1. NATURAL ATTENUATION VÍT MATĚJŮ ROBIN KYCLT
ENVISAN-GEM, a.s. Department of Biotechnology, Radiová 7, Praha 10, THE CZECH REPUBLIC

2. OVERVIEW OF NATURAL ATTENUATION
Terms and synonyms
Definitions
History
Natural attenuation processes
Aerobic and anaerobic biodegradation
Electron acceptors
Implementation

3. NATURAL ATTENUATION IS NOT A „DO-NOTHING“ APPROACH
"Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents (Wiedemeier, et al 1996)."
NATURAL ATTENUATION IS NOT A „DO-NOTHING“ APPROACH

5. NATURAL ATTENUATION Intrinsic (bio)remediation Natural attenuation
Monitored natural attenuation
Enhanced (natural) attenuation
Passive remediation
Natural restoration
Natural recovery
Enhanced passive
remediation

6. NATURAL ATTENUATION -

7. NATURAL ATTENUATION - DEFINITIONS
Monitored Natural Attenuation - Reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The 'natural attenuation processes' that are at work in such a remediation approach include a variety of 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; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants. (U.S. Environmental Protection Agency, 1999)

8. NATURAL ATTENUATION - DEFINITIONS
Natural Attenuation - Reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes, such as; biodegradation, dispersion, dilution, adsorption, and volatilization. (American Society for Testing and Materials, 2003)

9. NATURAL ATTENUATION - HISTORY
1994 – AIR FORCE CENTER FOR ENVIRONMENTAL EXCELLENCE (AIR FORCE CENTER FOR ENGINEERING AND ENVIRONMENT) – The Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater

10. NATURAL ATTENUATION - HISTORY
Oil companies – various protocols for NA
AMERICAN SOCIETY FOR TESTING AND MATERIALS – National Standard for NA
AFCEE – Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water

11. NATURAL ATTENUATION - HISTORY

12. POLLUTANTS Chlorinated solvents (PCE, TCE, chlorinated alkanes) BTEX
Organic acids and their salts
Alcohols, aldehydes and ketones
TNT, RDX (explosives)
Chlorobenzenes
Mixed contamination (leachate water from landfills)
Nitrates
Sulfates
Radionuclides
Heavy metals
MTBE
Tar oil
and many more……….

13. THE LENGTH OF CONTAMINANT PLUME
Mineral oil hydrocarbons
PAH
BTEX
Chlorinated ethenes

14. CONTAMINANT PLUME LENGHT
The two primary factors that control source longevity are:
mass of contaminants present in the source area, and
availability of electron acceptors, of which oxygen is the most important.
The larger the contaminant mass, the longer the period of time required for it to be completely degraded. Across a wide range of concentrations, the rate of biodegradation of petroleum hydrocarbons follows a hyperbolic rate law:

15. NATURAL ATTENUATION PROCESS
Biological degradation; advection; dispersion; sorption; diffusion; chemical reaction (transformation); volatilization (evaporation); abiotic and biotic transformation (mineralization and biodegradation); dilution, sorption/desorption; ion exchange; complexation; and, plant and animal uptake.

16. Fate of Organic Contaminants
in the Subsurface

17. POLLUTANT FORMS AT A CONTAMINATED SITE
DISSOLVED
SORBED
VOLATILIZED
FREE PHASE OF PRODUCT

19. WHERE

20. ADVECTION
Advection
Advective transport is the process by which dissolved contaminants are transported by flowing groundwater at its average linear velocity. Flow rates are determined by the relationship:
Vx = - K i /n
where: Vx is the average linear groundwater velocity (L/T), K is the hydraulic conductivity (L/T), i is the hydraulic gradient (L/L) and n is the porosity of the subsurface material (dimesionless).
(The symbols in brackets represent dimensions e.g. L=length, T=time).

21. ADVECTION

22. DISPERSION Hydrodynamic Dispersion
Hydrodynamic dispersion is the process whereby a contaminant plume spreads out in directions that are longitudinal and transverse to the groundwater flow path. Hydrodynamic dispersion is the result of two physical processes, namely: mechanical dispersion and molecular diffusion. Mechanical dispersion occurs only under ground water flow conditions, whereas, molecular diffusion occurs under both flowing and stagnant conditions.
Hydrodynamic dispersion D (L2/T), incorporates the effect of both dispersion and molecular diffusion and can be written as:
D = αVx + De

23. DISPERSION
Where α is the dispersivity of the porous medium, Vx is the average linear groundwater velocity and De is the diffusion coefficient (L2/T) (a property of the dissolved contaminant).

24. DISPERSION Mechanical Dispersion
Mechanical dispersion occurs because of the tortuosity in flow paths within a porous medium. That is, when the particles in a porous medium prevent groundwater and any dissolved contaminants from flowing in a straight line. This typically results in longitudinal and transverse spreading of the contaminant plume. The component of hydrodynamic dispersion contributed by mechanical dispersion is given by the relationship:
Mechanical Dispersion = aVx
Mechanical dispersion is the dominant mechanism causing hydrodynamic dispersion at typical groundwater velocities.

25. DISPERSION

26. DIFFUSION
Molecular diffusion results in the movement of chemicals from zones of higher concentrations to zones of lower concentrations. Diffusion is dependent only on concentration gradients and will occur even in materials of low hydraulic conductivities. Diffusion as a contaminant migration mechanism is most important when groundwater velocities are low, typically less than a few centimetres a year. The mass flux of a diffusing chemical is represented in the hydrodynamic dispersion equation as De. Diffusion is likely insignificant compared to dispersion in most settings.

27. (AD)SORPTION
Adsorption is a process that can remove potential contaminants from solution in groundwater and bind them temporarily to the surface of solid particles. This process is referred to as hydrophobic bonding and is an important factor in controlling the fate of many organic contaminants. Two components of an aquifer have the greatest effect on sorption: organic matter and clay minerals. In most aquifers, the organic fraction tends to control the sorption of petroleum hydrocarbons. This process results in the overall delay in contaminant movement with respect to the average velocity of groundwater and is referred to as "retardation". The amount of retardation that a dissolved chemical experiences depends on its characteristics and the properties and composition of the material through which it is migrating. For linear, reversible adsorption, the retardation factor can be expressed as:

28. (AD)SORPTION R = 1 + Kd pb/n (Freeze and Cherry, 1979)
where pb is the dry bulk density of the soil (M/L3), n is the porosity, and Kd is the distribution coefficient (L3/M) which describes the partitioning of a dissolved solute between the aqueous and sorbed phases. The distribution coefficient is unique to individual contaminants and is dependent upon the chemical characteristics of the porous medium and groundwater.
The degree of adsorption of organic contaminants is directly related to the organic carbon content of the soil.
It has been found that an empirical relationship exists between the distribution coefficient (Kd), the fractional mass of organic carbon in the soil (foc) and the organic carbon
distribution coefficient (Koc) (L3/M) as follows:

29. (AD)SORPTION Kd =( foc )( Koc)
This relationship is of importance because it is difficult to measure the distribution coefficient, Kd, directly, but foc can be measured or estimated and Koc values are available in the literature for many compounds. So it is the way how Kd obtain.

30. VOLATILIZATION
Volatilization is the process by which chemicals dissolved in groundwater are transferred from the aqueous phase into the vapour phase. The chemical then migrates through the capillary fringe and into the unsaturated or vadose zone. This process is dependent on the Henry's Law constant (a coefficient describing the partitioning of a chemical between the gas and aqueous phases), the concentration of the chemical, the depth to the water table, and the temperature. Generally, the higher the vapour pressure and Henry's Law constant, the more likely the chemical will be released from the soil or groundwater to the soil gas. The physico-chemical properties of the BTEX compounds give them relatively high Henry's Law constant compared to many other organic compounds.

31. AEROBIC AND ANAEROBIC DEGRADATION
Biodegradation of BTEX parameters by indigenous subsurface microbes appears to be the primary mechanism for intrinsic remediation at sites where natural attenuation has been documented. During biodegradation, microbes transform available nutrients, including hydrocarbons, into substances useful for energy and cell reproduction. Microbes obtain this energy by facilitating the transfer of electrons from electron donors to electron acceptors. This results in the oxidation of electron donors and the reduction of electron acceptors. Electron donors include natural organic material and petroleum hydrocarbons. Electron acceptors in groundwater include dissolved oxygen, nitrate, Fe (III), sulphate and carbon dioxide.

32. AEROBIC AND ANAEROBIC BIODEGRADATION
The use of electron donors by microbes begins with dissolved oxygen (aerobic conditions) followed by anaerobic (absent or minimal dissolved oxygen). The natural bioattenuation for aromatic hydrocarbons can follow both aerobic and anaerobic pathways.

33. AEROBIC BIODEGRADATION
Biodegradation of petroleum hydrocarbons is often an aerobic process that occurs when indigenous hydrocarbon-degrading micro-organisms are supplied with oxygen and nutrients necessary to utilize fuel hydrocarbons as an energy source. The supply of oxygen in a petroleum hydrocarbon impacted aquifer is constantly renewed by the influx of oxygenated, upgradient flow and the vertical diffusion of oxygen from the unsaturated soil zone into the groundwater (Borden and Bedient, 1986). Aquifers consisting of sandy soil and characterized by moderate to high hydraulic conductivities typically support effective aerobic biodegradation.

34. AEROBIC BIODEGRADATION
Aerobic biodegradation is typically found within aquifers with dissolved oxygen (DO) concentrations greater than 1 to 2 mg/L. Dissolved oxygen concentrations are used to estimate the mass of contaminant that can be biodegraded aerobically. In this process, the microbes utilize dissolved oxygen as an electron acceptor to degrade the hydrocarbon substrate. Under aerobic conditions, dissolved oxygen levels are reduced as aerobic respiration occurs.

35. AEROBIC BIODEGRADATION
Anaerobic bacteria generally cannot function at DO levels greater than 1 mg/L. As documented by Chiang et al. (1989), an inverse correlation between DO and aromatic hydrocarbon concentrations has been observed suggesting that aerobic biodegradation is the most significant mechanism for natural attenuation of BTEX. In general, dissolved oxygen concentrations will be lower than background conditions in groundwater impacted by BTEX compounds. Thus a reduction in DO concentrations within an existing BTEX groundwater plume provides strong indication that microbes are already established and actively biodegrading petroleum hydrocarbons by aerobic respiration.

37. ANAEROBIC BIODEGRADATION
Typically, depletion of dissolved oxygen is observed in the impacted groundwater plume as a result of increased levels of microbial respiration. This depletion of dissolved oxygen results in the development of anaerobic conditions within and near the margins of the groundwater plume.
When oxygen is absent or exists at low levels, then nitrate, iron (III), sulphate, and carbonate can serve as terminal electron acceptors in the following order of reduction:

39. ANAEROBIC BIODEGRADATION
Denitrification
Nitrate can be used as an electron acceptor by anaerobic microbes to mineralize the BTEX compounds via denitrification, after almost all of the dissolved oxygen has been consumed and anaerobic conditions have developed. Nitrate concentrations are used to estimate the mass of contaminant that can be biodegraded by denitrification processes. The denitrification occurs in the following sequence:

40. ANAEROBIC BIODEGRADATION
In areas where denitrification is occurring, there will be a strong correlation between areas of high BTEX concentrations and areas depleted of nitrate relative to measured background concentrations.
The following equation describes the overall stoichiometry of an aromatic hydrocarbon degradation during denitrification
6 NO3-(nitrate) + 6 H+(hydrogen) + C6H6 (benzene)
6 CO2(g)(carbon dioxide)+ 6 H2O (water)+ 3 N2(g)(nitrogen gas)

41. ANAEROBIC BIODEGRADATION
The balanced equation indicates that six moles of nitrate are required to metabolize one mole of benzene. Thus in the absence of microbial cell production, each 1.0 mg/L of ionic nitrate consumed by
microbes
will mineralize
approximately
0.21 mg/L of
benzene or
BTEX
compounds.

42. ANAEROBIC BIODEGRADATION
Iron (III) Reduction
Once the available dissolved oxygen and nitrate in the aquifer have been depleted, iron (III) can be used as an electron acceptor. During this process, iron (III) is reduced to iron (II), which is relatively soluble in water. Iron (II) concentrations are used as an indicator of anaerobic degradation of hydrocarbon compounds. By knowing the volume of contaminated groundwater, the background iron (II) concentration and the iron (II) measured in the contaminated area, it is possible to estimate the mass of BTEX lost to biodegradation through iron (III) reduction.

43. ANAEROBIC BIODEGRADATION
The oxidation of the BTEX compounds coupled with the reduction of iron (III) may result in the high concentrations of iron (II) in groundwater within and near the contaminated plume. Most of the iron (III) that is reduced to iron (II) is subsequently precipitated when the groundwater mixes with oxygenated groundwater downgradient of the plume.
The following equation describes the overall stoichiometry of benzene oxidation by iron reduction to carbon dioxide, iron (II) and water under anaerobic biodegradation:
30 Fe(OH)3 (iron (III)) + 60 H+(hydrogen) + C6H6 (benzene)
6 CO2(g)(carbon dioxide)+ 78 H2O (water) Fe2+(iron (II))

44. ANAEROBIC BIODEGRADATION
The balanced reaction indicated that thirty (30) moles of iron (III) are required to metabolize one mole of benzene. Thus in the absence of microbial cell production, 1.0 mg/L of benzene or BTEX results in the production of 21.8 mg/L of iron (II) during the iron (III) reduction.

45. ANAEROBIC BIODEGRADATION
Sulphate Reduction
Following nitrate and iron (III) reduction, sulphate may be used as an electron acceptor for anaerobic biodegradation. This process is termed sulphate reduction and results in the production of sulphide. The presence of decreased concentrations of sulphate and increased concentrations of sulphide relative to background concentrations indicates that hydrocarbon biodegradation may be a result of sulphate reduction. By knowing the volume of contaminated groundwater, the background sulphate concentration and the concentration of sulphate measured in the contaminated area, the mass of BTEX lost to biodegradation through sulphate reduction can be calculated.

46. ANAEROBIC BIODEGRADATION
The following equation describes the overall stoichiometry of benzene oxidation by sulphate reduction to carbon dioxide, sulphide and water under anaerobic biodegradation:
3.75 SO42- (sulphate) H+(hydrogen) + C6H6 (benzene)
6 CO2(g)(carbon dioxide)+ 3 H2O (water)+ 3.75
H2S0(sulphide)
The balanced equation indicates that 3.75 moles of sulphate are required to metabolize one mole of benzene. Thus in the absence of microbial cell production, 1.0 mg/L of sulphate consumed by microbes results in the destruction of approximately 0.21 mg/L of benzene or BTEX compounds.

47. ENERGY GAINED FROM DIFFERENT REDOX PROCESSES

48. ABIOTIC TRANSFORMATION PROCESSES
• Hydrolysis
• Elimination
• Oxidation and Reduction via Fe (III) and Mn (III/IV)
• Oxidation by Hydroxyl Radicals
• Reduction via Organic Matter and Excess Reactants
• Photolysis

49. U.S. EPA LINES OF EVIDENCE FOR NATURAL ATTENUATION
1. Primary line: documented loss of contaminants Historical groundwater and/or soil chemistry data that demonstrate clear trends of decreasing contaminant mass (concentration) that is not the result of migration
Hydrogeologic and geochemical data that are indirect indicators of attenuation mechanisms
Data from field and microcosm studies that directly demonstrate certain attenuation mechanisms

50. U.S. EPA LINES OF EVIDENCE
2. Secondary line: documented NA processes
Means: geochemical indicators, biochemical indicators (electron balance), application of screening models, presence of metabolites
Hydrological and geochemical data can be used in an analytical screening model to demonstrate indirectly the type of natural attenuation process active at the site and the rate at which such processes will reduce contaminant concentrations to required levels.

51. U.S. EPA LINES OF EVIDENCE
3. Third line: documented microbial activity
Means: additional laboratory or field data such as RNA-DNA analysis, enzyme data, bacterial counts, bacterial strains, functional analysis of genes
Data from field or microcosm studies which directly demonstrate the occurrence of particular natural attenuation processes at the site and its ability to degrade the contaminants of concern.

52. WEIGHT OF EVIDENCE APPROACH
All of the AFCEE Protocols (Including EPA, 1998) Rely on Independent and Converging Lines of Evidence to Document and Quantify Natural Attenuation

53. ATTENUATION CAPACITY
The natural attenuation capacity of the aquifer is defined as the sum of the processes that are active within the aquifer being evaluated.
The rate of these processes is a function of contaminant concentration and other parameters that can vary spatially and, in some cases, temporally.

54. MLR > AC No Natural Attenuation
ATTENUATION CAPACITY
MLR – Mass Loading Rate (input of pollutant(s) into a contaminant plume)
AC – attenuation capacity
MLR > AC No Natural Attenuation
MLR = AC ?
MLR < AC Natural Attenuation could be employed

55. ATTENUATION CAPACITY
MASS LOADING RATE: dissolution of pollutant from free phase product, desorption, wash-out, ……
ATTENUATION CAPACITY: all processes of natural attenuation
If AC > MLR, then the development of a contaminant plume should be

56. The four stages describing the development of a contaminant plume (adapted from Newell and Conner 1998)
source
EXPANDING
STABLE
SHRINKING
former source
EXHAUSTED I.
II.
III.
IV.
Time evolution of a plume if it undergoes attenuation

57. SHRINKING CONTMINANT PLUME

58. STABLE CONTAMINANT PLUME

59. EXPANDING CONTAMINANT PLUME

60. NATURAL ATTENUATION

61. EVALUATION OF NATURAL ATTENUATION
Various protocols (U.S. EPA, AFCEE, American Petroleum Institute, EPA (GB), Umweltbundesamt (Germany), U.S. Geological Survey) with various scoring systems serving as a tool for decission making exist.

62. EVALUATION OF NATURAL ATTENUATION

63. EVALUATION OF NATURAL ATTENUATION
1) Review available site data and develop a preliminary conceptual model. Determine if receptor pathways have already been completed. Respond as appropriate.
2) If sufficient existing data of appropriate quality exist, apply the screening process to assess the potential for natural attenuation.
3) If preliminary site data suggest natural attenuation is potentially appropriate, perform additional site characterization to further evaluate natural attenuation. If all the recommended screening parameters like
- Local geologic and topographic maps,
- Geologic data,
- Hydraulic data,
- Biological data,
- Geochemical data, and
- Contaminant concentration and distribution data)
have been collected and the screening processes suggest that natural attenuation is not appropriate based on the potential for natural attenuation, evaluate whether other processes can meet the cleanup objectives for the site (e.g., abiotic degradation or transformation, volatilization, or sorption) or select a remedial option other than MNA.

64. EVALUATION OF NATURAL ATTENUATION
4) Refine conceptual model based on site characterization data, complete pre-modeling calculations, and document indicators of natural attenuation.
5) Simulate, if necessary, natural attenuation using analytical or numerical solute fate and transport models that allow incorporation of a biodegradation term.
6) Identify potential receptors and exposure points and conduct an exposure pathways analysis.
7) Evaluate the need for supplemental source control measures. Additional source control may allow MNA to be a viable remedial option or decrease the time needed for natural processes to attain remedial objectives.

65. EVALUATION OF NATURAL ATTENUATION
8) Prepare a long-term monitoring and verification plan for the selected alternative. In some cases, this includes monitored natural attenuation alone, or in other cases in concert with supplemental remediation systems.
9) Present findings of natural attenuation studies in an appropriate remedy selection document, such as a CERCLA Feasibility or RCRA Corrective Measures Study. The appropriate regulatory agencies should be consulted early in the remedy selection process to clarify the remedial objectives that are appropriate for the site and any other requirements that the remedy will be expected to meet. However, it should be noted that remedy requirements are not finalized until a decision is signed, such as a CERCLA Record of Decision or a RCRA Statement of Basis.

66. ABBREVIATIONS
CERCLA = The Comprehensive Environmental Response, Compensation, and Liability Act
RCRA = Resource Conservation and Recovery Act

67. IMPLEMENTATION
The OSWER directive “Use of Monitored Natural attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites.”(OSWER; US EPA, 1999) recognizes that natural attenuation processes occur in all soil and groundwater systems and act, to varying degrees, on all contaminants. Thus, a decision to rely on natural attenuation processes as part of a site remediation strategy does not depend on the occurrence of natural attenuation processes, but on their effectiveness in meeting site-specific remediation goals.
OSWER = Office of Solid Waste and Emergency Response, U.S. EPA

68. IMPLEMENTATION

69. IMPLEMENTATION First step:
1. Removal of source zone by active remediation followed by natural attenuation or enhanced attenuation.
This is necessary to stop or substantially reduce the mass loading rate to the contanminant plume.

70. IMPLEMENTATION 2. Enhanced attenuation Second step
Enhanced Attenuation is a plume remediation strategy to achieve groundwater restoration goals by providing a “bridge” between source zone treatment and MNA and/or between MNA and slightly more aggressive methods. Enhanced attenuation provides an organized, scientific, and disciplined approach to implement treatment technologies at appropriate sites and at appropriate times. Various remediation technologies can be designed to reduce the source flux and/or increase the attenuation capacity/rate in the plume to assure the plume will stabilize and shrink.

71. IMPLEMENTATION 3. Monitored Natural Attenuation Third step
MNA is based on natural attenuation processes with “no human intervention,” Enhanced Attenuation adds human intervention to boost existing attenuation processes. This intervention enables less energy-intensive attenuation processes to achieve cleanup goals where natural attenuation processes are insufficient to reach those goals in the required timeframe.

72. IMPLEMENTATION Site and plume characteristic:
In general, the level of site characterization necessary to support a comprehensive evaluation of natural attenuation is more detailed than that needed to support active remediation.
Site characterizations for natural attenuation generally warrant
a quantitative understanding of source mass;
ground water flow;
contaminant phase distribution and partitioning between soil,
ground water, and soil gas;
rates of biological and non-biological transformation;
an understanding of how all of these factors are likely to vary with time.

73. IMPLEMENTATION
Demonstrating the efficacy of this remediation approach likely will require analytical or numerical simulation of complex attenuation processes. Such analyses, which are critical to demonstrate natural attenuation’s ability to meet remedial action objectives, generally require a detailed conceptual site model as a foundation.

74. MONITORING
All monitoring programs should be designed to accomplish the following:
• Demonstrate that natural attenuation is occurring according to expectations;
• Identify any potentially toxic transformation products resulting from biodegradation;
• Determine if a plume is expanding (either downgradient, laterally or vertically);
• Ensure no impact to downgradient receptors;
• Detect new releases of contaminants to the environment that could impact the effectiveness of the natural attenuation remedy;
• Demonstrate the efficacy of institutional controls that were put in place to protect potential receptors;
• Detect changes in environmental conditions (e.g., hydrogeologic, geochemical, microbiological, or other changes) that may reduce the efficacy of any of the natural attenuation processes; and
• Verify attainment of cleanup objectives.

75. MONITORING

76. NATURAL ATTENUATION – ADVANTAGES
Less intrusion as few surface structures are required;
Potential for application to all or part of a given site, depending on site conditions and remediation objectives;
Use in conjunction with, or as a follow-up to, other (active) remedial measures; and
Potentially lower overall remediation costs than those associated with active remediation.

77. NATURAL ATTENUATION – ADVANTAGES
As with any in situ process, generation of lesser volume of remediation wastes, reduced potential for cross-media transfer of contaminants commonly associated with ex situ treatment, and reduced risk of human exposure to contaminants, contaminated media, and other hazards, and reduced disturbances to ecological receptors;
Some natural attenuation processes may result in in-situ destruction of contaminants;

78. NATURAL ATTENUATION - DISADVANTAGES
Longer time frames may be required to achieve remediation objectives,compared to active remediation measures at a given site;
Site characterization is expected to be more complex and costly;
Toxicity and/or mobility of transformation products may exceed that of the parent compound;
Long-term performance monitoring will generally be more extensive and for a longer time;
Institutional controls may be necessary to ensure long term protectiveness;

79. NATURAL ATTENUATION - DISADVANTAGES
Potential exists for continued contamination migration, and/or cross-media transfer of contaminants;
Hydrologic and geochemical conditions amenable to natural attenuation may change over time and could result in renewed mobility of previously stabilized contaminants (or naturally occurring metals), adversely impacting remedial effectiveness; and
More extensive education and outreach efforts may be required in order to gain public acceptance of MNA.

80. NATURAL ATTENUATION
MATHEMATICAL
MODELLING

81. CONCEPTUAL SITE MODEL
A conceptual site model is a three-dimensional representation that conveys what is known or suspected about contamination sources, release mechanisms, and the transport and fate of those contaminants. The conceptual model provides the basis for assessing potential remedial technologies at the site. “Conceptual site model” is not synonymous with “computer model;“
Computer models, which simulate site processes mathematically, should in turn be based upon sound conceptual site models to provide meaningful information.

82. CONCEPTUAL SITE MODEL System, process and effect describing model:
Representation of a conceptual idea of the system with all important features of the site (e.g. geological conditions/ structure of layers, groundwater stockworks, groundwater damage, direction of groundwater flow, substance sources, concentrations, protected objects, known and/or assumed effect of natural attenuation processes), uses, requirements to model results.
The conceptual site model is not a mathematical model but the preliminary intellectual stage of a numerical, in simple cases also analytical model:

83. GEOLOGICAL STRUCTURE MODEL
Geological structure Description and representation of the geological Model conditions: lithology, stratigraphy and genesis.
These elements are represented in their spatial position to each other as lines, areas and/or bodies

84. HYDROLOGICAL STRUCTURE MODEL
Hydrogeological properties (e.g. porosity, storage model coefficient, hydraulic permeability) are
assigned to the geological structures / units. If necessary, geological units are united with identical hydrogeological structures or conversely also further differentiated. Here, the geohydraulic and geochemical transfer and storage properties are described.
The hydrogeological structure model forms the basis for the process-related models mentioned hereinafter

85. NUMERICAL GROUNDWATER FLOW MODEL
The numerical groundwater flow model converts the flow model flow processes into mathematical relations. Indicating starting and marginal conditions either a stationary flow field (i.e. groundwater levels and flow speeds calculated from them) or in the case of transient (non-stationary) model simulations groundwater levels and flow speeds and directions varying temporarily are calculated.

86. NUMERICAL SUBSTANCE TRANSPORT MODEL
Based on the flow speed field calculated the numerical transport model, substance transport model calculates the transport of substances in subsoil determined by processes such as advection, hydrodynamic dispersion, diffusion, substance storage (sorption/desorption, precipitation/ dissolution), if necessary, volatilization and by reaction processes (e. g. biological degradation).

87. MULTI-SPECIES MODEL FOR REACTION SYSTEMS
The multi-species model considers the reaction processes, reaction systems with their reaction parameters showing the transport and reaction behaviour of a system of interacting substances

88. HOW OFTEN IS NATURAL ATTENUATION EFFECTIVE?
It has been estimated by the EPA (J. Wilson, 2001) that natural attenuation will be effective as the sole remedy at approximately 20% of all chlorinated solvent sites. It has also been estimated that natural attenuation may serve as a portion of the remedy at an additional 50% of all chlorinated solvent sites (Ellis et al., 1996).

89. ANY QUESTIONS

90. THANK YOU
FOR YOUR
ATTENTION !!