text: Aside from bioreduction, biosorption, bioaccumulation and biomineralization, which are bacterial strategies for natural attenuation of radioactive contamination, there are also human methods that increase the efficiency or speed of microbial processes. This accelerated natural attenuation involves an intervention in the contaminated area to improve conversion rates of radioactive waste, which tend to be slow. There are two variants: biostimulation and bioaugmentation.
  Biostimulation is the addition of nutrients with trace elements, electron donors or electron acceptors to stimulate activity and growth of natural indigenous microbial communities. It can range from simple fertilization or infiltration (called passive biostimulation) to more aggressive injections to the ground, and is widely used at US DOE sites. Nitrate is used as nutrient to biostimulate the reduction of uranium, because it serves as very energetically favorable electron acceptor for metal-reducing bacteria. However, many of these microorganisms (Geobacter, Shewanella or Desulfovibrio) exhibit resistance genes to heavy metals that limit their ability to bioremediate radionuclides. In these particular cases, a carbon source such as ethanol is added to the medium to promote the reduction of nitrate at first, and then of uranium. Ethanol is also used in soil injection systems with hydraulic recirculations: it raises the pH and promotes the growth of denitrifying and radionuclide-reducing bacteria, that produce biofilms and achieve almost 90% decrease in the concentration of radioactive uranium.
  A number of geophysical techniques have been used to monitor the effects of in situ biostimulation trials including measurement of: spectral ionization potential, self potentials, current density, complex resistivity and also reactive transport modelling (RTM), which measures hydrogeological and geochemical parameters to estimate chemical reactions of the microbial community.
  Bioaugmentaton, on the other hand, is the deliberated addition to the environment of microorganisms with desired traits to accelerate bacterial metabolic conversion of radioactive waste. They are often added when necessary species for bioremediation do not exist in the treatment place. This technique has shown in field trials over the years that it does not offer better results than biostimulation; neither it is clear that introduced species can be distributed effectively through the complex geological structures of most subsurface environments or that can compete long term with the indigenous microbiota.