CWD Geology & Groundwater Hydrology 3 — Questions and Answers
Question 1: What does the specific yield of an unconfined aquifer represent?
- The volume of water released per unit drop in hydraulic head per unit area (Correct answer)
- The total porosity of the aquifer material
- The rate of vertical recharge
- The pumping rate of a well
Correct answer: The volume of water released per unit drop in hydraulic head per unit area
Specific yield is the volume of water that drains by gravity from a unit volume of aquifer material per unit decline in the water table.
Question 2: A perched water table occurs when:
- The main water table is under artesian pressure
- Groundwater accumulates above a localized impermeable layer above the regional water table (Correct answer)
- Two aquifers merge at depth
- Tidal fluctuations raise water levels seasonally
Correct answer: Groundwater accumulates above a localized impermeable layer above the regional water table
A perched aquifer forms when a lens of low-permeability material intercepts downward-percolating water, creating a localized saturated zone above the main water table.
Question 3: Which geologic process is responsible for creating the primary porosity in sandstone aquifers?
- Dissolution by acidic water
- Tectonic fracturing
- Depositional pore spaces between grains (Correct answer)
- Volcanic vesicle formation
Correct answer: Depositional pore spaces between grains
Primary porosity in sandstone results from the spaces between sand grains that were preserved during original sediment deposition and cementation.
Question 4: The vadose zone is best defined as:
- The saturated zone below the water table
- The unsaturated zone between the land surface and the water table (Correct answer)
- The zone of capillary rise just above the water table
- The zone of groundwater discharge to streams
Correct answer: The unsaturated zone between the land surface and the water table
The vadose zone (unsaturated zone) extends from the land surface to the water table and contains both air and water in pore spaces.
Question 5: What is transmissivity in hydrogeology?
- The rate at which water enters the aquifer from precipitation
- The product of hydraulic conductivity and saturated aquifer thickness (Correct answer)
- The ratio of groundwater withdrawn to total storage
- The vertical movement of water through the vadose zone
Correct answer: The product of hydraulic conductivity and saturated aquifer thickness
Transmissivity (T = K × b) describes the ability of an aquifer to transmit water horizontally and depends on both the material's conductivity and the saturated thickness.
Question 6: Stream gauging data shows baseflow increases after no recent rainfall. This MOST likely indicates:
- Surface runoff is delayed
- Groundwater is discharging into the stream (gaining stream) (Correct answer)
- The stream is recharging the aquifer
- Evapotranspiration has increased
Correct answer: Groundwater is discharging into the stream (gaining stream)
A gaining stream receives groundwater discharge from adjacent aquifers, which can sustain or increase streamflow during dry periods.
Question 7: Which grain size class generally has the highest hydraulic conductivity?
- Clay
- Silt
- Fine sand
- Coarse gravel (Correct answer)
Correct answer: Coarse gravel
Coarse gravel has large, well-connected pore spaces that offer minimal resistance to flow, resulting in the highest hydraulic conductivity of common sediment types.
CWD Geology & Groundwater Hydrology 3
What does the specific yield of an unconfined aquifer represent?