2.5 Production of Gas in the Subsurface
In addition to gases derived from the atmosphere, groundwater may contain gases that are produced in the subsurface. The most commonly produced subsurface gases are carbon dioxide ([latex]{CO}_{2}[/latex]), methane ([latex]{CH}_{4}[/latex]), and nitrogen ([latex]N_{2}[/latex]). [latex]{CO}_{2}[/latex] is produced in the unsaturated zone by the oxidation, i.e., decay, of organic matter and by respiration from organisms including plants and microbes. The concentration of [latex]{CO}_{2}[/latex] in the unsaturated zone can be orders of magnitude higher than in the atmosphere; this, in turn, drives weathering reactions (Drever, 1988; Garrels & Christ, 1965; Solomon & Cerling, 1987). Weathering reactions, decay of organic matter, and fermentation reactions can all produce [latex]{CO}_{2}[/latex] below the water table. While dissolved [latex]{CO}_{2}[/latex] concentrations in groundwater can be significant, the partial pressure of [latex]{CO}_{2}[/latex] often remains at low to moderate levels because of its high solubility in water.
When molecular oxygen is not available, the decay of organic matter continues by a series of reactions that can involve various electron acceptors. Fermentation reactions—in concert with fermentative organisms—can convert organic matter into [latex]{CH}_{4}[/latex], and [latex]{CO}_{2}[/latex]. In the deep subsurface, thermal maturation of hydrocarbons can also produce [latex]{CH}_{4}[/latex]. Because of its low solubility in water, even modest concentrations of dissolved [latex]{CH}_{4}[/latex] can lead to significant gas pressures. In shallow aquifers that receive recharge from agricultural lands or human/animal wastes, high concentrations of dissolved nitrate ([latex]{NO}_{3}^{-})[/latex]can occur. Under appropriate biogeochemical conditions, [latex]{NO}_{3}^{-}[/latex]can be used as an electron acceptor to oxidize carbon to [latex]{CO}_{2}[/latex] and form nitrogen gas ([latex]N_{2}[/latex]). This process, known as denitrification, can result in elevated concentrations of dissolved [latex]N_{2}[/latex] which in turn can produce modest partial pressures of [latex]N_{2}[/latex].
When gas is produced in the subsurface, it is possible to develop TDGP that exceeds the fluid pressure—especially when the fluid pressure is lowered such as near pumping wells. When this happens, it is possible to form bubbles that can be trapped in pore throats. In surface water, buoyant gas bubbles can migrate to the surface and escape to the atmosphere. However, in a porous medium the interaction between interfacial forces, capillary pressures, and the physical size of pore throats makes it difficult for bubbles to migrate. As a result, bubbles tend to accumulate in the porous media.
Although a bubble may form due to excess pressure of just one or several gases, once the bubble is formed, it can strongly influence the concentration of other gases as they can now partition into the bubble as discussed with respect to the dimensionless Henry Coefficient (Section 2.1). This partitioning depends on the solubility of each gas and affects low solubility (high volatility) gases such as [latex]He[/latex] and [latex]Ne[/latex] more than high solubility (low volatility) gases.