Luc Lebbe

GROUNDWATER REMEDIATION AND THE COST EFFECTIVENESS OF PHYTOREMEDIATION

In 1999, phytoremediation was applied at the site of a Belgian car factory to contain two BTEX plumes. This case study evaluates the cost effectiveness of phytoremediation compared to other remediation options, applying a tailored approach for economic evaluation. Generally, when phytoremediation is addressed as being cost effective, the cost effectiveness is only determined on an average basis. This study however, demonstrates that an incremental analysis may provide a more nuanced conclusion. When the cost effectiveness is calculated on an average basis, in this particular case, the no containment strategy (natural attenuation) has the lowest cost per unit mass removed and hence, should be preferred. However, when the cost effectiveness is determined incrementally, no containment should only be preferred if the value of removing an extra gram of contaminant mass is lower than €320. Otherwise, a permeable reactive barrier should be adopted. A similar analysis is provided for the effect determined on the basis of remediation time. Phytoremediation is preferred compared to ‘no containment’ if reaching the objective one year earlier is worth €7 000.

Global sampling to assess the value of diverse observations in conditioning a real‐world groundwater flow and transport model

The use of additional types of observational data has often been suggested to alleviate the ill-posedness inherent to parameter estimation of groundwater models and constrain model uncertainty. Disinformation in observational data caused by errors in either the observations or the chosen model structure may, however, confound the value of adding observational data in model conditioning. This paper uses the global generalized likelihood uncertainty estimation methodology to investigate the value of different observational data types (heads, fluxes, salinity, and temperature) in conditioning a groundwater flow and transport model of an extensively monitored field site in the Netherlands. We compared model conditioning using the real observations to a synthetic model experiment, to demonstrate the possible influence of disinformation in observational data in model conditioning. Results showed that the value of different conditioning targets was less evident when conditioning to real measurements than in a measurement error-only synthetic model experiment. While in the synthetic experiment, all conditioning targets clearly improved model outcomes, minor improvements or even worsening of model outcomes was observed for the real measurements. This result was caused by errors in both the model structure and the observations, resulting in disinformation in the observational data. The observed impact of disinformation in the observational data reiterates the necessity of thorough data validation and the need for accounting for both model structural and observational errors in model conditioning. It further suggests caution when translating results of synthetic modeling examples to real-world applications. Still, applying diverse conditioning data types was found to be essential to constrain uncertainty, especially in the transport of solutes in the model.

MODFLOW procedure to simulate axisymmetric flow in radially heterogeneous and layered aquifer systems

A procedure is outlined to simulate axisymmetric groundwater flow in radially heterogeneous and layered aquifer systems using the unmodified version of MODFLOW. The procedure is straightforward, as it only requires correction of some of the input parameters. In contrast to other MODFLOW procedures to simulate axisymmetric flow, no restrictions are imposed on the type of flow, the discretization of radial distance, or the parameter values. Hence, the method can deal with both confined and unconfined flow, wellbore storage, and axisymmetric aquifer inhomogeneities including effects of finite-thickness skin and gravel pack. Several test cases are presented, which compare the calculated results with existing analytical solutions, the analytic element solver TTim, and the axisymmetric, finite-difference model MAxSym. It is concluded that the MODFLOW procedure is capable of simulating accurately axisymmetric flow in radially heterogeneous multi-aquifer systems.RésuméUne procédure est présentée pour simuler l’écoulement axisymétrique d’eau souterraine dans des systèmes aquifères hétérogènes radialement et stratifiés au moyen de la version non modifiée de MODFLOW. La procédure est simple car elle ne requiert que la correction de quelques paramètres d’entrée. Contrairement à d’autres procédures MODFLOW destinées à simuler un écoulement axisymétrique, aucune restriction n’est imposée au type d’écoulement, à la discrétisation de la distance radiale ou aux valeurs des paramètres. Par conséquent, la méthode peut s’appliquer tant aux écoulements libres que captifs, à l’effet de capacité au puits et aux hétérogénéités de l’aquifère axisymétrique, incluant les effets de skin d’épaisseur finie et du massif de gravier. Plusieurs cas d’étude sont présentés qui comparent les résultats calculés avec des solutions analytiques existantes, le solveur d’éléments analytiques TTim et le modèle aux différences finies axisymétrique MAxSym. Il en est conclu que la procédure MODFLOW est capable de simuler précisément l’écoulement axisymétrique dans des systèmes multi-aquifères radialement hétérogènes.ResumenSe describe un procedimiento para simular los flujos de agua subterránea axisimétricos en sistema acuíferos estratificados y radialmente heterogéneos usando la versión no modificada del MODFLOW. El procedimiento es directo ya que solamente requiere la corrección de alguno de los parámetros de entrada. En contraste con otros procedimientos de MODFLOW para simular flujos axisimétricos, no se imponen restricciones en el tipo de flujo, la discretización de las distancias radiales, o los valores de los parámetros. Por lo tanto, que el método puede tratar tanto con flujo no confinado como confinado, almacenamiento en el pozo, e inhomogeneidades axisimétricas del acuífero incluyendo los efectos de películas de espesores finitos y empaque de gravas. Se presentan varios casos de ensayos, que comparan resultados calculados con soluciones analíticas existentes, la resolución del elemento analítico TTim, y el modelo axisimétrico de diferencias finitas MAxSym. Se concluye que el procedimiento de MODFLOW es capaz de simular con exactitud el flujo axisimétrico en sistemas multiacuíferos radialmente heterogéneos.摘要本文概述了采用未更改的MODFLOW软件模拟放射状非均匀和层状含水层系统中轴对称地下水流的过程。过程非常简单,因为只需要修改一些输入的参数。与其它MODFLOW模拟轴对称流的过程相比,对水流类型、放射状距离的离散化或参数值没有什么限制。因此,这个方法可以处理承压和非承压水流,钻井孔存储、以及轴对称含水层非均质性包括有限厚度--和砾料的影响。展示了几个实验案例,与用目前的解析方法,解析元TTim方法和轴对称有限差模型MAxSym得到的计算结果进行了对比。结论是MODFLOW的过程能够准确模拟放射状非均匀和层状含水层系统中轴对称流。ResumoDescreve-se o procedimento utilizado para simular o escoamento assimétrico de águas subterrâneas em sistemas aquíferos multi-camada radialmente heterogéneos, usando a versão não modificada do MODFLOW. O procedimento é simples, na medida em que apenas requer a correção de alguns dos parâmetros de entrada. Em contraste com outros procedimentos do MODFLOW para simular o escoamento assimétrico de eixos, aqui não é imposta nenhuma restrição nem para o tipo de escoamento, nem para a discretização da distância radial, nem para os valores dos parâmetros. Assim, o método pode lidar com ambas as situações de escoamento confinado e não confinado, armazenamento do poço, e heterogeneidades assimétricas do aquífero, incluindo efeitos de espessura de pele de poço (zona de baixa permeabilidade circundante ao furo) e de filtro de areão calibrado. São testados vários casos que comparam os resultados calculados com soluções analíticas existentes, o elemento analítico da solução TTim, e a simetria axial do modelo de diferenças finitas MAxSym. Conclui-se que o procedimento do MODFLOW é capaz de simular com precisão o escoamento assimétrico em sistemas multi-aquífero radialmente heterogéneos.

Bioremediation: How to deal with removal efficiency uncertainty? An economic application

Bioremediation is a remediation strategy, which has considerable strength but also certain limitations. Complex and uncertain relationships among biomass, contaminants, and nutrients lead to an uncertain level of removal efficiency. The uncertainty inherent to a bioremediation strategy should be addressed in the remediation selection process. In order to evaluate the bioremediation strategy economically, this study takes into account the reversibility of a decision. A decision tree structures the different remediation strategies, thus giving the possible courses of action open to the decision maker. The option value indicates the importance of having the possibility to reverse a previously made decision. Compared with conventional economic evaluation tools, more information to ground the selection made is revealed.