Geology & Groundwater Hydrology Flashcards

What does a negative water-level trend in a long-term monitoring well most likely indicate?

Answer: Aquifer depletion where withdrawals exceed recharge

A persistent downward trend in water levels indicates the aquifer is being mined—long-term withdrawals exceed natural recharge rates.

Which of the following BEST describes a gaining reach of a stream in relation to the local aquifer?

Answer: The stream receives groundwater discharge because the water table is higher than the stream

In a gaining reach, the water table in adjacent banks is higher than the stream stage, driving groundwater into the stream channel.

Effective porosity differs from total porosity because it:

Answer: Excludes isolated pores and pores occupied by immobile water

Effective porosity (kinematic porosity) counts only interconnected pores through which water can actually flow, excluding dead-end and isolated pore spaces.

A well drilled in a valley-fill alluvial aquifer adjacent to a perennial stream is likely to exhibit what behavior?

Answer: Induced recharge from the stream when pumped heavily

Heavy pumping can reverse the hydraulic gradient between the stream and aquifer, inducing the stream to recharge the aquifer—a process called induced infiltration.

What is the primary reason shale acts as a confining unit for aquifers?

Answer: Very low hydraulic conductivity due to fine grain size and high clay content

Shale's extremely fine clay particles pack tightly together, resulting in very low permeability that restricts vertical groundwater movement between aquifer units.

Regional groundwater flow from recharge areas to discharge areas is driven primarily by:

Answer: Differences in hydraulic head between topographically high and low areas

Groundwater flows from areas of high hydraulic head (elevated recharge zones) to areas of low hydraulic head (discharge zones) following the potentiometric gradient.

When a driller encounters a sudden increase in drilling rate (fast drilling) in otherwise tight formation, this MOST likely indicates:

Answer: A fault zone or fracture with open voids

A sudden drilling rate increase in hard rock typically signals an open fracture, fault zone, or void that offers no resistance to the drill bit.