A new model for polluted soil risk assessment
Introduction
In recent years, the problem of evaluating risk related to soil pollution has become increasingly important, worldwide. The increasing number of polluted soils in industrialised counties has required the formalisation of well-defined methodologies for defining the technical and economical limits of soil remediation. In many situations, these limits are defined in terms of general threshold values that, often, cannot be reached even with the so-called best available technology (BAT). This occurs, for example, as a consequence of the characteristics of the pollutants or of the affected soil, or of the extremely high cost or duration of the remedial intervention.
For these reasons, both in North American and European countries, many alternative methodologies based on systematic and scientifically well-founded approaches have been developed in order to determine the real effects of the pollution on receptor targets. Typically, these methodologies are organised into different levels of detail, the so-called “Tiers” (ASTM, 1998). Tier 1 is based on a conservative estimation of the risk for the targets that come from very general and “worst case” general situations. Tier 2 is based on a more detailed and site-specific estimation of the hazard, evaluated by the use of semi-empirical, analytical formulas for the source characterisation, the transport of the pollutant and the target exposition evaluation. Tier 3 is the more detailed and site-specific level of application of the risk assessment methodologies and requires the use of numerical methods with many detailed data from the site and on the receptors (e.g. chemical/physical parameters of the pollutants, hydro-geological data, exposition data, etc.).
Section snippets
The general characteristics of risk assessment methodologies
The increasing use and application of risk assessment procedures has produced a diversification and specialisation of different methodologies, depending on the case under consideration. The polluted soil risk assessment methodologies differ from each other mainly in the way they model the targets and effect of the pollutants (e.g. damages to the human life, chronic health effects, acute health effects, damages to the ecosystems, possible genetics modifications, etc.)
Generally speaking, the risk
The pollutant fate and transport model
Now, we want to focus our analysis on the aspect of the transport of the pollutant from the sources to the receptor sites and on the importance of the correct modelling of the pollutant fate. In particular, we are going to describe in detail a simulation model for the pollutants redistribution among phases at the source points and for the simulation of the effect of pollutants absorption by the receptors. The model we would analyse in detail is an innovative simulation model for the pollutants
The fluid dynamical layer
The flow of a fluid in a saturated porous medium is governed by the Darcy equation, whose generalisation for a multiphase flow is given by the equation:where qα (m s–1) is the volumetric flow rate per unit volume, k (m2) the intrinsic permeability of the solid matrix (it depends on the shape and the size of the pores through which the phase flows), and pα (Pa), krα, μα (Pa s) and ρα (kg m–3) are the pressure, the relative permeability, the viscosity and the density of the
Modelling the transport of chemicals
In the polluted soils, chemicals can then be present in all the phases: as gas in soil air, as a dissolved chemical in the soil water and as a dissolved chemical in the nonaqueous phase, adsorbed to the soil organic matter. Consequentially, the total chemical concentration Ct can be written aswhere Ca is the adsorbed chemical concentration (expressed as the mass of sorbant per mass of dry soil), Cl the dissolved chemical concentration in the wetting phase (expressed as the
Chemical reaction modelling
Typically, many different chemical reactions take place in soils: chemicals contained in a phase can react with other chemicals to form both soluble and/or insoluble (in a certain phase) products (it must be noted that products which are insoluble in water could be soluble in the NAPL phase and vice-versa); soluble species in water may also enter into oxidation/reduction, acid/base reactions, chelations, complexations, and several other types of association that result in soluble products. It
Some possible application to detailed risk analysis evaluation
Besides the possibility of application of this model to the project and analysis of “in situ” bioremediation interventions (Villani et al., 2001), our model can be usefully applied in detailed risk analysis evaluation. For example, it can be used for detailed simulation of the shape of the contamination source and for the simulation of the fate of the pollutant through the migration pathways. In this way, it is possible to integrate the results of this simulation model with the procedures for
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