How does rainwater become acidic enough to dissolve limestone?

Rainwater becomes acidic enough to dissolve limestone primarily through the formation of carbonic acid. As rainwater infiltrates the ground, it absorbs carbon dioxide from the atmosphere and, more significantly, from the soil, where biological processes can produce carbon dioxide concentrations 10 to 40 times higher than those found in the free atmosphere.12

When this carbon dioxide dissolves in water, it forms a dilute solution of carbonic acid. This acidic water then reacts with the calcite in limestone bedrock, converting the insoluble calcium carbonate into a water-soluble compound, calcium bicarbonate, which is then carried away in solution. This process of chemical dissolution is the fundamental mechanism responsible for the creation of solution caves and karst landscapes. While industrial pollutants like sulfur dioxide and nitrogen oxides can further lower the pH of rainwater, the natural enrichment of carbon dioxide in soil remains the primary driver for the formation of these geological features.23415