2.2 Deep Chemically Aggressive Water Forms Interconnected Conduits

Occurrence of water undersaturated in calcite at large depth and distance into the subsurface system requires the presence of carbon dioxide gas which can develop when groundwater transports organic carbon into the system that is then oxidized to form carbon dioxide gas. Both aqueous and gaseous carbon dioxide (CO₂) play a role in development of karst because CO₂ produces carbonic acid that results in dissolution of calcite.

The six mechanisms for instigation of calcite dissolution are:

1. variation in groundwater temperature because calcite solubility decreases with rising temperatures unlike most rock forming minerals;
2. production of sulfuric acid along groundwater flow paths, as suggested by Moore and Nicholas (1964) during which oxidation of small amounts of sulfide minerals, especially pyrite, results in sulfuric acid (H₂SO₄);
3. floods in surface streams or rapid snowmelt causing exceptionally large rapid recharge of undersaturated water;
4. mixing of dissimilar waters at fracture/joint intersections, because when waters of different chemistry (each saturated in calcite but with different CO₂ partial pressure or temperatures) mix, the mixed water can be slightly undersaturated in calcite (Wigley and Plummer, 1976; Thraikill, 1968);
5. in-situ formation of CO₂ deep in the vadose zone by particulate and dissolved organic transport (Wood and Petraits, 1984); Wood, 1985); and,
6. degassing of groundwater that is thermodynamically saturated with calcite as it reaches caves (Cao et al., 2021) resulting in precipitation of calcium carbonate and formation of stalactites and stalagmites.

Another less common source of CO₂ or SO₄ at depth occurs near magmatic areas or oil fields and can create chemically aggressive water.

Once chemically aggressive water is present at depth, karst openings are enlarged by dissolution of rock surrounding permeable openings which then become more permeable, thus higher flow volumes and velocity bring more aggressive water in contact with the soluble rocks and the karstification process proceeds.