Monitored Natural Attenuation
Over the past several years natural attenuation has received increasing attention as it became more and more clear that naturally occurring processes acted to remediate contaminated groundwater. The term Natural Attenuation (NA) refers to “naturally occurring processes in soil and groundwater environments that act without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants in those media”.
The most important destruction mechanism during natural attenuation is microbial mediated degradation. Non-destructive attenuation mechanisms include sorption, dispersion, dilution and volatilisation. During the former process the contaminants are broken down ultimately into harmless components such as water, chlorine, and carbon dioxide. In the latter processes the contaminants remain but the concentrations in the groundwater decrease. Inorganic contaminants like heavy metals, arsenic and cyanide can be attenuated by chemical and physical processes but cannot be degraded.
Monitored Natural Attenuation MNA is defined as “the monitoring of groundwater to confirm whether NA processes are acting at a sufficient rate to ensure that the wider environment is unaffected and that remedial objectives will be achieved within reasonable timescale; this well be typically be less than one generation or 30 years”
In a series of well-described and investigated cases, trends can be observed in the development of a plume and plumes can be grouped in different categories (figure 1):
· expanding: residual source present. Mass flux of contaminants exceeds the assimilative capacity of the aquifer,
· stable: insignificant changes in both plume dimensions and mass,
· fading: residual source (nearly) exhausted. Significant reduction in plume mass as inferred from either plume dimensions or concentrations,
· Fading and-or loosening: average plume concentration very low and almost unchanging over time. Plume may loosen from the original source zone.
A stabilised or fading plume is direct and irrefutable evidence of
natural attenuation. Several studies addressed the changes in plume length and
concentration for dissolved petroleum hydrocarbon plumes and plumes of
chlorinated solvents (Mace et al. 1997, Newell et al.1990, Rice et al.
1995). Studies have shown that BTEX plumes tend to be
shorter than plumes of chlorinated ethenes or other chlorinated solvents (Newell and Connor 1998). Most of the studies on plume behaviour were carried
out at US sites where aerobic conditions prevail. Generally aerobic conditions
are favourable for the degradation of petroleum hydrocarbons including BTEX. In
Figure 1: Plume life cycle
increased interest in applying MNA as part of a remedial strategy, a number of
guidelines, protocols and recommendations have been developed or are currently
being developed. Much information can be found on the websites listed in the
literature list. Most of the protocols are updated and adapted regularly to
keep up with scientific innovations and extending expertise in the field of
MNA. An overview of protocols is given by Rittmann et al. 2000. Also in
The protocols and guidelines all have two important criteria to allow monitored natural attenuation as a remedial strategy:
· MNA has to be protective to human health and the environment,
All protocols state that to support remediation by MNA, the site owner must demonstrate convincingly that attenuation, or even degradation, of site contaminants is occurring at rates sufficient to be protective for human health and the environment. The data and type of information that can be used to underpin the efficacy of NA can be classified in three “lines of evidence”. Whereas in most protocols and guidelines the first two lines of evidence are indispensable, the third line is usually considered as additional. This third line is brought up as additional evidence, for instance, in case of complex situations and compounds, or to narrow the bandwidth of predictions, or to reduce uncertainty.
1. Primary line: documented loss of contaminants
Means: historical contaminant data for the plume or singles wells
Historical groundwater data and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points. In the case of a groundwater plume the decreasing concentrations should not be solely the result of plume migration. In the case of inorganic chemicals the primary attenuation mechanism should also be understood.
line: documented NA process
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.
3. Third line: documented microbial activity
Means: additional laboratory or field data such as RNA-DNA analysis, enzyme data, bacterial counts
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.
In order to implement MNA at a site, the authorities have to formally approve MNA as a remedial strategy. Also there have to be clear-cut agreements on the monitoring strategy, the performance criteria, the contingency plan, the remedial objectives and the timeframe of the remediation. The formal approval of the authorities depends on the country or “county” or “provincie” or “region” or “Bundesland”; in some countries a formal contract has to be signed by both parties while in others the authorities approve officially a proposed remedial plan (NL: “beschikking op saneringsplan”).
In most countries one should provide:
o Full site description including historical data and a conceptual model,
o Risk assessment,
o Fate and transport model,
o Remedial objectives and timeframe,
o Monitoring strategy,
o Performance criteria,
o Contingency plan.
Site investigation data should first be used to develop a site-specific conceptual model representing the origin of the contaminant, preferably both in time and location, and a hypothesis on how the contaminants behaved after liberation into the environment. Crucial parameters such as type of contaminant (DNAPL, LNAPL), mass, groundwater transport and type of natural attenuation processes have to be included.
There is some confusion concerning where MNA is a remedial measure for plumes only or also for source zones. Actually this is very site-specific and depends on the balance between the mass flux out of the source zone, the rate of NA and the location of the plume. If, despite the presence of a source zone, the plume is stable, there is no scientific reasoning against MNA. The NA processes can keep up with the supply of contaminants from the source zone and the plume will remain in position. However, the presence of a source zone may increase the duration of the MNA significantly.
The remedial objectives should be risk-based. It should be ascertained that the site-specific remediation is protective of human health and the environment. Remediation objectives could include preventing exposure to contaminants, minimising further migration of contaminants from source areas, minimising further migration of the groundwater contaminant plume, reducing contamination in soil or groundwater to specific clean-up levels appropriate for current use.
Also the criterion of cost-effectiveness is important. If the costs for a full clean-up are too high in relation to its economical and societal benefits, the balance may switch over in favour of a prolonged MNA process.
In many cases a fate and transport model is used with either analytical or numerical solutions to predict the effectiveness of MNA into the future. Preferably site-specific data and parameters are used in the model. The model can be used to identify the key processes by a sensitivity analysis of input parameters. The model also forms the basis for the monitoring strategy as it indicates the plume behaviour both in time and space, the expected trends in concentration of parent and daughter products, and timescale for remedial targets to be achieved.
The model predicts the optimum monitoring well position and sampling frequency.
For the requirements that should be set for the monitoring strategy, the EPA advises can be taken as point of departure. The monitoring program should specify the location, frequency, and type of samples and measurements to evaluate remedy performance as well as define the anticipated performance objectives of the remedy. In addition an adequate monitoring program should:
o identify any potentially toxic transformation products resulting from biodegradation,
o determine if a plume is expanding (either down-gradient, laterally or vertically),
o ensure adequate warning of potential impact to down-gradient receptors,
o detect new releases of contaminants to the environment that could have an impact on the effectiveness of the natural attenuation remedy,
o demonstrate the efficacy of institutional controls that were put in place to protect potential receptors, and
o detect changes in environmental conditions (e.g., hydrogeological, geochemical, microbiological, or other changes) that may reduce the efficacy of any of the natural attenuation processes.
The evaluation of the efficacy of the MNA remedial action should be based on pre-defined performance criteria. These criteria should be realistic and process based. The performance criteria should take into account the normal variations in concentration measurements. These variations can be the result of natural processes such as (temporal) non-equilibrium situations due to, for instance, groundwater fluctuations, heavy rainfall or temperature shifts. Also man-induced variations occur due to sampling procedures or minor analytical variations.
The implementation documents for MNA should include a contingency plan in case the MNA does not meet the performance criteria and is not as effective as expected or even failing. Important aspects that should be included in the contingency plan are:
definition of the activation procedure for the contingency measures.
Preferably, the decision criteria should be based on process parameters and not purely on concentration data. The contingency plan should be activated, for example, when several wells or sequential measurements indicate a deviation from the assumed trend. Obviously, concentrations exceeding the expectations in a single well cannot be the basis for activation of the contingency plan.
o Type of contingency measures. It should be indicated at least which contingency measures could be used. The contingency measures often refer to “conventional” techniques such as pump and treat or hydrological containment to prevent the plume from migration and/or further expansion. In some cases also an additional active remediation of the source zone could be considered.
o The changed use of the site that might result in an increase of the risk should automatically start a process of re-evaluating the possibilities of MNA. This concerns both the altered use (e.g., housing instead of industry) and changes in hydrogeological conditions (e.g., increase or decreased groundwater extraction regime, long lasting changes in groundwater level, changes in groundwater fluctuations).
In some countries MNA is relatively new and there is little experience with evaluating and implementing it as remedial strategy. Essential for the implementation of MNA is the early involvement of relevant stakeholders such as the site owner and/or polluter, whoever is being affected by pollution, the service provider and the regulator and planner. However, other stakeholders can also be influential such as:
· Site users, workers (possibly unions), visitors,
· Financial community (banks, founders, lenders, insurers),
· Site neighbours (tenants, dwellers, visitors, local councils),
· Campaigning organisations and local pressure groups,
· Other technical specialists and researchers.
Stakeholders will have their own perspective, priorities, concerns and ambitions regarding any particular site. Given the range of stakeholder interests, consultation can be a time consuming and expensive process, particularly if approaches are only made at a late stage in decision making.
MNA research projects have been carried out in several
European countries (