**5. Potential issues with additives use in bioremediation**

Bioremediation has been demonstrated to be an effective technology for degrading chlorinated solvents and fuel hydrocarbons and has been used widely in groundwater and soil remedia‐

**Figure 3.** Schematic of a permeable reactive barrier (PRB) composed of organic carbon based materials (biowall) for bioremediation of contaminated groundwater (Adapted from [15])

tion. A number of issues associated with additives use in bioremediation have been encoun‐ tered and need to be taken into consideration when considering bioremediation at a site.

## **5.1. Biofouling**

*4.3.3. Trenching*

improve performance [23-24].

Solid substrate treatment systems are typically deployed in an excavated trench in the form of a permeable reactive barrier (PRB, e.g., biowall) (Figure 3). This treatment method relies on the natural flow of groundwater through solid substrates within the PRB to promote contact with slowly released soluble organic matter. This method is particularly suitable for sites with low permeability. Sometimes, perforated pipes can be laid on the top or bottom of the organic fill material to amend the organic fill material with liquid substrates or other amendments to

Trenches may be installed using either continuous one-pass trenchers designed for installing subsurface utilities or hydraulic excavators. Trench depths are limited by type of equipment used, stability of the formation, and ability of the equipment to excavate the formation.

Bioremediation has been demonstrated to be an effective technology for degrading chlorinated solvents and fuel hydrocarbons and has been used widely in groundwater and soil remedia‐

Continuous trenching is not practical in hard and consolidated bedrock [23, 27].

**5. Potential issues with additives use in bioremediation**

**Figure 2.** Schematic of a recirculation system in bioremediation (Adapted from [15])

156 Advances in Bioremediation of Wastewater and Polluted Soil

Biofouling refers to failing or decreasing effectiveness of a remediation system due to excessive growth of biomass of non-targeting microorganisms. Biomass buildup in wells could decrease fluid injection rates. Biofouling of injection or recirculation wells has been observed at several sites due to growth of biomass or biofilms within well screens and surrounding sand packs. Biofouling in injection wells may impact the ability to effectively inject and distribute substrate.

Several approaches have been used to prevent or mitigate the biofouling issue. Preventive measures typically include well rehabilitation techniques such as surging and pumping, high pressure jetting, pulsed injection, injection of carbon dioxide under pressure, use of a clean water push to remove substrate residue, chemical methods such as surging and scrubbing with hydrogen peroxide or use of non-oxidizing biocides (e.g., Tolcide®) [16,25,39].

#### **5.2. Stalling**

Stalling in bioremediation refers to the incomplete degradation of targeted compounds due to accumulation of degradation daughter products. Anaerobic reductive bioremediation of PCE and TCE, for example, may undergo incomplete degradation to DCE or VC resulting in the accumulation of these daughter products. Stalling has been observed in a number of scenarios:


#### **5.3. System bypassing or short circuiting**

System bypassing or short circuiting is a common issue with in situ remedial technologies that rely on injection and distribution of amendments within the subsurface. Aquifer permeability and preferential pathways are two major factors that may impact distribution of substrates throughout a DNAPL source zone.

Short circuiting of substrates in the vadose zone may occur during the injection of liquid substrates. Whether injections are conducted through permanent wells or by direct-push methods, injection pressures must remain relatively low to avoid unintentionally fracturing the formation. Injection pressures greater than the overburden pressure may cause fracturing of the aquifer formation. This may lead to preferential flow of substrates along open fractures resulting in nonuniform distribution and short circuiting.

Another factor that can result in substrate bypassing is aquifer heterogeneity. Any liquid substrate, including aqueous substrate mixtures, will migrate along the pathway of least resistance (highest permeability). In heterogeneous systems, substrate distribution may bypass lower permeability aquifer zones. In these situations, multiple well points screened in each lithologic layer or zone may be required to avoid short circuiting of substrate to higher permeability zones.

## **5.4. Reduction of hydraulic conductivity**

and TCE, for example, may undergo incomplete degradation to DCE or VC resulting in the accumulation of these daughter products. Stalling has been observed in a number of scenarios: **1.** Reductive dechlorination is most effective and efficient under sulfate-reducing to methanogenic conditions. In attempts to increase substrate utilization and reduce competition for hydrogen (e.g., by methanogenesis), hydrogen concentrations are controlled in some remediation systems. This may result in significant portions of the treatment zone remaining insufficiently reducing for complete dechlorination to occur, leading to "stalling" at intermediate degradation products such as *cis*-DCE or VC [24,

**2.** Microorganisms generally gain more energy from dechlorination of more highly chlori‐ nated contaminants such as PCE and TCE. Dechlorination of daughter products such as DCE and VC may not start until after parent products are sufficiently depleted [23].

**3.** *Dehalococcoides mccartyi* is the only known bacteria that can degrade chlorinated ethenes completely [41-42]. However, this bacterium may not be present at populations required to achieve complete dechlorination. Limitations in microbial populations create the potential for incomplete degradation and the buildup of daughter products such as *cis*-

**4.** Co-contaminants or other geochemical conditions such as low pH, inadequate electron donor availability, or unfavorable geochemical conditions may inhibit microbial popula‐

System bypassing or short circuiting is a common issue with in situ remedial technologies that rely on injection and distribution of amendments within the subsurface. Aquifer permeability and preferential pathways are two major factors that may impact distribution of substrates

Short circuiting of substrates in the vadose zone may occur during the injection of liquid substrates. Whether injections are conducted through permanent wells or by direct-push methods, injection pressures must remain relatively low to avoid unintentionally fracturing the formation. Injection pressures greater than the overburden pressure may cause fracturing of the aquifer formation. This may lead to preferential flow of substrates along open fractures

Another factor that can result in substrate bypassing is aquifer heterogeneity. Any liquid substrate, including aqueous substrate mixtures, will migrate along the pathway of least resistance (highest permeability). In heterogeneous systems, substrate distribution may bypass lower permeability aquifer zones. In these situations, multiple well points screened in each lithologic layer or zone may be required to avoid short circuiting of substrate to higher

tions and lead to incomplete degradation of chlorinated solvents.

40].

DCE or VC.

permeability zones.

**5.3. System bypassing or short circuiting**

158 Advances in Bioremediation of Wastewater and Polluted Soil

resulting in nonuniform distribution and short circuiting.

throughout a DNAPL source zone.

Reduction in hydraulic conductivity may cause contaminated groundwater to flow around the treatment zone, impacting the ability of the targeted compound(s) to come into contact with the organic substrate and also impacting the ability to effectively further distribute soluble organic substrates. During enhanced bioremediation, hydraulic conductivity may be adverse‐ ly impacted by several factors:


In the case of solid substrates used in trenches, permeability of solid substrate mixtures must remain equal to or higher than that of the surrounding formation. In practice, coarse sand and pea gravel are usually mixed with the substrate to maintain a sufficiently high permeability.

#### **5.5. Displacement and dilution**

In groundwater remediation, some mixing of injected substrate with contaminated ground‐ water will occur due to advection and dispersion. In order to achieve widespread distribution of substrates, injecting large volumes of soluble substrate mixed with potable water is necessary in some cases. However, injection of large volumes of substrate may cause significant displacement and dilution of the contaminant plume. One way to address the displacement and dilution issue is to inject a low volume/high concentration substrate mixture and to rely on advection and dispersion for mixing, although this requires relatively high rates of advection and dispersion to occur. The use of recirculation systems can aid in avoiding this issue.

#### **5.6. pH and secondary water quality issues**

When applying organic carbon in bioremediation, caution is required to not supply too much substrate to the subsurface because excess organic substrate generates organic acids and causes decreases in the pH of groundwater. In addition, anaerobic reductive dechlorination generates HCl that could also decrease groundwater pH.

A decrease in pH to the acidic range could potentially mobilize metals (notably iron, manga‐ nese) and metalloids (arsenic), creating secondary water quality issues at a site. The release of these metals is also impacted by the prevailing redox conditions at a site. A decrease in pH could also inhibit growth of bacteria communities such as *Dehalococcoides*, thereby stopping the bioremediation process.

Changes in redox conditions can also enhance solubilization of metals and promote the formation of the following undesirable products (e.g., hydrogen sulfide and methane gases):

