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Monitored Natural Attenuation Process variations & synonyms In situ/ex situ Natural attenuation Enhanced natural attenuation Intrinsic (bio)remediation

7.2.11 Natural attenuation


Soil/groundwater/vapour In situ Risk management role Outcome

Typically in pathway management, may also remove source Combination of destruction, dispersal and stabilisation

OUTLINE Under certain conditions, the risk presented by subsurface contamination may decrease as a result of natural processes. Monitored natural attenuation acknowledges the existence of such processes and monitors their effects to confirm that risk is likely to decrease within a specified time scale. Risk reduction may be as a result of a decrease in contaminant concentration, volume, mobility or toxicity. Processes include dispersion, dilution, sorption, volatilisation, biodegradation, chemical or biological stabilisation, transformation or destruction of contaminants. The contaminants remain in place whilst these processes take place. MNA is not a ‘do nothing’ option; there is a need to demonstrate likely and actual effectiveness at all stages. Extensive site investigation is required to characterise: the geological and groundwater conditions; contaminant characteristics (types, distribution, concentration, mobility); and degradation potential of subsurface geochemical conditions (e.g. redox potential (Eh), pH, presence of electron acceptors). Lines of evidence are used to demonstrate natural attenuation include plume status (shrinking/ stable/expanding); geochemical concentrations (e.g. contaminants, daughter products, electron acceptors, dissolved oxygen and direct laboratory data). Long-term monitoring is required to prove efficacy of processes and warn if identified receptors are likely to be impacted. Natural attenuation can be enhanced by creation of oxygenated zones to increase area and rate of aerobic biodegradation, or by creating highly reducing conditions for halogenated organics using techniques like in situ flushing (Table 7.2.4) PRBs (Table 7.2.5) or redox amendments (Table 7.2.6). Risk management may be a combination of several of these techniques, and a zone of enhanced natural attenuation downstream in the aquifer. Natural attenuation may also be used as ‘pathway management’ in tandem with an intervention for source reduction. STRENGTHS • Cost of site investigation & long-term monitoring is likely to be considerably cheaper than installation & operation of many remediation options • Potential high environmental merit due to low inputs and often transformation of contaminants into innocuous byproducts • Limited intrusive work required; site can be put to productive use • May be used in conjunction with, or after, other remedial measures • Can be used to manage risks from contaminants (e.g. DNAPLs) which are hard to remove

WEAKNESSES • May take long time to reach remediation goal • Requires considerable site investigation • Needs long-term monitoring (years/decades) • Subsurface conditions may change, resulting in renewed mobility of contaminants • Potential for continued contamination and cross-media transfer of contaminants • Heterogeneity of subsurface geological and geochemical conditions may make site characterisation and prediction of natural attenuation difficult • Some intermediates or by-products may be as or more toxic than the original contaminants • Rates of attenuation may drop as natural capacities are exceeded

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Organic cyanides Gravel >2 mm








? e

Pesticides/herbicides Dioxins/furans


Volatile metals


Non-volatile metals






Cyanides Corrosives Asbestos


Organic corrosives




Halogenated semivolatile Non-halogenated volatile Non-halogenated semivolatile



Halogenated volatile


APPLICABLE CONTAMINANTS AND MATERIALS – Outcome: combination of destruction, dispersal and stabilisation

Oxidisers Reducers

Sand 0.06–2 mm

Silt 2–60 µm Clay <2 µm Peat g ? ? ? Key: Usually applicable; Potentially applicable; ? May be applicable; not treatable; may worsen situation. See Tables 4.9.2 and 4.9.3 for contaminant classifications. Notes

a. b. c. d. e. f. g.

Efficacy of MNA for chlorinated solvents depends on the conditions at the site Primarily BTEX, also others, e.g. acetone Some will attenuate, e.g. phenols, methanol Some will attenuate, e.g. acetic acid Old chlorinated types resist degradation. More modern ones tend to attenuate Cannot be destroyed, but in some cases can be immobilised Contaminants tend to adsorb on to the clay matrix and be temporarily immobilised. However, these contaminants may be released slowly, with the clay acting as a long-term source (albeit at reduced concentrations) of contamination h. Radionuclides cannot be destroyed, but radioactivity is ultimately self-limiting, and relying on this attenuation combined with institutional controls has been used to manage land suffering from acute radiological problems LINES OF EVIDENCE The ‘lines of evidence’ approach is based on gathering complementary data that together establish that the processes of natural attenuation are occurring and that it will be protective of human health and the environment. Although the precise terms and details vary in general, the lines of evidence used are: 1. Documented loss of contaminant mass at the field scale (note: plume trend may not be clear from plotting data. It may be necessary to quantify the trend using statistical methods) 2. Presence and distribution of geochemical/biochemical indicators of natural attenuation 3. Direct microbiological evidence

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Chlorinated solvents â&#x2030;Ľ3 chlorine atoms Chlorinated solvents, <3 chlorine atoms





Reductive dehalogenation (contaminant as electron acceptor)


Sorption & binding

Anaerobic degradation (contaminant as electron donor)


Degradative attenuation mechanisms Aerobic degradation (contaminant as electron donor)

Contaminant type

Precipitation & cation exchange


Non-degradative attenuation mechanisms




Petroleum hydrocarbons

BTEX, middle distillates

Fuel oxygenates


Heavy metals (cationic)

Hg, Cd, Zn, Pb, Ni, Sr, Co

Heavy metals (anionic)

CrO4, AsO4, AsO3, TcO4


Ca, Mg, Si


PO4, BO3


NH3, NH4+








Phenols & phenolics


Need to consider specific substance




= primary importance; = secondary importance; ? = some doubt exists over the importance of the process. NB: Dispersion and diffusion are aplicable to all contaminants and have, therefore been excluded.

Dominant attenuation mechanisms for key groups of contaminants (from Environment Agency 2000)

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EXAMPLE: BTEX NATURAL ATTENUATION (from Biodegradation, volatilisation, sorption, dispersion and dilution (by recharge). Note: chemical degradation is insignificant



Nitrate (NO3 ) 3+


Iron (Fe and Fe ) 2--


Trend inside the plume






Fe decreases as 2+ Fe increases


Sulphate (SO4 )



Carbon dioxide (CO2)

Groundwater/soil gas


Methane (CH4)

Groundwater/soil gas

Increases (in absence of oxygen)


Requirements for installation of monitoring wells

Operation and maintenance

Well maintenance

Monitoring progress

Collection and analysis of groundwater samples

Site requirements

Access; monitoring wells


Monitoring wells

Potential environmental impacts

Contamination may reach existing or new receptors before attenuation is complete. Subsurface conditions may change, affecting the progress of natural attenuation, in the worst case leading to the release of adsorbed or absorbed contaminants

Health and safety issues

Potential long-term contamination exposure of residents must be assessed at all sites

Practical depth of treatment

No limit established


Dependent on the initial conditions. E.g. risk assessment may indicate that no action is required to protect downstream receptors. Modelling may indicate that after source Availability in UK reduction, natural attenuation will lead to effective disappearance of residual contamination over several years, with plume shrinkage evident after one year

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REFERENCES Learning Alvarez, P.J. and Illman, W.A. (2005) Bioremediation and Natural Attenuation: Process Fundamentals and Mathematical Models. Wiley, ISBN 0471650439. American Petroleum Institute (2003) Groundwater Remediation Strategies Tool. Regulatory and Scientific Affairs Department. Publication Number 4730. December 2003. Environment Agency (1996) Evaluation of Remedial Actions for Groundwater Pollution by Organic Solvents. R&D Tech. Report P9, Bristol. ISBN 1857050525. Environment Agency (2004) Mobilising Nature’s Armoury: Monitored Natural Attenuation – Dealing With Pollution Using Natural Processes. Booklet SCHO0104BHTD-E-E, Environment Agency, Bristol. eapublications. storefront/ Evans, D., Jefferis, S.A., Thomas, A.O. and Cui, S. (2001) Remedial Processes for Contaminated Land: Principles and Practice. C549. CIRIA, London. ISBN 0 86017 549 9. IAEA (1999) Technologies for Remediation of Radioactively Contaminated Sites. IAEATECDOC-1086. IAEA, Vienna, via Land Quality Management Ltd (2001–2004) The HowToMNA website The National Research Council (2000) Natural Attenuation for Groundwater Remediation. The National Academies Press, Washington, DC. ISBN 0309516471. Sinke, A. and le Hecho, I. (1999) Monitored Natural Attenuation: Review of Existing Guidelines and Protocols. TNO-Nicole report, TNO-MEP R99/313A. TNO Institute of Environmental Sciences, Energy Research and Process Innovation, Apeldoorn. Suthersan, S.S. (1999) Remediation Engineering Design Concepts. CRC Press Inc., Boca Raton, Florida (CD-ROM). ISBN 0849321689. United States Department of Energy (2004) Compatibility of Alternative Chlorinated Solvent Source Treatment Strategies with Monitored Natural Attenuation – Looney, B.B. and Vangelas, K.M. United States Geological Survey (2003) Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation – Francis H. Chapelle, Mark A. Widdowson, J. Steven Brauner, Eduardo Mendez III and Clifton C. Casey. Water Resources Investigation Report 03-4057. Wiedemeier, T.H et al. (1999) Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface. Wiley. ISBN 0471197491.

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Handbooks Air Force Center for Environmental Excellence – AFCEE (2004) Implementing Monitored Natural Attenuation and Expediting Closure at Fuel-release Sites – Brauner, J. et al. AFCEE, Brooks City-Base, Texas. NTIS: ADA426387. Environment Agency (2000) Guidance on the Assessment and Monitoring of Natural Attenuation of Contaminants in Groundwater – Carey, M.A., Finnamore, J.R., Morrey, M.J. and Marsland, P.A. R&D Publication 95, Environment Agency, Bristol. ISBN 1857052632. NOBIS (1998) Decision Support Model for Natural Attenuation, Phase 2. Report 98-1-21. CUR/NOBIS, Gouda. CUR/NOBIS, Gouda, The Netherlands. United States Environmental Protection Agency (1998) Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water – Wiedemeier, T.H. et al. EPA/600/R-98/128. 5FAttenuation/cat/Guidance/ United States Environmental Protection Agency (2004) Performance Monitoring of MNA Remedies for VOCs in Ground Water. EPA 600-R-04-027. default.focus/sec/Natural%5FAttenuation/cat/Guidance/ United States Environmental Protection Agency (2005) Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites. EPA 600-R-04-179. cat/Guidance/ Case studies and demonstrations Barr, D., Finnamore , J.R., Bardos, R.P., Weeks , J.M. and Nathanail, C.P. (2002) Biological Methods for Assessment and Remediation of Contaminated Land: Case Studies. Project RP625. CIRIA Report C575. CIRIA, London. Environment Agency (2003) SIReN: Phase 2B Further Investigation (Phase 2b) of the SIReN (Site for Innovative Research on Natural Attenuation) Site – Lethbridge, G., Scott, P. and Earle, R. R&D Technical Report P2-208/TR/3. Environment Agency, Bristol, UK. ISBN 1844321061. storefront/ United States Environmental Protection Agency (2004) Performance Monitoring of MNA Remedies for VOCs in Ground Water. EPA/600/R-04/027. reports/600R04027/600R04027.pdf United States Environmental Protection Agency (2004) FRTR Cost and Performance Remediation Case Studies and Related Information. CD-ROM. EPA 542-C-04-004, (has a comprehensive listing of case studies), and also on US Federal Remediation Technologies Roundtable (FRTR) Cost and performance web page:

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