Groundwater, Soils, and Sediments practice questions

Answer: A) Fraction of aquifer volume that drains by gravity as the water table falls

Specific yield is the volume of water an unconfined aquifer gives up by gravity drainage per unit volume per unit decline of the water table. It equals total porosity minus specific retention.

• ACorrect. Specific yield is the drainable water volume per unit aquifer volume for a unit drop in the water table. It equals porosity minus specific retention.
• BIncorrect. This is total porosity. Some pore water is held against gravity, so specific yield is less than porosity.
• CIncorrect. This is specific retention, the part of porosity that does not drain. Specific yield is the part that does drain.
• DIncorrect. This is specific storage, which controls storage in confined aquifers and is far smaller than specific yield.

Answer: B) 4 × 10⁻⁴ m/s

Rearranging Darcy's law, Q = KiA, gives K = Q/(iA) = (2.0 × 10⁻⁶)/(0.5 × 0.01) = 4 × 10⁻⁴ m/s.

• AIncorrect. This is Q/A = 2.0 × 10⁻⁶ / 0.01. It is the Darcy flux, and the hydraulic gradient has been left out. K is the Darcy flux divided by the gradient.
• BCorrect. Darcy's law Q = KiA gives K = Q/(iA) = 2.0 × 10⁻⁶ / (0.5 × 0.01) = 4 × 10⁻⁴ m/s, which is a typical value for clean sand.
• CIncorrect. This is Q·i/A. The gradient was multiplied instead of divided, which inverts its role in Darcy's law.
• DIncorrect. This comes from dividing by the gradient twice (Q/(A·i²)). Darcy's law has the gradient to the first power only.

Answer: B) 100 m³/(day·m)

• AIncorrect. Divide, don't multiply.
• BCorrect. Q/s = 500/5.
• CIncorrect. That's inverted.
• DIncorrect. Divide, don't subtract.

Answer: A) Drawdown at each well increases

• ACorrect. This is well interference.
• BIncorrect. Interference adds drawdown.
• CIncorrect. They still produce.
• DIncorrect. Confinement is geologic.

Answer: B) Dissolved chlorinated ethenes such as TCE, removed by reductive dechlorination

Zero-valent iron PRBs passively treat plumes of chlorinated solvents such as TCE. As the iron oxidizes, it reductively dechlorinates the contaminants, releasing chloride.

• AIncorrect. ZVI is a reductant, not an oxidant. Petroleum hydrocarbons such as benzene and toluene are not the target of ZVI barriers. Any BTEX removal near these barriers comes from biodegradation, not reaction with the iron.
• BCorrect. Oxidation of the iron supplies electrons that strip chlorine from chlorinated solvents such as TCE as groundwater flows passively through the barrier.
• CIncorrect. A PRB treats dissolved contaminants carried by groundwater flow. It does not skim free-phase light NAPL, which floats on the water table.
• DIncorrect. Iron grains are not an ion-exchange medium for salts. Chloride is actually released as a product when the iron dechlorinates solvents.

Answer: C) 0.30

• AIncorrect. That's the solids fraction.
• BIncorrect. That's the void ratio, V_v/V_s.
• CCorrect. n = V_voids/V_total.
• DIncorrect. That's inverted.

Answer: A) Greater retardation

• ACorrect. More sorption slows its movement.
• BIncorrect. Sorption increases retardation.
• CIncorrect. Sorbing solutes move slower.
• DIncorrect. K_d measures sorption.

Answer: A) At the potentiometric surface, above the top of the aquifer

A confined aquifer is pressurized, so water in a well rises above the aquifer top to the potentiometric surface. When that surface is above land surface, the well is a flowing artesian well.

• ACorrect. Water in a confined aquifer is under pressure greater than atmospheric, so it rises in the well to the potentiometric surface, above the aquifer top. If that surface is above ground level, the well flows (flowing artesian).
• BIncorrect. If the head were only at the aquifer top, the aquifer would be at atmospheric pressure at its upper boundary. A confined aquifer's pressure pushes the water level above that boundary.
• CIncorrect. The confining layer separates the two systems hydraulically. The confined aquifer's head is set by its own recharge and pressure, not by the shallow water table.
• DIncorrect. A water level inside the aquifer describes unconfined (water-table) conditions. A confined aquifer stays fully saturated, with its head above the aquifer top.

Answer: D) Water table

• AIncorrect. It's the water table.
• BIncorrect. That bounds a confined aquifer.
• CIncorrect. That's usually the bottom.
• DCorrect. It's at atmospheric pressure.

Answer: A) 3

• ACorrect. 1 + 1.6(0.5)/0.4 = 1 + 2.
• BIncorrect. Remember to add 1 to ρ_b K_d/n.
• CIncorrect. Divide by n, don't multiply.
• DIncorrect. Recheck: 0.8/0.4 = 2.