Hydrogen Isotopes

Dissolved hydrogen (H_2(aq)) is an important intermediate produced and consumed by microorganisms during the anaerobic decomposition of organic matter in many subsurface environments. Because H₂ may undergo isotopic fractionation reactions as a result of microbial cycling, the stable hydrogen-isotope (²H) value of ground water in equilibrium with H₂ may be changed from ambient conditions. However, field-scale observations and isotopic dilution and isotopic fractionation calculations suggest that microbial cycling of H₂ does not affect the ²H value of ground water, even in strongly reducing methanogenic conditions characterized by high H₂ concentrations. These results imply that the widespread application of ²H and ¹⁸O values to deduce the climatic history of ground-water recharge is valid, even if applied in areas where the ground water may have received recharge from areas affected by petroleum-hydrocarbon releases or natural accumulations of organic matter, attendant anaerobic conditions, and active H₂ cycling.

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