GROUNDWATER AND LEACHATE: IN-SITU BIOLOGICAL TREATMENT
Natural Attenuation
Natural Attenuation ("No Action")
Essentially an in situ biological remediation as the apt nutrients, moisture content, temperature and oxygen can all occur naturally within the ground.
**No action means no cost, no addition of harmful chemicals, no pollution and no machinery.** The process would be monitored until contaminant concentrations had been reduced to acceptable levels.
These native microorganisms would simply reproduce by themselves and reduce the concentration of contamination in the appropriate environment...e.g. if there is no contaminant movement (zero plume growth despite diffusion, dilution or dispersion otherwise occurring) the contaminants are being bio-degraded.
Note:
Evidence for natural attenuation:
Uses:
Advantages:
Disadvantages:
For further information regarding: technical protocols, lines of evidence, site characterisation & modelling, remedial objectives and duration, contaminant fate and transport modells, monitoring strategy, performance criteria and the contingency plan, visit:
http://www.eugris.info/FurtherDescription.asp?e=27&Ca=2&Cy=0&T=Monitored%20Natural%20Attenuation
**No action means no cost, no addition of harmful chemicals, no pollution and no machinery.** The process would be monitored until contaminant concentrations had been reduced to acceptable levels.
These native microorganisms would simply reproduce by themselves and reduce the concentration of contamination in the appropriate environment...e.g. if there is no contaminant movement (zero plume growth despite diffusion, dilution or dispersion otherwise occurring) the contaminants are being bio-degraded.
Note:
- Natural attenuation is not a "technology" per se.
- Requires modelling and evaluation of the contaminant degradation rates that would occur (especially when plume is still expanding/migrating) to ensure bio-degradation would occur quickly enough and reduce the contaminant concentrations to below regulatory standards.
- Long term monitoring must be conducted throughout to confirm degradation rates are meeting the cleanup objectives.
- Significant research and on-going debates about its use at hazardous waste sites. The benefits are recently being realised and its use is still evolving.
Evidence for natural attenuation:
- Stable or shrinking contaminant plume – zero plume growth (net effect) and no movement through the ground shows natural attenuation to be acting at the same rate as contaminant flow
- Shorter plume than expected
- Low levels of Oxygen, Nitrate or Sulphate – depletion of these electron acceptors show the process is occurring as they are naturally used during the bioremediation process (especially is below the WT, where anaerobic bacteria gets all of its oxygen/water from existing chemicals). E.g. bioremediation of BTEX/non-chlorinated compounds would result in lower levels of Oxygen
- Presence of metabolic by-products – e.g. Ferrous Iron, with less Iron content than its Ferrite Iron
- Presence of active, heterophobic bacteria (non-self sustaining) – these prove bio-remediation because they only occur as part of the bio-remediation activity.
Uses:
- VOCs, SVOCs and fuel hydrocarbons are commonly evaluated for natural attenuation.
- Some pesticides also can be allowed to naturally attenuate - but generally less effective.
- Only if natural attenuation processes results in a change in the valence state of the metal would it results in immobilisation of a metal contaminant (e.g., chromium) (no actual treatment)
Advantages:
- Less generation & transfer of waste.
- Less intrusive (only ground monitoring wells required).
- May be applied to part/or all of a contaminated area (depending on site conditions, cleanup objectives and allowable treatment time) e.g. can be used with or as a 'polish' treatment after other (active) remedial measures.
- Generally lower cost than active remediation - just for modelling (whether feasible) & performance monitoring (until sufficient contaminant levels have been reached).
Disadvantages:
- The process may be too slow if require rapid remediation, or have fast groundwater flow.
- More education & communication efforts are required to gain public acceptance of MNA.
- Toxicity and/or mobility of contaminant may be too great.
- Long-term, more extensive performance monitoring reqd.
- Longer time to achieve clean-up objectives. Typically requires several years.
- Site characterisation (modelling/evaluation) may be more complex and costly.
For further information regarding: technical protocols, lines of evidence, site characterisation & modelling, remedial objectives and duration, contaminant fate and transport modells, monitoring strategy, performance criteria and the contingency plan, visit:
http://www.eugris.info/FurtherDescription.asp?e=27&Ca=2&Cy=0&T=Monitored%20Natural%20Attenuation