The Effect of Grain Size on Porewater Radiolysis

J. DeWitt* (Corresponding Author), S. McMahon* (Corresponding Author), J. Parnell

*Corresponding author for this work

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The radiolysis of porewaters by uranium, thorium, and potassium in mineral grains is a recognized source of molecular hydrogen in rock-hosted and sediment-hosted fluids. This radiolytic hydrogen is of geomicrobiological interest as a potential energy source (electron donor) for microbial metabolism, especially in energy-limited settings such as the marine deep biosphere or the subsurface of Mars. Previous efforts to predict the production of radiolytic hydrogen from columns of rock and sediment have tended to rely upon analytic models that cannot account for the attenuation of mineral radiation by grains larger than ∼30 µm. To address this, we have developed a Monte Carlo method to simulate the physics of mineral radiation and evaluate the production of H2 as a function of mineral grain size and radioisotope composition. The results confirm that grain size is a major control on radiolytic H2 yield. For example, using the standard geological classification of grain sizes, we find that clay can produce up to an order of magnitude more H2 per unit time than sand. The magnitude of this effect is illustrated using compositional data from real geological units in order to demonstrate the dependence of radiolytic hydrogen flux on natural radionuclide concentration and bulk porosity.

Original languageEnglish
Article numbere2021EA002024
Number of pages21
JournalEarth and Space Science
Issue number6
Early online date9 Jun 2022
Publication statusPublished - 9 Jun 2022

Bibliographical note

Funding Information:
The authors greatly appreciate the helpful discussions with Eric Benton at Oklahoma State University and Regina DeWitt at East Carolina University. S.M. also thanks Barry B. McMahon for helpful discussions about the geometry in Text S2 in Supporting Information S1.

Data Availability Statement

Supporting Information:
Supporting information can be found in the online version of this article.

Data Availability Statement:
The radiolytic hydrogen production data used for the grain size analysis in this study are available via CC BY-SA 4.0 (DeWitt et al., 2022). The Fortran program used for the simulation of porewater radiolysis is available via CC BY-NC-ND 4.0 and developed openly at East Carolina University (DeWitt, 2022)


  • geomicrobiological
  • marine deep biosphere
  • microbial
  • porewaters
  • radiolysis
  • radiolytic hydrogen


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