CES Groundwater & Hydrogeology 2 — Questions and Answers
Question 1: A dense non-aqueous phase liquid (DNAPL) released to the subsurface will most likely:
- Sink through the saturated zone and pool at the top of an aquitard (Correct answer)
- Float on the water table as a separate phase lens
- Dissolve completely in groundwater before reaching the saturated zone
- Volatilize entirely in the vadose zone and never reach groundwater
Correct answer: Sink through the saturated zone and pool at the top of an aquitard
DNAPLs have densities greater than water (e.g., chlorinated solvents like PCE and TCE) and will migrate downward through both the vadose and saturated zones until they encounter an impermeable layer, forming a pooled source.
Question 2: The retardation factor (R) for a contaminant in groundwater accounts for which process?
- Sorption of the contaminant onto aquifer solids, slowing its migration relative to advective water flow (Correct answer)
- Radioactive decay reducing contaminant mass over time
- Biodegradation converting the contaminant to non-toxic byproducts
- Volatilization transferring the contaminant from groundwater to soil vapor
Correct answer: Sorption of the contaminant onto aquifer solids, slowing its migration relative to advective water flow
The retardation factor R = 1 + (ρb × Kd) / n describes how much slower a sorbing contaminant travels compared to the average groundwater velocity; sorption onto soil organic matter or minerals is the primary mechanism.
Question 3: Hydrodynamic dispersion in a groundwater contaminant plume consists of:
- Mechanical dispersion caused by velocity variations and molecular diffusion (Correct answer)
- Advection driven by the hydraulic gradient alone
- Sorption and retardation of hydrophobic compounds
- Volatilization at the capillary fringe interface
Correct answer: Mechanical dispersion caused by velocity variations and molecular diffusion
Hydrodynamic dispersion combines mechanical (advective) dispersion—due to heterogeneous pore-scale velocity variations—with molecular diffusion, together causing spreading and dilution of the contaminant plume.
Question 4: A cone of depression forms around a pumping well because:
- Pumping removes water faster than the aquifer recharges locally, lowering the water table or potentiometric surface radially (Correct answer)
- The well casing blocks lateral groundwater flow, creating a stagnant zone
- Volatilization of pumped water reduces aquifer pressure near the well
- Dense contaminants accumulate near the well screen, reducing permeability
Correct answer: Pumping removes water faster than the aquifer recharges locally, lowering the water table or potentiometric surface radially
Pumping creates a head gradient directed toward the well; as water is extracted, head declines radially outward from the well, forming a characteristic cone-shaped depression in the potentiometric surface.
Question 5: The Theis equation is used in groundwater science primarily to:
- Determine aquifer transmissivity and storativity from pumping test drawdown data (Correct answer)
- Calculate the hydraulic gradient between monitoring wells
- Estimate recharge rates from precipitation data
- Predict contaminant retardation in the saturated zone
Correct answer: Determine aquifer transmissivity and storativity from pumping test drawdown data
The Theis (1935) equation describes transient drawdown around a pumping well in a confined aquifer; by matching observed drawdown-time data to the Theis type curve, T and S can be determined from a pumping test.
Question 6: Specific yield (Sy) is an important parameter for which aquifer type, and what does it represent?
- Unconfined aquifers; the volume of water released by gravity drainage per unit aquifer volume per unit head decline (Correct answer)
- Confined aquifers; the elastic compression of the aquifer matrix under reduced pressure
- Fractured rock aquifers; the proportion of secondary porosity to total rock volume
- Perched aquifers; the ratio of infiltration to evapotranspiration at the surface
Correct answer: Unconfined aquifers; the volume of water released by gravity drainage per unit aquifer volume per unit head decline
Specific yield (also called drainable porosity) applies to unconfined aquifers and represents water released by gravity drainage when the water table declines; it is typically much larger than the storativity of confined aquifers.
Question 7: Which in-situ remediation technology is most appropriate for a LNAPL gasoline plume in a shallow unconfined aquifer?
- Dual-phase extraction (soil vapor extraction combined with groundwater pump-and-treat) (Correct answer)
- Permeable reactive barrier using zero-valent iron
- Air sparging beneath the contaminated zone to strip dissolved chlorinated solvents
- Deep soil mixing with Portland cement to stabilize heavy metals
Correct answer: Dual-phase extraction (soil vapor extraction combined with groundwater pump-and-treat)
Dual-phase extraction removes both vapor-phase VOCs from the vadose zone via SVE and liquid-phase LNAPL/dissolved-phase hydrocarbons via simultaneous groundwater extraction, making it highly effective for fuel hydrocarbons near the water table.
CES Groundwater & Hydrogeology 2
A dense non-aqueous phase liquid (DNAPL) released to the subsurface will most likely: