What geologic condition would produce slow drainage within the cone of depression of an unconfined aquifer?

• A. Sandstone aquifers
• B. Glacial drift
• C. Unfavorable porosity
• D. Basaltic flow

Answer: (B)

When a cone of depression forms in an unconfined aquifer, the speed at which it drains back after pumping stops depends on how easily groundwater can move through the surrounding materials—its hydraulic conductivity. Glacial drift often includes clay-rich layers and fine-grained sediments that have very low permeability. Those low-permeability pockets act as barriers to flow, so water moves slowly toward the pumped well and the cone refills (drains) slowly. In contrast, sandstone aquifers typically have higher permeability, so drainage would be quicker, and basaltic flows vary but aren’t inherently slow unless heavily fractured or altered. Porosity speaks more to how much water can be stored than how quickly it can move; therefore, the condition that leads to slow drainage is glacial drift with its low-permeability components.