9: Monitored Natural Attenuation Flashcards
Describe monitored natural attenuation. Explain the mechanisms that contribute to it.
Monitored natural attenuation (MNA) is regular sampling, testing and documenting the performance of a variety of natural physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation, dispersion, dilution, sorption, chemical reaction, volatilization and sorption.
Explain how MNA is different than doing nothing.
MNA requires a regular sampling, testing and documenting plan to verify the degradation of target contamination by natural processes, and is accountable to stakeholders. Doing nothing does not monitor or verify the contamination so it is free to spread or remain undetected.
Describe the advantages and disadvantages of MNA compared to more active treatment technologies.
MNA can be significantly less costly than active treatment technologies, but may not be applicable to some sites and/or contaminants.
Describe all the lines of evidence for MNA in your own words.
Documented mass loss of contaminant
This evidence involves developing a model accounting for site conditions and the initial contaminant mass (concentration x area x specific discharge). Routine sampling of monitoring wells located throughout the suspected plume monitor any change in contaminant concentration and calculated mass.
Geochemical fingerprints showing operation of biodegradation
Sampling for background and plume levels of terminal electron acceptors (O2, NO3, Fe(III) SO4, etc) and products of election reducers (Fe(II), and CH4) to show secondary evidence bacterial growth is occurring at different TEA levels.
Microcosm studies that show direct evidence of biodegradation
A field sample of a contaminated area is taken and microbiological tests are conducted to identify bacteria activity in a lab to simulate in situ conditions.
Explain why additional sampling often required beyond that for a standard site assessment is necessary for the site assessment and monitoring of MNA.
Additional sampling is often be required due to a contamination plume extending beyond the standard site assessment area. Monitoring of the entire contaminant plume is necessary to understand the natural attenuation process of the contaminant.
Perform assimilative capacity calculations on polluted aquifers.
Assimilative capacity (AC) estimates the concentration of a compound that can be theoretically degraded by natural processes. AC is computed as follows
AC = sum(R*[Cplume-Cbase])
Explain why the assimilative capacity of an aquifer is difficult to calculate for reductive dehalogenation or halorespiration.
Dehalogenation is most effective in the sulfate reducing and methanogenic conditions. These conditions occur after oxygen reducing and nitrate reducing processes and sampling results may only be available low concentrations spread over a wide area that makes it difficult to capture the full contaminant plume.
