Which of the following is a geologic or hydrogeologic factor that can dictate contamination movement?

• A. Depth to groundwater
• B. Ground water
• C. Soil characteristics
• D. Topography

Answer: (A)

The movement of contaminants in the subsurface is governed by how the geology and groundwater system arrange themselves, especially the vertical distance to the water table. Depth to groundwater directly controls how long a pollutant in the unsaturated zone must travel before reaching the aquifer. If groundwater is shallow, contaminants can enter the water supply quickly, creating a plume with little time for natural filtration. If the groundwater is deep, the travel path is longer, increasing opportunities for sorption to soils, biodegradation, and dilution, which can slow or lessen the impact on the aquifer. While soil properties and topography also influence transport, the vertical separation to groundwater is the key factor that dictates whether and how fast contamination can reach the groundwater.