Alpha: A Fortran program for simulating porewater radiolysis

The radiolysis of porewaters by uranium, thorium, and potassium in mineral
grains is a recognised source of molecular hydrogen in rock- 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 microns. 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.