ASBOG Hydrogeology 5 — Questions and Answers
Question 1: Which of the following conditions will cause the water table to decline even in the absence of pumping wells?
- Increase in recharge from heavy precipitation
- Evapotranspiration exceeding recharge in a water-table aquifer
- Construction of an impermeable surface over the recharge area
- Both B and C (Correct answer)
Correct answer: Both B and C
Both high ET losses from the capillary fringe and reduction in recharge due to impervious cover can cause the water table to decline without pumping.
Question 2: A well installed in a leaky (semi-confined) aquifer is pumped. Compared to the Theis solution for a confined aquifer, drawdown in the leaky aquifer will:
- Be greater at all times because leakage increases stress on the system
- Reach a steady state at smaller drawdown due to vertical leakage from the aquitard (Correct answer)
- Exhibit delayed yield similar to an unconfined aquifer response
- Show no difference from the Theis solution at early time
Correct answer: Reach a steady state at smaller drawdown due to vertical leakage from the aquitard
Leakage from an overlying or underlying aquitard provides additional water, limiting drawdown and allowing a steady state to be reached at a shallower level than Theis predicts.
Question 3: The specific capacity of a well is defined as:
- The maximum pumping rate before the well goes dry
- Pumping rate divided by drawdown in the pumped well (Correct answer)
- Total volume of water the aquifer can supply per day
- Transmissivity times the well radius
Correct answer: Pumping rate divided by drawdown in the pumped well
Specific capacity (Q/s) is pumping rate divided by the drawdown in the well; it is a practical field measure of well performance.
Question 4: Darcy's Law becomes invalid at high groundwater velocities primarily because:
- Molecular diffusion overwhelms advection
- Turbulent flow develops, violating the linear relationship between flux and gradient (Correct answer)
- The Reynolds number drops below unity
- Capillary forces become negligible at high velocity
Correct answer: Turbulent flow develops, violating the linear relationship between flux and gradient
At high velocities (Reynolds number > ~1–10 in porous media), inertial forces cause turbulence and the flux-gradient relationship becomes nonlinear, invalidating Darcy's Law.
Question 5: Which map product is used to show the configuration of the potentiometric surface of a confined aquifer and delineate groundwater flow directions?
- Water-table map based on unconfined well measurements
- Potentiometric surface map based on piezometric levels in the confined aquifer (Correct answer)
- Isopach map of the confining layer
- Geologic cross-section through the aquifer
Correct answer: Potentiometric surface map based on piezometric levels in the confined aquifer
A potentiometric surface map contours the static water levels in wells completed in a confined aquifer, with flow directed perpendicular to contours from high to low head.
Question 6: A NAPL (non-aqueous phase liquid) that is denser than water (DNAPL) behaves differently than LNAPL in the subsurface because it:
- Floats on top of the water table and spreads laterally
- Sinks through the saturated zone and may pool atop aquitards below the water table (Correct answer)
- Volatilizes rapidly and is removed in the vadose zone
- Has no residual saturation and migrates as a continuous phase indefinitely
Correct answer: Sinks through the saturated zone and may pool atop aquitards below the water table
DNAPLs sink through the unsaturated and saturated zones due to their high density, pooling on low-permeability layers deep in the aquifer and forming persistent source zones.
Question 7: The cone of depression created by a pumping well will be steeper (greater drawdown per unit distance) in an aquifer with:
- High transmissivity and high storativity
- Low transmissivity and low storativity (Correct answer)
- High transmissivity and low storativity
- Low transmissivity and high storativity
Correct answer: Low transmissivity and low storativity
Low transmissivity means greater head loss is needed to convey water to the well, and low storativity means little water is released per unit head decline, both steepening the cone.
ASBOG Hydrogeology 5
Which of the following conditions will cause the water table to decline even in the absence of pumping wells?