Hydrogeology
What to know
Porosity is storage; permeability (hydraulic conductivity, K) is the ability to transmit water. An unconfined aquifer is bounded by a water table; a confined aquifer's potentiometric surface may lie above the top of the unit. Aquitards retard flow but do not stop it.
Darcy's law: Q = K·i·A, where i is the hydraulic gradient (dimensionless). Transmissivity T = K·b (units of m²/day) describes the whole saturated thickness b. Storativity S is dimensionless: for confined aquifers it is small (roughly 10⁻⁵ to 10⁻³, from compressibility), while for unconfined aquifers it approximates specific yield (0.1–0.3). Specific retention is water held against gravity.
Well hydraulics: pumping creates a cone of depression; drawdown versus time is analyzed with Theis or Cooper–Jacob methods; boundary conditions (recharge, faults) bend the curves. Water-quality work uses advection–dispersion, sorption retardation (R = 1 + ρbKd/θ), and decay. Coastal aquifers follow the Ghyben–Herzberg relation.
Traps
Mixing K and T; assigning large storativity to a confined aquifer; using water-table elevation as head in a confined system; confusing porosity with permeability (clay is porous but nearly impermeable); forgetting that the gradient is measured between head measurements, not along the pipe.
Rule of thumb
Ghyben–Herzberg: freshwater lens thickness ≈ 40 × water-table height above sea level (for fresh 1.000 and seawater 1.025 g/cm³).
Worked example: K = 10 m/day, i = 0.001, A = 100 m² → Q = 10 × 0.001 × 100 = 1 m³/day.