ABSTRACT
Figure 1 is an idealized example of groundwater occupation of two porous medium aquifer units sandwiched between three aquicludes in an escarpment setting typical of moderately deformed sedimentary rocks. Geological exposure dictates the extent of the recharge area across the crest of the escarpment. Springs have developed at the lowest topographic points of each aquifer. The upper aquifer has filled beneath the overlying or confining aquiclude. An unconfined aquifer has a water table for its upper boundary; a confined aquifer has an aquiclude as its upper boundary. As a result, the water in a confined aquifer is pressurized and the hydraulic head is greater than the elevation of the upper boundary of the aquifer. It is convenient to consider a virtual water table termed a “potentiometric surface”, the level to which water would rise if a well were drilled into the aquifer. It is quite possible for the potentiometric surface to be above the ground surface, in which case, artesian springs and flowing wells may occur. If recharge is highly seasonal, then the water table and potentiometric surface may also oscillate. The upper spring on the gentle dip slope may therefore be seasonal, only flowing when the water table reaches that elevation. The lower aquifer in Figure 1 is isolated from surface recharge and protected from surface-derived contamination. Recharge may occur by slow seepage from the upper aquifer, but the water would be expected to have a long residence time. Flow from the lower perched spring might be more steady, and the water more mineralized than for the upper perched spring.
