Natural Attenuation: an efficient strategy for the remediation of contaminated groundwater.

For four years, PhD student Phil Ham from the University of Utrecht, The Netherlands has conducted research into the transport and fate of contaminated groundwater plumes. Here he speaks to edie about his findings.

His PhD thesis, conducted under the supervision of Dr Ruud Schotting, has received a great deal of interest, not only because of its importance in terms of scientific research but also because the results of some of his studies provide useful modelling tools for engineers, particularly in the assessment of monitored natural attenuation (MNA).

The focus of many hydrogeology studies is upon aquifer remediation, or 'attenuation', by describing transport and (reactive) mixing of solutes in aquifers, or other porous media.

Natural attenuation (NA) and enhanced natural attenuation (ENA) can play an important role in the remediation of contaminated groundwater.

Under polluted sites, for example landfill and industrial sites, there is often talk of mobile contaminated groundwater plumes, made up of dissolved pollutants.

Although the movement of these plumes is determined largely through the hydrology of the underlying site, it is the interplay between physical, chemical and biological processes, which determines the long-time behaviour of such plumes, ie. whether plumes grow, shrink, or become stationary.

In the last two cases, expensive remediation strategies can be avoided because naturally occurring processes reduce, or completely remove, the threat of polluted groundwater to people and to the environment.

Risk-based strategy

Active remediation techniques generally involve the injection or emplacement of particular chemicals and/or micro-organisms into the subsurface to enhance remediation.

One example of active remediation is through the use of bioscreens, ie. permeable reactive barriers artificially created in the subsurface to enhance natural remediation.

Conversely, natural attenuation alone is purely the monitoring of groundwater remediation, where it is believed that the ambient biogeochemical composition of a contaminated aquifer is sufficient to degrade contaminants over time.

To this extent, natural attenuation is not only a collective term for naturally occurring biological and geochemical processes but it is also a risk-based engineering strategy for aquifer restoration.

It is the term "risk-based" which leads to uncertainty and, therefore, only through a thorough understanding of the fundamental processes governing attenuation, is it possible to accurately quantify risks of implementing such strategies.

From Scientific Research to Engineering Tools

The aim of Mr Ham's research project was to gain a fundamental understanding into competing mixing processes involved in natural/enhanced attenuation; processes which occur chiefly in the transition zone between fresh and contaminated groundwater, ie. the plume fringe.

Analytical and numerical methods, in combination with laboratory and field data, were employed to quantify these mixing processes.

In particular, the focus of this work was on transverse dispersion, believed to be the most important mechanism resulting in the mixing of electron acceptors (EA) and electron donors (ED).

Most importantly, a new two-dimensional analytical solution was derived, proving conclusively that transversal dispersion is indeed the most important dispersion mechanism leading to the attenuation of steady-state contaminant plumes where species (e.g. EA and ED) mix and instantaneously react together.

Such cases are prevalent in the subsurface, especially when organic contaminants such as petroleum hydrocarbons, are concerned.

In this case, the length of a contaminant plume can be expressed as a simple algebraic formula, dependent on the concentrations of EA and ED, the porosity of the medium, the groundwater velocity and transversal dispersivity.

The analytical model was applied to field data for a well-documented petroleum hydrocarbon plume, consisting chiefly of a non-reactive benzene plume and a quasi steady-state, reactive toluene plume.

Extracting parameters from data profiles for conservative species (benzene in this case), it was possible to accurately predict plume lengths of the reactive toluene plume.

With the help of fellow researcher Mariene Gutierrez-Neri, this model was extended generically to further include anaerobic core degradation processes, which have been demonstrated to play an important role in some remediation cases.

This occurs when core degradation rates are comparable to fringe degradation, which is often the case when EA availability is limited.

Generic Policy Development and a Health Warning!

Since, globally, environmental issues are of increasing importance it is not surprising that groundwater pollution is a focus of concern.

Unfortunately the term 'contamination' is ambiguous at best, since environmental regulations differ from country to country.

In this way, it is the role of independent environmental authorities to determine if groundwater and aquifers are contaminated and, if so, whether polluted sites should be remediated.

The need for remediation is determined by the risk a polluted site poses, or could pose, to the local population, which is typically a question concerning the possibility of amounts of specific elements or molecular compounds exceeding acceptable or safe limits.

The threat of groundwater contamination and the remediation of already polluted aquifers, along with the need for a generic policy for tackling groundwater remediation, is therefore an important issue that can only be adequately addressed through better scientific understanding of geochemical processes and hydrological (transport) mechanisms.

Analytical models provide quick-to-implement techniques to provide first estimates of plume characteristics, e.g. plume lengths, concentration profiles.

Such models are therefore an important part in the formulation and assessment of an MNA framework, since from an engineering viewpoint, the interest is usually on maximum plume lengths and maximum concentrations at given points.

However, it's important to remember that analytical models use simplifying assumptions to describe an environmental system, which may or may not be valid depending on the site in question.

In most cases, analytical models like those considered Mr Phil Ham, may act as a pre-cursor to more detailed numerical modelling exercises, which can more accurately describe the complexities of the subsurface.

However, the point is, that these simple models provide us with a simple "yes/no" answer as to whether natural attenuation occurs or not, which could be a good basis for the development of a general policy for groundwater restoration.

Dr. Philip Ham can be contacted at ham@geo.uu.nl

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