Groundwater hydrology
Also known as: hydrogeology
Groundwater hydrology is the study of how water occurs and moves beneath the earth's surface through aquifers — porous layers of soil and rock. It covers the water table, permeability, Darcy's law, and well hydraulics.
Groundwater hydrology studies the water stored in and flowing through the voids of soil and rock below the ground surface. Water infiltrates downward until it reaches the water table, below which all pores are saturated. A geologic unit that stores and transmits usable quantities of water is an aquifer; low-permeability layers that impede flow are aquitards.
Aquifers come in two forms. An unconfined aquifer's upper boundary is the water table itself, free to rise and fall with recharge. A confined aquifer is sandwiched between impermeable layers and held under pressure, so water in a well penetrating it rises above the aquifer top — sometimes flowing at the surface as an artesian well.
Flow through porous media is governed by Darcy's law: Q = KiA, where K is hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional flow area. Porosity measures how much void space a material has, while permeability measures how easily water moves through it — clay is porous but nearly impermeable. Pumping a well lowers the water level around it, producing drawdown and a cone of depression whose shape underlies well-yield equations for confined and unconfined aquifers.
The FE Civil exam tests groundwater hydrology within water resources engineering — expect Darcy's law calculations, hydraulic gradient and seepage velocity problems, aquifer classification, and well drawdown analysis.
Key takeaways
- Groundwater fills the saturated zone below the water table; aquifers store and transmit it.
- Unconfined aquifers are bounded by the water table; confined aquifers are pressurized between impermeable layers.
- Darcy's law, Q = KiA, governs flow through porous media.
- Porosity is void space; permeability is ease of flow — clay has high porosity but very low permeability.
- The FE Civil exam tests Darcy's law, aquifer behavior, and well drawdown in its water resources section.