Abstract

Irreversible water degassing, connected with periodic entrapment of pore air and formation of bubbles after the wetting of unsaturated sediments, followed by the subsequent release of air from the bubbles after the sediments dry, is proposed as a possible mechanism for the precipitation of calcium carbonate in the vadose zone. The process involves an initial stage during rainy periods, when pore air may be entrapped due to preferential water flow in large pores. Equilibration of the water-air system causes pressure in the entrapped-air bubbles to decrease to a value close to the sum of atmospheric and capillary pressure (40-95% of atmospheric pressure). This pressure decrease leads to degassing of pore water adjacent to the bubbles, bubble expansion, and a shift in the equilibrium of all aqueous reactions involving dissolved gases. In particular, the degassing must accelerate carbonate precipitation.

A second stage develops when entrapped air is released during lowering of the groundwater table and/or drying of sediment. Release of entrapped air that has been subjected to a pressure lower than atmospheric, should draw in a portion of free pore air towards the water-air interface. That free pore air has a lower gas concentration compared to that in the released air, which causes fast mixing between the two. In addition, transfer of a portion of the degassed water towards the groundwater occurs. In combination, these processes cause the degassing and corresponding geochemical processes in the zone of air entrapment and release to be irreversible.

The proposed degassing mechanism of carbonate precipitation may take place in the wetting-front zone that passes through unsaturated sediments, from the land surface downwards, after rainy periods. This mechanism may also occur at the upper boundary of the fluctuating capillary fringe of phreatic aquifers, including perched groundwater periodically formed within unsaturated sediments that lie upon more clayey (less permeable) layers. For a sandy capillary fringe, the minimum rate of carbonate precipitation by this mechanism is estimated in a wide range of 120 to 175,200 mg m−2 yr−1. At these rates, within a 1 m zone of capillary-fringe fluctuations, sand might become cemented until pourability is lost for from 800 to 1,170,000 years.

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