Which factor leads to the highest acidity in water just below the water table, promoting dissolution in limestone?

• A. Rainwater acidity and percolation through organic debris
• B. Higher salinity at depth
• C. Increased temperature near the surface
• D. Exposure to atmospheric oxygen

Answer: (A)

The key idea is that groundwater acidity that drives limestone dissolution comes from carbonic acid formed when rainwater infiltrates through soil and organic matter. As rainwater percolates, it picks up carbon dioxide from the air and from soil respiration and decomposition. The dissolved CO2 forms carbonic acid (H2CO3), which dissociates to release H+ ions. Those hydrogen ions attack calcite (CaCO3) in the limestone, dissolving it to calcium and bicarbonate: CaCO3 + H2CO3 → Ca2+ + 2 HCO3−. The presence of organic debris adds additional organic acids, further lowering pH and enhancing dissolution just below the water table where this infiltrating, CO2-rich water is most active. Higher salinity at depth, increased temperature near the surface, or exposure to atmospheric oxygen don’t provide the same strong, localized source of acidity that carbonic acid from infiltrating rainwater does, so they’re less effective at promoting limestone dissolution in this context.