Why Springs Bubble: A Framework for Gas Discharge in Groundwater

The mechanisms leading to the formation of bubbles in springs and wells have received minimal attention beyond causation. Free-phase gas quantification provides insight into a range of topics, from hazards that need to be managed to the economic value of soda waters. The presence of free-phase gas or vapor bubbles in groundwater may provide valuable information about the subsurface flow system. Additionally, free-phase gas or vapor bubbles may strip dissolved gasses from solution and introduce error into the assessment of gasses in springs, affecting analyses such as groundwater dating techniques. Similar to Meinzer's (1927) classification of springs by discharge rate, this paper proposes a framework for discussing free-phase gas and bubble phenomenon in groundwater to organize literature and foster future research. The naming, description, and categorization of free-phase gas or bubbles in groundwater provided in this review use seven common bubble manifestations or facies. These facies are based upon the processes in which the groundwater is charged with a gaseous species and the processes that bring the gas out of solution. Gas bubbles found in groundwater rarely occur as a single gas; therefore, these facies may overlap, but the proposed structure provides a useful framework. A glossary of bubble terms is provided to assist in clarity of describing free-phase gas phenomena.