The production rate and isotopic composition of H
2 derived from radiolytic reactions in H
2O were measured to assess the importance of radiolytic H
2 in subsurface environments and to determine whether its isotopic signature can be used as a diagnostic tool. Saline and pure, aerobic and anaerobic water samples with pH values of 4, 7, and 10 were irradiated in sealed vials at room temperature with an artificial γ source, and the H
2 abundance in the headspace and its isotopic composition were measured. The H
2 concentrations were observed to increase linearly with dosage at a rate of 0.40 ± 0.04 molecules (100 eV)
−1 within the dosage range of 900 to 3500 Gray (Gy; Gy = 1 J Kg
−1) with no indication of a maximum limit on H
2 concentration. At ∼2000 Gy, the H
2 concentration varied only by 16% across the experimental range of pH, salinity, and O
2. Based upon this measured yield and H
2 yields for α and β particles, a radiolytic H
2 production rate of 10
−9 to 10
−4 nM s
−1 was estimated for the range of radioactive element concentrations and porosities typical of crustal rocks. The δD of H
2 was independent of the dosage, pH (except for pH 4), salinity, and O
2 and yielded an αD
H2O-H2 of 2.05 ± 0.07 (αD
H2O-H2 = (D/H)
H2O to (D/H)
H2), slightly less than predicted radiolytic models. Although this radiolytic fractionation value is significantly heavier than that of equilibrium isotopic exchange between H
2 and H
2O, the isotopic exchange rate between H
2 and H
2O will erase the heavy δD of radiolytic H
2 if the age of the groundwater is greater than ∼10
3 to 10
4 yr. The millimolar concentrations of H
2 observed in the groundwater of several Precambrian Shields are consistent with radiolysis of water that has resided in the subsurface for a few million years. These concentrations are well above those required to support H
2-utilizing microorganisms and to inhibit H
2-producing, fermentative microorganisms.
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