MTBE was detected in the unconfined aquifer. Which hydrogeologic unit is this?

• A. Confined aquifer
• B. Aquitard
• C. Unconfined aquifer
• D. Surface water

Answer: (C)

MTBE in groundwater under the surface indicates a unit where the saturated zone is in direct hydraulic contact with the surface—the upper boundary is the water table. That describes an unconfined aquifer, which is open to recharge from above and lacks a continuous impermeable cap above the saturated zone. A confined aquifer would be overlain by an impermeable layer (an aquitard or aquiclude), keeping it isolated from surface interactions, while an aquitard is a low-permeability layer itself, not a groundwater body, and surface water is not a groundwater unit. So the detected MTBE is in an unconfined aquifer.