The effects of dissolution are well illustrated by the formation of caves, sinkholes (dolines) and other karst features. Atmospheric and soil carbon dioxide, produced largely by organic respiration and bacterial decomposition, combine with water to produce carbonic acid, which progressively dissolves carbonate or evaporite minerals (Figure).
Karst features are particularly common in warm, humid climates where abundant water and biotic activity combined with high temperatures favor dissolution. As acidic groundwater flows along joints, faults and bedding planes, dissolution gradually enlarges them. This produces a positive feedback loop in which enlarged conduits permit more groundwater flow, which produces more dissolution. Dissolution occurs during relatively rapid flow in the vadose zone above the water table, in the zone near the fluctuating top of the water table and possibly during slower flow in the phreatic zone below the water table. One result of such large-scale dissolution is the formation of networks of large cavities in the form of caves that frequently contain underground streams that enter the subsurface down dissolution features and emerge as cave springs.
Sinkholes (dolines) are circular to ovoid depressions (Figure B12.1b(ii)) that form by (1) gradual loss of surface soluble rocks by accelerated dissolution; (2) gradual surface sinking of less soluble overburden (soil, sediment) by infiltration into cavities produced by dissolution of subsurface rock layers; or (3) sudden collapse of the surface by the collapse of rock or soil into an underlying cavity. The features and processes associated with karst development present many natural hazards that can be compounded by human activities. Natural karst hazards include severe flooding of sinkholes and karst valleys, continued dissolution and subsidence and/or sudden collapse of the surface during sinkhole formation.
Many human activities accelerate subsidence and collapse. Diversion of water into underground systems can accelerate dissolution, leading to collapse of bedrock or soil into underground cavities. Overextraction of water from karst aquifers lowers the water table to the point where loss of water pressure triggers the collapse of surface materials into underlying cavities. In situations where collapse is imminent, additional surface loading during construction projects may trigger subsidence or collapse. Building houses, roads and commercial structures in karst regions presents real challenges. In addition, surface pollutants that rapidly enter groundwater in karst areas due to high recharge rates are quickly dispersed, thus polluting wells, springs and underground rivers over wide areas. Extreme care must be used when disposing of any hazardous materials in karst areas.
