Groundwater: Aquifers, Hydraulic Head, and Darcy's Law
The organizing idea
Groundwater questions center on hydraulic head and Darcy's law: flow moves from high head to low head, and discharge is proportional to hydraulic gradient and conductivity. Master head, gradient, conductivity, and storage, and most items resolve.
Concepts to master
- Aquifer types: confined versus unconfined; a confined aquifer is bounded by low-permeability layers and has a potentiometric surface; an unconfined aquifer has a water table that is also its upper boundary.
- Porosity, specific yield, storativity: porosity is total void space; specific yield is the drainable fraction; confined storage comes mainly from compressibility, unconfined storage mainly from drainage.
- Darcy's law: Q = K·A·(Δh/ΔL), where K is hydraulic conductivity and Δh/ΔL is the hydraulic gradient. Flow direction follows the head gradient, not the land surface.
- Well hydraulics: drawdown, cone of depression, pumping interference, and the distinction between steady and transient response.
- Contamination and recharge: travel times are slow, so contamination plumes move with groundwater flow; recharge and discharge areas define flow paths.
Traps
Confusing porosity with permeability, assuming water flows toward the nearest stream rather than down-gradient, and mixing confined and unconfined storage concepts.
Worked rule of thumb
Darcy check: K = 10 m/day, A = 100 m², head drops 2 m over 50 m. Gradient = 0.04, so Q = 10 × 100 × 0.04 = 40 m³/day. Write the gradient as a dimensionless ratio first; this prevents the most common unit and inversion errors.