ASBOG Hydrogeology 2 — Questions and Answers
Question 1: Which dimensionless number is used to compare advective transport to diffusive transport of solutes in groundwater?
- Reynolds number
- Peclet number (Correct answer)
- Froude number
- Damköhler number
Correct answer: Peclet number
The Peclet number (Pe) is the ratio of advective to diffusive solute transport; high Pe means advection dominates.
Question 2: A confined aquifer has a storage coefficient of 5×10⁻⁴. This value indicates that water release is primarily due to:
- Gravity drainage of pore water
- Compression of the aquifer and expansion of water (Correct answer)
- Evapotranspiration at the water table
- Tidal fluctuations in the aquifer
Correct answer: Compression of the aquifer and expansion of water
In confined aquifers, the storativity reflects elastic compression of the aquifer skeleton and expansion of pore water under pressure change.
Question 3: Which of the following best describes a perched aquifer?
- An aquifer below a regional confining layer
- A saturated zone above the regional water table, separated by an unsaturated zone (Correct answer)
- An aquifer recharged exclusively by tidal forcing
- A fractured rock aquifer with secondary porosity
Correct answer: A saturated zone above the regional water table, separated by an unsaturated zone
A perched aquifer is a locally saturated zone sitting atop a low-permeability lens above the main water table.
Question 4: In the context of aquifer testing, what does the term 'skin effect' refer to?
- The effect of aquifer boundaries on drawdown
- Near-wellbore damage or improvement affecting wellbore efficiency (Correct answer)
- Thickness variation of the unsaturated zone
- Capillary fringe effects on pumping
Correct answer: Near-wellbore damage or improvement affecting wellbore efficiency
Skin effect describes additional head loss (positive skin = damage, negative skin = improvement) caused by near-wellbore conditions.
Question 5: The Ghyben-Herzberg relation describes the depth of the freshwater-saltwater interface below sea level as approximately how many times the freshwater head above sea level?
- 10
- 20
- 40 (Correct answer)
- 100
Correct answer: 40
The Ghyben-Herzberg relation gives the interface depth as 40 times the freshwater head above sea level, derived from the density ratio of freshwater to saltwater.
Question 6: Which aquifer property controls the rate at which water-level changes propagate through a confined aquifer?
- Specific yield
- Hydraulic conductivity alone
- Hydraulic diffusivity (T/S) (Correct answer)
- Capillary pressure
Correct answer: Hydraulic diffusivity (T/S)
Hydraulic diffusivity (transmissivity divided by storativity) governs how fast pressure perturbations travel through a confined aquifer.
Question 7: A water sample from a spring shows tritium activity of less than 0.8 TU. This result most likely indicates the water:
- Recharged after 1952 nuclear testing
- Is modern, post-bomb water
- Has a mean residence time greater than about 50 years (Correct answer)
- Has been contaminated with industrial tritium
Correct answer: Has a mean residence time greater than about 50 years
Tritium levels below ~0.8 TU indicate the water predates 1952 thermonuclear testing, implying residence times exceeding ~50 years.
ASBOG Hydrogeology 2
Which dimensionless number is used to compare advective transport to diffusive transport of solutes in groundwater?