How is the effect of artesian pressure best described?

The effect of artesian pressure is best described as excess pressure due to elevated water levels. This phenomenon occurs when water in a confined aquifer is under pressure because it is trapped between layers of impermeable rock or clay. When the water level is higher than the level of the aquifer, it creates pressure that can cause the water to rise spontaneously to the surface if a well is drilled into the aquifer. This is a result of the natural hydraulic gradient and the elevation of the water in the surrounding area, which leads to conditions where water can flow without the need for pumping.

In contrast, mechanical compaction refers to the squeezing and reduction of volume in sediment or rock, typically affecting porosity and permeability, rather than pressure dynamics in aquifers. Gas injection involves injecting gas into formations, which alters pressure but is distinct from the natural processes of artesian systems. Finally, thermal expansion is related to the increase in pressure or volume due to temperature changes, but does not specifically address the water movement and pressure buildup in artesian wells. Therefore, the choice highlighting excess pressure due to elevated water levels encapsulates the essence of artesian pressure accurately.