Perrochet, Pierre

2001, Guyonnet, Dominique, Perrochet, Pierre, Come, Bernard, Seguin, J J, Parriaux, Aurèle

In the context of municipal solid waste and hazardous waste disposal, the notion of "equivalence" between different barrier designs appears in regulatory documents from several industrialized countries. While in the past, equivalence has been thought of mainly in terms of contaminant travel times, in recent years it has been defined more in terms of the magnitude of a disposal site's potential impact on groundwater resources. This paper presents some original analytical solutions to the problem of contaminant migration through a multi-layered mineral barrier. The solutions account for the two major mechanisms of subsurface contaminant migration, namely, advection and diffusion-dispersion. An example application using the proposed solutions and a numerical model illustrates how one multi-layered mineral barrier can be considered superior to another from a strictly hydro-dispersive viewpoint. The influence of partial saturation of the mineral barrier is investigated using a numerical solution to the Richards equation for unsaturated flow. It is emphasized that conclusions relative to the superiority of one multi-layered barrier, with respect to another, should not only consider hydro-dispersive aspects, but also other processes such as the mechanical and chemical evolutions of the different barrier components. Although such phenomena are poorly addressed by existing models, failure to take them into account, at least in a qualitative fashion, may lead to unconservative conclusions with respect to barrier equivalence. (C) 2001 Elsevier Science B.V. All rights reserved.

1999, Guyonnet, Dominique, Come, Bernard, Perrochet, Pierre, Parriaux, Aurèle

The Monte Carlo method is a popular method for incorporating uncertainty relative to parameter values in risk assessment modeling. But risk assessment models are often used as screening tools in situations where information is typically sparse and imprecise. In this case, it is questionable whether true probabilities can be assigned to parameter estimates, or whether these estimates should be considered as simply possible. This paper examines the possibilistic approach of accounting for parameter value uncertainty, and provides a comparison with the Monte Carlo probabilistic approach. The comparison illustrates the conservative nature of the possibilistic approach, which considers all possible combinations of parameter values, but does not transmit (through multiplication) the uncertainty of the parameter values onto that of the calculated result. In the Monte Carlo calculation, on the other hand, scenarios that combine low probability parameter values have all the less chance of being randomly selected. If probabilities are arbitrarily assigned to parameter estimates, without being substantiated by site-specific field data, possible combinations of parameter values (scenarios) will be eliminated from the analysis as a result of Monte Carlo averaging. This could have a detrimental impact in an environmental context, when the mere possibility that a scenario may occur can be an important element in the decision-making process.

1998, Guyonnet, Dominique, Seguin, J J, Come, Bernard, Perrochet, Pierre

This paper presents analytical solutions for estimating the potential impact of waste disposal sites (landfills) on groundwater. The solutions yield estimates of the average groundwater concentrations in an aquifer located below the landfill, and of the pollutant flux at the interface between a mineral barrier and the aquifer. Several source boundary conditions are considered: a constant source concentration, an exponentially decreasing source concentration, and a source concentration that is controlled by the diffusive release of pollutants from the waste. The latter is of particular relevance to the case of stabilized-waste landfills. The solution for the case of stabilized-waste is compared to two numerical models, and developed in terms of dimensionless variables that are combinations of the real model parameters. These variables are then used to generate type curves for estimating peak aquifer concentrations. An example application of the proposed type curves illustrates their usefulness for evaluating the performance of stabilized-waste landfills, in terms of potential impact, for different landfill designs.