Reactive Transport Model for investigating contamination problems

Reactive transport models are considered a fundamental tool for study environmental contamination problems. The application of reactive transport models provides diagnostic and prognostic tool for the comprehension of otherwise complex dynamics of natural environments. For this reason, they were applied successfully to several field sites contaminated by inorganic and organic pollutants.

[Raffaella Meffe. IMDEA Agua]

In the course of the last century, particularly with the advent of the intensive industrialization and population growth, the stresses on groundwater resources have increased (IAEA, 1998). The presence of contaminants in groundwater is of particular concern when the groundwater is used for drinking water production. The comprehension of the occurrence, behavior and fate of these contaminants in the environmental system is extremely important for groundwater management and remediation issues. The application of reactive transport models (RTMs) to study groundwater contamination problems at a selected field site is a valuable tool for quantitative and prognostic evaluations.

Several studies exist where RTMs were used to model field observations in aquifers contaminated by inorganic and organic pollutants (Meyer et al., 2001; Brun et al., 2002). Let us consider a real study case where a RTM was found to be useful to interpret biogeochemical processes occurring in a contamination plume. The field site is a landfill in the Netherlands where a leachate plume was migrating through the heterogeneous phreatic aquifer. The leachate contained high concentrations of dissolved organic carbon (DOC), alkalinity, methane, nitrate, ammonium, iron and other major ions (except sulfate and nitrate). To obtain quantitative insight into the reactions contributing to the changes in leachate composition downstream of the landfill site and therefore to simulate observed data, a combination of reactions had to be incorporated in the model. Reactions such as degradation of DOC, precipitation of minerals, cation exchange, sorption, proton buffering, degassing were considered the main process attenuating the pollution at the field site. Observed data were well reproduced and interpreted by the applied RTM. The study demonstrated that the application of a RTM allowed the identification of the relevance and the impact of various biogeochemical processes on landfill leachate plume evolution (van Breukelen et al., 2004).

In general, a model can be considered as a description or approximation of the physical systems by means of mathematical equations. In the context of the Earth Sciences, reactive transport models (RTMs) have been intensively developed since the 1970s. They study mainly solute transport in the subsurface under the influence of different interacting processes, such as advection, dispersion, diffusion and chemical reactions.

The growing interest towards reactive transport simulations is related mainly to the increasing attention to groundwater quality that determined the necessity of developing additional tools for a better comprehension of environmental systems (Zheng and Bennett, 2002).

The rapid development of computer technology has considerably reduced the computational times of the simulations and therefore, numerous RTMs have been developed and applied to natural phenomena, and especially to groundwater contamination problems. These models can be used either qualitatively or quantitatively to provide insight into natural phenomena. Quantitative approaches are used to validate or invalidate ideas by introducing real values for environmental parameters into the model and comparing measured data to simulated data to evaluate if the working hypothesis is correct or has to be rejected or modified. Qualitative approaches provide insight into the general features of a particular phenomenon, rather than specific details (Lichtner et al., 1996).

Several natural processes such as for example mineral precipitation-dissolution, cation exchange, biodegradation, sorption and desorption can be generally described by coupling fluid flow, solute transport and biogeochemical interactions (Regnier et al., 2003). The application of RTMs provides diagnostic and prognostic tool for the comprehension of otherwise complex dynamics of natural environments and they are used mainly to understand the fate and the transport of a selected target compound within a given compartment at local or regional scale.

RTMs are also applied for integrating new experimental, observational and theoretical knowledge about geochemical, biological and transport processes (Regnier et al., 2003).

Concerning groundwater contamination, RTMs are usually addressed to answer to the following questions:

- What will the concentration of the contaminant be at a given location?

- When will the concentration of the contaminant at a certain point reach a given level?

- Will the remediation design achieve the target reduction in contaminant concentration during a given period of time?

RTMs can also be applied to reconstruct the history of a contamination event, to estimate the contaminant concentration to which populations have been exposed, and the time period of that exposure (Zheng and Bennett, 2002).

The modeling process involves at least four fundamental steps (Boudreau, 1997):

- Problem identification

- Model formulation

- Model solution

- Interpretation of model results

Problem identification refers to the definition of the topics and objectives that have to be investigated through simulations. Once the problem to investigate is identified, models are developed to answer the specific questions arose in the problem identification step. During model formulation, all the available data concerning physical and chemical properties of the porous media are inserted in the model; a series of boundary conditions and parameters related to the investigated chemical process (biodegradation, oxidation, sorption, etc.) are considered. Once the model is completed, the simulation can be run and model results can be interpreted and compared to measured data to evaluate if the working hypotheses are correct or have to be modified.

Such a description suggests that a model simulation is a very simple procedure that can solve many problems for hydrogeologists and engineers. In reality, the application of a model simulation to an environmental system could be a very difficult task because the environmental system that is intended to be studied is generally complex and heterogeneous. One of the major concerns about the applicability of RTMs is whether or not they can be used with confidence in predicting future evolution of groundwater systems. Even though models may correctly integrate physical and chemical equations, uncertainties related to the characterization of environmental systems could still remain. Very often, researchers deal with a scarce database if compared to the heterogeneity of the system that they want to model and therefore approximations have to be considered.

These uncertainties are often cited as a reason for avoiding simulation; but in fact, simulations offer the only realistic approach to these difficulties. Uncertainties related to model parameters and/or processes can be evaluated during model simulations by analyzing the effect of parameters and/or processes variations on the model results and comparing them to observed data.

The results of model simulations may influence political decisions that have to be taken in the context of environmental management. Therefore, it is extremely important that models are applied and tested in a variety of environments so that the confidence, the limitations and the reliability of model predictions are properly evaluated before taking decisions that can irreversibly affect the environment.

References

Boudreau, B.P., 1997. Diagenetic models and their implementation. Springer Berlin, pp:414.

Brun, A., Engesgaard, P., Christensen, T.H., Rosbjerg, D., 2002. Modelling transport and biogeochemical processes in pollution plumes: Vejen landfill, Denmark. J. Hydrol. 256 (3-4), 228-247.

IAEA (International Atomic Energy Agency), 1998. Application of isotope techniques to investigate groundwater pollution. IAEA Tecdoc-1046, Vienna.

Lichtner P.C., Steefel, C.I., Oelkers, E.H., 1996. Reactive transport in porous media. Mineralogical Society of America, pp: 438.

Mayer, K.U., Benner, S.G., Frind, E.O., Thornton, S.F., Lerner, D.N., 2001. Reactive transport modeling of processes controlling the distribution and natural attenuation of phenolic compounds in a deep sandstone aquifer. J. Contami. Hydrol. 53 (3-4), 341-368.

Regnier, P., Jourtabchi, P., Slomp, C.P., 2003. Reactive transport modeling as a technique for understanding coupled biogeochemical processes in surface and subsurface environments. Netherlands Journal of Geosciences, 82 (1), 5-18.

Van Breukelen, B.M., Griffioen, J., Röling, W.F.M., van Verseveld, H.W., 2004. Reactive transport modelling of biogeochemical processes and carbon isotope geochemistry inside a landfill leachate plume. J. Cont. Hydrol. 70, 249-269.

Zheng, C., Bennett, G.D., 2002. Applied contaminant transport modelling, 2nd ed. John Wiley & Sons, New York, pp: 621.

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