man-made aquifer in laboratory standard when a fungus was applied as a biological
barrier. They reported that the fungus can be used as a barrier (permeable reactive
barrier) in natural aquifers. When a permeable reactive barrier was applied to treat
the contaminated groundwater by chlorinated solvents, the main constraint was the
carbonate precipitated in the iron zone rather than the velocity precise measurement
of groundwater. For the performance of permeable reactive barrier, maintaining
barrier reactivity and preserving the permeability of barrier are crucial for its success
and can be achieved via retaining the suitable size distribution of particles (Vogan
et al. 1999; Mumford et al. 2014). The most critical challenges in this method are
decrease in barrier reactivity, zero-valent iron, inability to use the method on
contaminated site for some recalcitrant compounds, reduction of porosity, and
chlorinated hydrocarbons. Polyhydroxybutyrate as a biodegradable polymer showed
a slow-release nutrient capability (carbon capacity) and can improve the biological
activity as a barrier and enhance the treatment of chlorinated compounds (Baric et al.
2014). Some studies (Henderson and Demond 2007) have showed that change in
climate conditions can make a complicated hydrogeological site characterization and
flaws of design can decrease the success of this method. The improved designs of
permeable reactive barrier and developed cost-effective techniques for site characterization enhance the efficiency of the method (Gibert et al. 2013). Moreover, to
remove the challenges like lack of permeability under the given geological situations, which are related to using of zero-valent iron, the use of iron sulfide (FeS)
barrier is useful (Henderson and Demond 2007).
Natural Attenuation
Natural attenuation, which is called intrinsic bioremediation, is an in situ
biotreatment method that includes passive treatment of contaminated sites, through
an absence of external forces (human intervention). The technique involves both
anaerobic and aerobic processes to biodegrade pollutions including those of recalcitrants (Azubuike et al. 2016). The lack of external force means that the method is
not costly as compared to other in situ methods. Based on the US National Research
Council, there are some challenges including proof of pollutions loss from polluted
sites, proof according to the analyses to show that the isolated microorganism from
polluted sites has the inherent potentials for biodegrading or transforming the present
pollutions at that polluted site, and proof of realization of biodegradation potentials
on-site (Philp and Atlas 2005). Adetutu et al. (2015) studied chlorinated compounds
and compared the efficiency different bioremediations including natural attenuation
to dechlorination of polluted groundwater by trichloroethene. They found a good
decrease in the concentration of trichloroethene, less than stipulated by the US
Environmental Protection Agency. Moreover, biodegradation is the principal mechanism of contaminant removal during natural attenuation. The disadvantage of this
method is that the intrinsic bioremediation takes more time to get the aimed
concentration level of contaminants. It was also reported that it does not result in
enough removal of polycyclic aromatic hydrocarbons and the corresponding
decrease in contaminated soil eco-toxicity (García et al. 2014).
474
M. Fatehi et al.
barrier. They reported that the fungus can be used as a barrier (permeable reactive
barrier) in natural aquifers. When a permeable reactive barrier was applied to treat
the contaminated groundwater by chlorinated solvents, the main constraint was the
carbonate precipitated in the iron zone rather than the velocity precise measurement
of groundwater. For the performance of permeable reactive barrier, maintaining
barrier reactivity and preserving the permeability of barrier are crucial for its success
and can be achieved via retaining the suitable size distribution of particles (Vogan
et al. 1999; Mumford et al. 2014). The most critical challenges in this method are
decrease in barrier reactivity, zero-valent iron, inability to use the method on
contaminated site for some recalcitrant compounds, reduction of porosity, and
chlorinated hydrocarbons. Polyhydroxybutyrate as a biodegradable polymer showed
a slow-release nutrient capability (carbon capacity) and can improve the biological
activity as a barrier and enhance the treatment of chlorinated compounds (Baric et al.
2014). Some studies (Henderson and Demond 2007) have showed that change in
climate conditions can make a complicated hydrogeological site characterization and
flaws of design can decrease the success of this method. The improved designs of
permeable reactive barrier and developed cost-effective techniques for site characterization enhance the efficiency of the method (Gibert et al. 2013). Moreover, to
remove the challenges like lack of permeability under the given geological situations, which are related to using of zero-valent iron, the use of iron sulfide (FeS)
barrier is useful (Henderson and Demond 2007).
Natural Attenuation
Natural attenuation, which is called intrinsic bioremediation, is an in situ
biotreatment method that includes passive treatment of contaminated sites, through
an absence of external forces (human intervention). The technique involves both
anaerobic and aerobic processes to biodegrade pollutions including those of recalcitrants (Azubuike et al. 2016). The lack of external force means that the method is
not costly as compared to other in situ methods. Based on the US National Research
Council, there are some challenges including proof of pollutions loss from polluted
sites, proof according to the analyses to show that the isolated microorganism from
polluted sites has the inherent potentials for biodegrading or transforming the present
pollutions at that polluted site, and proof of realization of biodegradation potentials
on-site (Philp and Atlas 2005). Adetutu et al. (2015) studied chlorinated compounds
and compared the efficiency different bioremediations including natural attenuation
to dechlorination of polluted groundwater by trichloroethene. They found a good
decrease in the concentration of trichloroethene, less than stipulated by the US
Environmental Protection Agency. Moreover, biodegradation is the principal mechanism of contaminant removal during natural attenuation. The disadvantage of this
method is that the intrinsic bioremediation takes more time to get the aimed
concentration level of contaminants. It was also reported that it does not result in
enough removal of polycyclic aromatic hydrocarbons and the corresponding
decrease in contaminated soil eco-toxicity (García et al. 2014).
474
M. Fatehi et al.