Groundwater Hydrology: Aquifers, Flow, and Well Hydraulics
Core concepts
Groundwater questions test aquifer types (confined, unconfined, perched, leaky), porosity versus specific yield versus specific retention, and hydraulic head. Darcy's law (Q = K·i·A) governs flow; transmissivity T = K·b (b = saturated thickness) and storativity S describe aquifer response. Confined aquifers have storativity from compressibility (typically 10⁻⁵ to 10⁻³); unconfined aquifers use specific yield (0.01–0.30).
Well hydraulics
Know the Thiem equation for steady radial flow, the Theis and Cooper-Jacob methods for transient pumping tests, and the concept of drawdown, cone of depression, radius of influence, and well efficiency. Specific capacity is discharge per unit drawdown. Capture zones and the distinction between the potentiometric surface (confined) and water table (unconfined) are essential.
Traps to watch
A classic trap is applying confined-aquifer formulas to an unconfined aquifer, or using storativity where specific yield belongs. Another is forgetting that drawdown in an unconfined aquifer reduces saturated thickness, so T changes. Watch for confusion between hydraulic conductivity (m/s) and intrinsic permeability (m²), and between total head and pressure head.
Worked example
Steady radial flow, Thiem: Q = 2πKb(h₂ − h₁)/ln(r₂/r₁). With K = 10 m/d, b = 20 m, h₂ − h₁ = 3 m, r₂/r₁ = 100: Q = 2π(10)(20)(3)/ln(100) = 3,770/4.605 ≈ 819 m³/d. Rule of thumb: ln(10) ≈ 2.303; a tenfold increase in radius adds about 2.3 to the denominator.