces, their connectivity, and the overall porosity of the geologic material.
Aquifers, which are geological formations able to store and yield significant amounts of water for use, tend to be permeable rocks such as sandstone or limestone, whereas impermeable rocks like shale, or un-fractured igneous and metamorphic rocks, act as barriers to water flow.