Modeling excess air and degassing in groundwater by equilibrium partitioning with a gas phase

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Abstract
Studies involving dissolved conservative gases in groundwater have usually found a gas excess above atmospheric solubility equilibrium (excess air), but sometimes also gas depletion (degassing). Both excesses and deficits of dissolved gases in groundwater can be explained by interactions with trapped gas bubbles. Excess air is formed when trapped air dissolves under increased pressure, degassing may occur if biogenic gases exsolve or pressure decreases. The concept of closed‐system equilibration between groundwater and trapped bubbles (CE‐model) proved successful in modeling excess air. Here we show that this concept can also be used to model the loss of dissolved gases and compare its performance to alternative models assuming a diffusion‐controlled degassing process. We find that for samples from laboratory tests and a shallow groundwater well that consistently exhibits gas undersaturation, the CE‐model provides the best available, albeit not perfect, description of degassing.