Groundwater & Hydrogeology Flashcards

A dense non-aqueous phase liquid (DNAPL) released to the subsurface will most likely:

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.

The retardation factor (R) for a contaminant in groundwater accounts for which process?

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.

Hydrodynamic dispersion in a groundwater contaminant plume consists of:

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.

A cone of depression forms around a pumping well because:

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.

The Theis equation is used in groundwater science primarily to:

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.

Specific yield (Sy) is an important parameter for which aquifer type, and what does it represent?

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.

Which in-situ remediation technology is most appropriate for a LNAPL gasoline plume in a shallow unconfined aquifer?

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.