What causes a cone of depression?

• A. Increased static water level due to aquifer recharge
• B. Thermal expansion of groundwater causing upward movement
• C. Differential pressure created between the zone of saturation, and the capillary fringe that pushes the water outward from the top of the well casing
• D. Collapse of the well casing due to corrosion

Answer: (C)

Pumping lowers the hydraulic head around the well, creating a pressure gradient that pulls groundwater from the surrounding saturated zone toward the well. The capillary fringe above the water table has a different pressure, and the differential between the saturated zone and this fringe drives water toward the pumped well. As water is withdrawn continuously, the water table declines around the well, forming a cone of depression that extends outward from the well. The other options describe processes that raise water levels, or are unrelated to pumping effects on the groundwater surface.