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An On-Line Version of a Column First Published in:
Environmental Technology May/June 1997 Vol. 7 No. 3 Pages 28-29

by: David B. Vance

Bioremediation was initially utilized as an active aggressive process, consisting of pump and treat technology used in conjunction with the injection of nutrients and sources of oxygen. More recently, it has become a critical component of the natural attenuation process. Biological Natural Attenuation (BNAT) (1), (2) is not a "free lunch" for those responsible for groundwater contamination. Assessment procedures must be completed that will support the reliance upon the BNAT process by: proving natural attenuation is taking place; defining the site specific natural attenuation mechanisms; quantifying the kinetics of the process and providing an estimated time for clean-up; and insuring that the site specific conditions will not allow unacceptable off-site migration of the contaminant plume. Given the above information a governing regulatory body can support a decision for a BNAT solution. The purpose of this column is to examine conditions under which the biological component of natural attenuation is inhibited or terminated, due to toxicity from heavy metals or certain hydrocarbons from: the impacting contaminants; other compounds associated with the contaminants (at non-regulated levels); or compounds associated with the native groundwater.

The effect of toxicity on the BNAT process is seen in three areas:

  • One, is simply inhibition of activity. The microbial population continues to function, but at a rate that is slower than what would occur under normal conditions.
  • Second, is a complete cessation of activity. The bacteria are killed outright, or more commonly they enter into a vegetative static state in which they are metabolically in- active. However, if the offending toxic material is removed from the system, bioactivity can resume.
  • Third is more subtle, but particularly important to the BNAT process, interference with degradational activity that is specifically focused on a contaminant of concern. The overall microbial activity is not inhibited, just degradation of the contaminant. This problem is more often seen in systems where xenobiotic hydrocarbons are the targets of co-metabolically generated degradation enzymes.

 To evaluate the impact of toxicity it is important to understand the mechanisms by which substances are toxic to microorganisms or interfere with the degradation process. The following bacteria cell components or processes are sensitive to toxic effects from the listed compounds:

  • The bacterial cell wall can be totally destroyed by heavy metals, phenols, and alcohols.
    • The permeability of the cell wall can be fatally increased by phenolic compounds, alcohols, detergents, and quaternary ammonium compounds.
  • The alteration of interior proteins (destroying their cellular function) is caused by halogens, phenols, and alcohols.
  • Interference with the action of intercellular enzymes will stop metabolic activity. Compounds toxic via this effect include: cyanide, strong oxidants (chlorine etc.), phenols, metals and metalloid's.
  • Interference with nucleic acid synthesis can prevent bacterial reproduction, heavy metals can act in this fashion.

 Environmental conditions also play a role in the impact that toxic materials have, particularly for metals. Toxicity of metals is governed by concentration, chemical form of the metal, pH and Eh of the environment, the type of microbial system involved (i.e. aerobic or anaerobic), and potential for adaptation of the bacteria. Toxic forms of metals include soluble salts and anionic complexes. Insoluble salts, oxides, or even elemental forms are not necessarily toxic. The best method to evaluate groundwater for the presence of potentially toxic metals is to collect and test filtered water samples.

  • The effect of pH is two-fold.
      • First, bacteria are sensitive to pH conditions. Most bacteria that are indigenous to groundwater systems and functional in the BNAT process thrive in the pH range of 6.3 to 7.5. A pH lower than 5.0 is toxic from the pH effects alone.
      • Lastly, lower pH will cause an increase in soluble metal concentrations, including compounds native to the aquifer.
  • The impact of redox potential is largely on the consortia of bacteria that will be active and the electron acceptor used by the system. Almost any range of Eh conditions has the potential to offer effective biodegradation. However, some anaerobic systems are more sensitive to toxic compounds.

 This column will end with a listing of concentrations at which certain compounds become toxic. It was collected from a survey of multiple literature sources. It is a general guide since site specific: environmental conditions; bacterial consortia; and bacterial adaptability can cause a wide variance of effect. In instances where a concentration range is given, the low value represents inhibition of microbial activity, and the high value represents a concentration that will kill or stop microbial activity.

Lastly, this list is not exhaustive with regards compounds that can be toxic to bacteria. It is focused on chemicals that may be commonly found at contaminated sites.

  • Copper 0.01 to 20 mg/L
      • Copper is probably the worst heavy metal with regards to impact on the BNAT process. It is extremely toxic, inhibitory at low concentrations, and can interfere with the co-metabolic degradation of chlorinated solvents at the lowest end of this range.
  • Zinc 0.3 to 10 mg/L
  • Cadmium 0.1 to 20 mg/L
  • Chromium 25 mg/L - Inhibition
  • Nickel 25 mg/L - Inhibition
  • Lead 900 mg/L - Inhibition
  • Cobalt 0.3 to 10 mg/L
  • Mercury 0.01 to 20 mg/L
  • Methanol 90 mg/L - Inhibition
  • Isopropanol 55 mg/L - Inhibition
  • Acetone 75 mg/L - Inhibition
  • Pentachlorophenol 1 to 200 mg/L
  • TNT 10 to 100 mg/L Cyanide 10 to 150 mg/L
  • Petroleum Hydrocarbons
      • The components of common petroleum hydrocarbons become inhibitory when soluble concentration ranges reach 200 to 500 mg/L. The upper range for lethal toxicity verges on free product, in which electron acceptor transport is more an issue than chemical toxicity.
  • Chlorinated Hydrocarbons
      • Most chlorinated solvents become inhibitory when soluble concentrations are in the 100 to 500 mg/L range. They become toxic at concentrations around 1,000 mg/L, due chemical attack on cell wall lipids.

 An assessment screen for compounds toxic to bacteria may be helpful when presenting arguments in support of natural attenuation. If found, it may be necessary to perform some level of bench scale testing to quantify the toxic effect of suspect compounds. This assessment process is still a much less expensive procedure than the typical "pump and treat" active groundwater remediation project that we have relied on in the past.

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