In groundwater near a fractured sandstone overlain by claystone layers, where would elevated uranium concentrations be expected?

• A. In a plume following fractures and joints
• B. Uniformly distributed
• C. Near claystone layer
• D. Near siltstone layer

Answer: (A)

The main idea here is that fracture-controlled flow governs how dissolved contaminants move in groundwater. In a fractured sandstone, the openings along fractures and joints create fast, connected pathways that channel groundwater and any dissolved species, including uranium. Because uranium in oxidizing groundwater tends to be soluble (as U(VI) species), it can be carried along these pathways and form an elongated plume aligned with the fracture network rather than spreading evenly in all directions.

The claystone layers above act as a barrier to vertical movement and have higher sorption capacity, which slows or halts movement into the overlying layers and limits lateral dispersion away from the fracture network. So the elevated uranium would be concentrated in a plume traveling along fractures and joints within the sandstone, not uniformly distributed or concentrated near the claystone or siltstone interfaces.