Groundwater Hydrology: Aquifer Properties, Darcy Flow, and Well Hydraulics
Aquifer properties
Distinguish porosity (total void fraction) from specific yield (drainable) and specific retention. Hydraulic conductivity K describes the medium–fluid combination; transmissivity T = K × saturated thickness b. Storativity S is the volume released per unit area per unit head decline: confined aquifers typically 10⁻⁵–10⁻³, unconfined aquifers approximate specific yield (0.01–0.3). Traps: using porosity as specific yield, and applying confined-aquifer storativity to an unconfined system.
Darcy's law and flow direction
Q = K·A·(Δh/ΔL), where Δh/ΔL is the hydraulic gradient. Flow moves from high to low hydraulic head, not necessarily downhill at the surface. Traps: reversing gradient sign, and forgetting that K units (m/s) require consistent area and length units.
Well hydraulics and drawdown
Steady radial flow to a well (Thiem): Q = 2πT(h₂ − h₁)/ln(r₂/r₁). Transient pumping follows the Theis solution; drawdown grows with pumping time and declines with distance from the well. Specific capacity = Q/s (discharge per unit drawdown). Traps: confusing drawdown with water level, and assuming a well's yield is independent of nearby pumping (interference).
Worked rule of thumb
A confined aquifer with K = 1×10⁻⁴ m/s and thickness b = 20 m has T = K·b = 2×10⁻³ m²/s. For a gradient of 0.001 across a 100 m-wide section, Q = K·A·i = 1×10⁻⁴ × (20×100) × 0.001 = 2×10⁻⁴ m³/s. Check units: m/s × m² × dimensionless = m³/s.