E5 Underground Flow and Aquifers

Topic

Groundwater occupies the pores and fractures of rocks and sediments saturated below the water table—the same surface previously described in the context of the unsaturated zone lying directly above it.

Unconfined aquifers have their upper boundary—the water table itself—in direct contact with atmospheric pressure; this direct contact allows them to respond rapidly to any recharge events they receive.

Confined aquifers, in contrast, are bounded at the top by an impermeable layer; the pressure this layer exerts on the confined water can cause the aquifer's piezometric level to rise above the aquifer's upper boundary, resulting in flowing wells that gush spontaneously upon drilling.

Water flow within any of these aquifers is described by Darcy's Law—previously mentioned—generalized here to three spatial dimensions and combined with the continuity equation to yield a hydraulic potential diffusion equation that governs the entire aquifer.

The Theis equation describes the drawdown of the piezometric level caused by pumping a single well in a confined aquifer; its solution—known as the Theis well function—allows for the calculation of both the aquifer's transmissivity and storage coefficient from data obtained in a single pumping test (the same two properties previously described regarding soil and subsoil characterization).

The interaction between a river and the underlying aquifer determines whether the river is a "gaining" stream (receiving water from the aquifer) or a "losing" stream (feeding water into the aquifer); the difference between the local water table and the river's water level determines which of these two situations occurs along any given stretch. The residence time of water within an aquifer can vary enormously—ranging from just a few days in the most active shallow aquifers to several million years in the fossil aquifers of certain deserts, such as the Ogallala or Nubian aquifers. Aquifer recharge is essentially the net flow of water moving from the unsaturated zone into the saturated zone, a process typically concentrated during rainy periods or in areas with vegetation cover that promotes infiltration.

The expected behavior of this entire system involves a very slow response of water table levels to recharge events, alongside a marked dampening of baseflow fluctuations in the rivers fed by these aquifers. Thus, from the saturated pores mentioned at the beginning of this document to the baseflow attenuation noted at the end—and covering unconfined and confined aquifers, the Theis equation, and river-aquifer interaction—groundwater’s slow movement underground ultimately buffers much of the variability associated with rainfall events at the surface.

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