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

• A. Sandstone bedrock
• B. Glacial drift
• C. Limestone karst
• D. Shale layer

Answer: (C)

The speed at which water drains toward a pumped well in an unconfined aquifer depends on how easily water can move through the surrounding materials. A shale layer within the aquifer acts as a low-permeability barrier, hindering flow and causing slower drainage toward the cone of depression. In contrast, limestone karst has many dissolved channels and high permeability, so drainage would be fast, not slow. Sandstone bedrock generally provides better flow than shale, leading to quicker drainage, and glacial drift is variable but often contains higher-permeability zones as well. So the condition that would produce slow drainage inside the cone of depression is a shale layer.