Usually, sampling of dissolved gases is not possible without atmospheric contamination. In case of reducing systems a correction of the atmospheric fraction is possible by the value of the oxygen concentration.
The gas mixtures in groundwater are defined by air components and hydrocarbons. The air components are:
- nitrogen (N2)
- oxygen (O2)
- carbon dioxide (CO2)
- argon (Ar)
- helium (He)
- other noble gases
- i- and n-butane
- other higher hydrocarbons.
Dissolved nitrogen in groundwater can be enriched via the degradation of nitrogen components and organic material as well as the atmospheric input. Assessments are typically done using the N2/Ar ratio and the isotope content of 15N-N2.
In groundwater with a high carbon dioxide content (sparkling mineralwater) the carbon dioxide mainly originates from volcanic sources. This can be evaluated by the isotopic signature (13C-CO2).
Elevated argon contents can be explained by atmospheric input or an admixture of radiogenic generated argon (40Arrad) as a product of the radioactive decay of potassium-40 (40K) or both. The ratio 40Ar/36Ar of the thermal water is a good indicator.
Atmospheric input by the means of precipitation, outgassing of the earth's mantle, and radiogenic production in rocks are potential root causes for the appearance of helium in groundwater. The determination of the helium isotopes (3He/4He) allows to distinguish between these origins and to quantify them.
Dissolved methane in a thermal water system is generated by a thermal formation or a microbial degradation of organic matter under anaerobic conditions or both. To differentiate between those formation processes the analysis of the isotope content 13C-CH4 and 2H-CH4 of methane and the higher hydrocarbons is needed.
Additional informations could be provided by the analysis of the carbon-14-content of methane (14C-CH4). It enables the determination of the formation age methane originated from bog and peat sediments or it helps answering specific questions regarding landfill gas.