6.1 Microbial Bioremediation and Removal of Groundwater Contaminants

Microbial bioremediation has become a widely used technology that is viewed as a more sustainable and economical approach compared to other treatment methods (e.g., pump and treat systems). It has been successfully implemented in many cleanup operations involving groundwater contaminated with organic pollutants such as petroleum hydrocarbons and chlorinated organic solvents as well as management and control of leachate pollution plumes from landfills (USEPA, 1998, 2002, 2013). Petroleum hydrocarbons contaminants span a wide range of reduced organic chemical substances, including BTEX compounds (benzene, toluene, ethyl benzene, and xylene) that are typically used as electron donors in microbial metabolism. The opposite is true of chlorinated solvents, such as carbon tetrachloride (PCE) and trichloroethene (TCE), which are highly oxidized and metabolized as electron acceptors.

Three different strategies are used in groundwater bioremediation. These include:

  • natural attenuation – where the autochthonous (natural) microbial community is left to eliminate the target contaminant without human intervention, relying on physical, chemical, and microbiological processes that occur naturally within a contaminant plume;
  • biostimulation – where the natural microbial community is stimulated to eliminate the target contaminant by the addition of essential nutrients; and
  • bioaugmentation – where, in addition to nutrients, select strains of bacteria may be injected into the subsurface to promote the elimination of the target contaminant.

The success of these strategies depends on the availability of appropriate electron acceptors and electron donors, whether the target contaminant is reduced (e.g., petroleum hydrocarbons) or oxidized (e.g., chlorinated solvents), and groundwater flow rates are slow enough to allow for degradation to occur (Christensen et al., 2000).

Aerobic biodegradation of reduced organic contaminants takes place in the presence of oxygen (Haritash and Kaushik, 2009; Bamforth and Singleton, 2005). This means the success of aerobic bioremediation is directly dependent on the availability of oxygen. If oxygen becomes limiting, as it often does in groundwater systems, it can be supplied directly to the subsurface by air sparging or through injection of a chemical oxidant (e.g., hydrogen peroxide) that decomposes to release oxygen.

In the absence of oxygen, anaerobic biodegradation may proceed using electron acceptors such as nitrate or sulfate for metabolic oxidation of a contaminant. This approach is widely employed at petroleum hydrocarbon contaminated sites where oxygen has been depleted (Chandra et al., 2013; Meckenstock et al., 2016; Varjani and Upasani, 2017). To stimulate anaerobic biodegradation, an amendment that contains an electron acceptor such as sulfate may be added to promote microbial degradation of petroleum hydrocarbons (USEPA, 2013).

Anaerobic conditions are also needed for the initial dechlorination of highly chlorinated organic solvents such as PCE and TCE, which serve as electron acceptors instead of electron donors (Hopkins et al., 1993; Meckenstock et al., 2015). As these compounds are metabolized, chloride atoms are removed and replaced by hydrogen atoms to form products (conjugate reductants) that are less oxidized than the original chlorinated compound. In some cases, the products are not sufficiently oxidized to serve as electron acceptors and can only be degraded further as electron donors in aerobic respiration. For example, PCE and TCE are highly oxidized and only undergo partial reductive dechlorination to less oxidized dichloroethane (DCE), vinyl chloride (VC), and chloroethane (CE) (Mohn and Tiedje, 1992; Kielhorn et al., 2000). Subsequent oxidation of DCE, VC, and CE to non-toxic forms ethylene, ethane, or ethanol requires aerobic conditions (e.g., Semprini et al., 1990; Semprini and McCarty, 1991; Hopkins et al., 1993).

Microorganisms can be used to remove inorganic contaminants such as nitrogenous nutrient compounds (e.g., nitrate or ammonia) that exist in groundwater in amounts above regulatory guidelines, as well as redox active toxic metals and metalloids. However, unlike organic contaminants that microbes can degrade by oxidation to carbon dioxide or nutrients that can be taken up and assimilated during metabolism, detoxification of metal and metalloid contaminants is accomplished through dissimilatory metabolic processes that remove or provide electrons for cellular energetics (Lovely and Coates, 1997; USEPA, 2013). Common applications include microbial reduction of Cr(VI), U(VI), and Se(VI) to more insoluble oxidation states: Cr(III), U(IV), and elemental Se, respectively. Other metals, such as Fe(II) or Mn(II), are subject to microbial oxidation and precipitation of insoluble oxides (Figure 21).

Photograph showing extensive precipitation of orange-colored insoluble hydrous ferric oxides by Fe(II)-oxidizing bacteria in a groundwater discharge zone near Deep River, Ontario, Canada.

Figure 21  Extensive precipitation of orange-colored insoluble hydrous ferric oxides by Fe(II)-oxidizing bacteria in a groundwater discharge zone near Deep River, Ontario, Canada.


Groundwater Microbiology Copyright © 2021 by F. Grant Ferris, Natalie Szponar, and Brock A. Edwards. All Rights Reserved.