Groundwater and Wells

Water below ground: aquifer types and properties, Darcy flow and seepage, and the drawdown around pumped wells and dewatering systems. It carries 4–6 of the 80 questions on the Water Resources & Environmental exam — a compact area with a small, well-defined set of relations.

What the problems look like

Short calculations on a section or well layout, plus concept checks. Recurring shapes — storage (specific yield in a water-table aquifer, storativity in a confined one) and water-level change from a volume budget; transmissivity of a layered aquifer; Darcy flux vs. seepage velocity and travel time between wells; flow-net seepage and exit gradient at a sheet-pile cutoff; Thiem (confined) and Dupuit (unconfined) steady-state drawdown; Cooper-Jacob test interpretation; and trench or well-ring dewatering. Which well flows at the surface, and why, is a typical concept check.

By exam day you should be able to —

• tell confined, unconfined, perched and artesian conditions apart, and pick S_y or S accordingly;
• compute head from casing elevation and depth to water, then the gradient;
• compute seepage velocity (flux ÷ effective porosity) and travel time;
• combine layers in parallel (along) and series (across);
• compute flow-net seepage and the exit gradient;
• apply Thiem and Dupuit to observation-well data, and read a Cooper-Jacob line.

Where people lose points

• Head is an elevation: subtract depth to water from the casing top; a difference in depths alone ignores differing casing elevations.
• Flux is not velocity — divide Darcy flux by effective porosity before computing travel time.
• Storage terms: a confined aquifer releases water elastically: the grain skeleton compresses and the water expands (S = S_s·b). Specific yield belongs to a falling water table, and specific retention is never released. Use the head decline, not the final depth.
• Layered soils: flow along layers takes the thickness-weighted K; flow across them, the thickness- weighted harmonic mean. Transmissivity is ΣK·b, and a confined T uses the aquifer thickness only.
• Flow nets: count flow channels and head drops as spaces, not lines, and keep N_f/N_d the right way up.
• Well equations: Thiem is 2πT and linear in drawdown; Dupuit is linear in h² with h measured from the aquifer base. Radius runs from the pumped well, not from the nearer observation well.

Have these at your fingertips

Darcy's law, the Thiem and Dupuit relations, the Cooper-Jacob straight line, and flow-net seepage — plus where they live in the Handbook. Two navigation notes: a search for "Dupuit" won't find the unconfined-well relation because of how the heading is spelled, so search "unconfined" instead; and flow nets are in the geotechnical chapter, not the groundwater section. Dewatering design (UFC 3-220-05) joins the supplied design standards only from the April 2027 exam; before then, work from the Handbook.

Going deeper

• WRE design standards: the April 2027 change
• Project Sitework — excavation dewatering in construction
• Hydrology — infiltration and recharge

Part of the Water Resources & Environmental exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.