ASBOG Hydrogeology 4 — Questions and Answers
Question 1: In a gaining stream, the relationship between groundwater and surface water is best described as:
- Stream water infiltrates into the aquifer along the entire reach
- Groundwater discharges into the stream, raising baseflow (Correct answer)
- The stream is perched above the regional water table
- There is no hydraulic connection between the stream and aquifer
Correct answer: Groundwater discharges into the stream, raising baseflow
A gaining stream receives groundwater discharge because the water table elevation near the stream is higher than the stream stage.
Question 2: The retardation factor (R) for a contaminant in groundwater is defined as:
- R = v_contaminant / v_water
- R = v_water / v_contaminant (Correct answer)
- R = porosity / bulk density
- R = dispersivity / advective velocity
Correct answer: R = v_water / v_contaminant
R = v_water / v_contaminant; a retardation factor greater than 1 means the contaminant travels slower than the groundwater due to sorption.
Question 3: Which of the following best describes the concept of 'hydraulic head'?
- The velocity of groundwater flow at a point
- The total mechanical energy per unit weight of water, expressed as an elevation (Correct answer)
- The pressure exerted by the overlying saturated column
- The gradient of the potentiometric surface
Correct answer: The total mechanical energy per unit weight of water, expressed as an elevation
Hydraulic head is the sum of pressure head and elevation head, representing total mechanical energy per unit weight expressed as a length (elevation).
Question 4: What is the primary cause of land subsidence associated with groundwater withdrawal in unconsolidated sedimentary basins?
- Dissolution of carbonate minerals by pumped water
- Compaction of fine-grained interbeds as pore pressure declines (Correct answer)
- Increase in seismic activity due to reduced pore pressure
- Expansion of clay minerals upon drying
Correct answer: Compaction of fine-grained interbeds as pore pressure declines
When pore pressure decreases in pumped systems, the effective stress on clay and silt interbeds increases, causing irreversible compaction and land subsidence.
Question 5: The Dupuit assumption in analytical groundwater models for unconfined aquifers states that:
- Flow is horizontal and hydraulic gradient equals the slope of the water table (Correct answer)
- Flow is vertical and pressure is hydrostatic throughout
- Hydraulic conductivity increases linearly with depth
- The water table is a no-flow boundary
Correct answer: Flow is horizontal and hydraulic gradient equals the slope of the water table
The Dupuit-Forchheimer assumption neglects vertical flow components and assumes flow is horizontal with the gradient equal to the water table slope.
Question 6: Which isotope ratio is most commonly used to distinguish between different recharge sources or paleoclimate conditions in groundwater studies?
- ³H/³He
- ¹⁸O/¹⁶O (expressed as δ¹⁸O) (Correct answer)
- ¹⁴C/¹²C
- ⁸⁷Sr/⁸⁶Sr
Correct answer: ¹⁸O/¹⁶O (expressed as δ¹⁸O)
δ¹⁸O (and δ²H) are the most widely used stable isotope tracers for identifying recharge sources, elevation effects, and paleoclimate signatures in groundwater.
Question 7: In a fractured rock aquifer, the primary porosity of the intact rock matrix differs from secondary porosity in that:
- Primary porosity stores water while secondary porosity (fractures) transmits it (Correct answer)
- Secondary porosity stores more water than primary porosity
- Primary porosity contributes more to hydraulic conductivity in crystalline rock
- Both primary and secondary porosity contribute equally to flow and storage
Correct answer: Primary porosity stores water while secondary porosity (fractures) transmits it
In dual-porosity systems, the low-permeability matrix stores the bulk of groundwater while the fracture network provides the main pathways for flow.
ASBOG Hydrogeology 4
In a gaining stream, the relationship between groundwater and surface water is best described as: