Solution in the vadose zone of fractured limestone during heavy precipitation results in pathways for the transportation of rainwater to the permanent water table. Where does the most solution occur?

• A. At the base of the water table
• B. At the bottom of the aquifer
• C. Near the surface
• D. At the top of the water table

Answer: (D)

The key idea is that dissolution of limestone in the vadose zone is driven by acidic, CO2-rich infiltrating water, and the strongest chemical weathering happens where this water first saturates the rock. At the top of the water table, infiltrating water spends more time in contact with fresh carbonate rock as it transitions from unsaturated to saturated conditions. This point—where the downward flow meets the beginning of the saturated zone—offers the greatest opportunity for CO2-rich water to dissolve CaCO3 along fractures, creating pathways that can channel water down to the permanent water table. Below that point, water is already saturated with dissolved carbonate, so the rate of further dissolution declines; higher up near the surface, contact time with rock is shorter and the water is more influenced by surface conditions, not the sustained contact that drives the most dissolution.