The study of abiotic reduction of nitrate and nitrite in Boom Clay

In Belgium, Boom Clay is studied as a reference host rock for the geological disposal of high-level and intermediate-level radioactive waste. Compatibility studies at the SCK
• CEN aim at investigating a perturbation of the capacity of Boom Clay to retard the migration of radionuclides to the biosphere, after disposal of Eurobitum bituminized radioactive waste in the clay (Valcke et al., 2009, Aertsens et al., 2009 and Bleyen et al., 2010).One of the geo-chemical perturbations is the possible oxidation of Boom Clay by the large amounts of nitrate that will be released by Eurobitum. A more oxidised Boom Clay could have a lower reducing capacity towards redox sensitive radionuclides, possibly enhancing their migration. As the conditions in the Boom Clay formation around a disposal gallery for Eurobitum are far from optimal for the growth of prokaryotes (limited space in the far-field, high pH in the near-field, gamma radiation by the waste during the first ∼300 years (effect limited to the primary and secondary waste package)), the impact of microbially mediated reduction of nitrate and nitrite is unclear. Therefore, batch tests are performed at the SCK
• CEN to study whether nitrate and nitrite can directly oxidise the main redoxactive components of Boom Clay (dissolved organic matter, kerogen, pyrite) without the mediation of prokaryotes.In a first series of batch tests, which are reported in this paper, the activity of denitrifying and nitrate reducing prokaryotes was inhibited by the addition of NaN3. NaN3 revealed to be an efficient inhibitor for these prokaryotes without affecting considerably the geochemistry of Boom Clay and/or Boom Clay pore water. Neither in batch tests with the Boom Clay slurries (with NaNO3 (0.1 and 1 M) or NaNO2 (0.1 M)) and with Boom Clay water (with 0.05 and 0.2 M NaNO3) a pure chemical nitrate or nitrite reduction was observed after respectively 3, 7 and 17 weeks and 1 year (Boom Clay slurries) and about 2 years (Boom Clay water). Furthermore, batch tests in which bacterial activity was allowed, demonstrated that the Boom Clay natural organic matter is a poor carbon source for (denitrifying and nitrate reducing) prokaryotes.

► Unfavourable conditions in the Boom Clay formation for the growth of prokaryotes.
► NaN3 is an efficient inhibitor for denitrifying and nitrate reducing prokaryotes.
► No abiotic nitrate or nitrite reduction observed in Boom Clay slurries/clay water.
► Boom Clay natural organic matter is a poor carbon source for prokaryotes.