MONITORED NATURAL ATTENUATION

MNA (also “intrinsic or passive” remediation) may be defined as naturally occurring physical/ chemical/ biological process/es that reduce concentration & mass of polluting substances in groundwater (DEC, 2004) and soil.  MNA is a process of field monitoring & laboratory analysis to confirm that the remediation is occurring as expected.  MNA processes can include:
• Biodegradation (consumption by microbes & conversion into less/ non-harmful substances)
• Physical processes (e.g. adsorption (soil adherence), evaporation etc)
• Chemical processes (e.g. hydrolysis, precipitation, cation exchange, oxidation/ reduction, aerobic/ anaerobic degradation, fermentation/ reductive dechlorination/  dehydrohalogenation)
• Dilution (mixing with unpolluted water)- non-destructive!.

However, MNA must:
• Demonstrate a reduction in mass/ concentration/ toxicity occurs.
• Be sufficiently rapid to prevent impact on receptors & minimise plume expansion
• Achieve remedial goals within one generation, ~30 years.
• Include monitoring over 2-3 years+ that MNA is occurring
• A contingency plan if MNA is not achieving remedial goals.

Typically MNA is part of a remedial strategy which includes removal of the source (e.g. leaking tanks/ PSH on the water table) & may be used with other active strategies (e.g. air sparging/ vacuum extraction/ oxidation agents etc)       
 

A Four Stage Process (WA DEC):

1) Screening Stage (Assess Viability)
• A soil & groundwater investigation establishing contaminant source, extent/ nature of the plume, aquifer characteristics & groundwater geochemistry
• Identification of sensitive gw receptors
• A feasibility assessment of MNA for the identified site characteristics, (e.g. Low to High)

2) Demonstration Stage (quantitative demonstration of the rate of MNA)
• (Primary evidence): historical monitoring data of concentration decreases with time/ distance
• (Secondary): geochemical mass balance assessment and/or observed data for a shrinking or stable plume.
• (Tertiary): Laboratory demonstration of degradation processes. 
This may require additional site investigation work.

3) Assessment Stage (Is MNA an acceptable remedial option?)
• Identify receptors to be protected, remedial targets to be achieved, & a remedial timescale)
• Modelling of the MNA process/es & aquifer environment, including predictive scenarios
• Reporting of the assessment results to DEC

Following acceptance of MNA as a suitable remedial option:

4) Implementation 
• Design of a monitoring program (Sampling & Analysis Plan)
• Monitoring reports submitted to DEC
• Recommendations whether remedial goals are achieved, or on-going monitoring is required.

Adopted from DoE 2004 (Use of MNA for Gw Remediation)
 
Advantages & Disadvantages:

Advantages:
• Transformation into innocuous by-products (e.g. CO2, ethene, H2O)
• Relatively non-intrusive, allowing ongoing use of infrastructure
• Uses natural process/es
• No waste production
• Lower energy use/ emissions

Disadvantages:
• Longer time-frames to remediate
• Longer monitoring periods
• Complex site characterisation
• Affected by environmental changes (e.g. renewed mobility)
• Secondary toxicity (of transformed chemicals, e.g. TCE>VC)
 

Typical Applications:
• Hydrocarbon contaminated sites (e.g. service stations/ underground storage tanks/ fuel farms, for TRH, BTEX, PAHs)- Chlorinated hydrocarbons (e.g. solvents)
• Landfill leachate impacts (e.g. NVOCs, NH4, Cl, phenols, OCs, PAHs etc)
• Non-point sources (e.g. OCPs)

Useful References:
www.claire.co.uk (SIReN)
www.sandia.gov/eesector/gs/gc/na/mnahome.html (US government MNA Toolbox)

Stuart Jeffries
HydroSolutions Pty Ltd

Comments are closed.