The paper deals with the description of the main processes that describe the transport and fate of pollu-tants in aquifers. Starting from a description of the advective, the diffusion and the dispersion fluxes, a balance equation is written in a Eulerian framework considering the adsorption or exchange of mass be-tween the pore water and the porous matrix, the decay/increase rate, the mass rate of injection or extraction wells. The parameters that determine the pollutant transport as the dispersivity and dispersion coefficients are introduced and their complexity is discussed as a function of the velocity field. Then the type and char-acteristics of the isotherm relationships that describe the exchange between the soil and the water are shown referring to the two main categories of equilibrium and no equilibrium isotherm. The complete statement of a mathematical model of transport phenomena in groundwater has been written. It is discussed if a cou-pled problem occurs or doesn’t occur; a coupled problem arises if the presence of contaminant modifies the density of the water. Then the concentration of the contaminant can induce gravity flow that influences the velocity distribution in the groundwater domain. In this case it is necessary to solve a system of differential equations made up of the pollutant balance, the Darcy equation, the water balance, the isotherm and the state equation that describes how changes in concentration can vary the density of the water. Then an introduction on the inverse problems concerning the groundwater pollution transport is carried out. An important problem is raised in the application of the theory of advection-dispersion to a real case of motion, i.e. on a real and, therefore, naturally heterogeneous terrain; in these cases, critical elements emerge that have led researchers to rethink the validity of classical theory. Indeed, many studies show that the lon-gitudinal dispersion values obtained from field measurements or obtained following the calibration of nu-merical models are very variable (different orders of magnitude) and, even within the same experiment, they show a growing variability in relation to the increase in the time spent in the aquifer. The explanation of this fact has to be found in the so called differentiated advection, i.e. in the continuous variation in the direction and absolute value of the water velocity consequent to the different permeability of natural soils, constituted by alternations of lenses of finite dimensions and different characteristics. It necessary, then to acquire a deep knowledge of the conductivity variability, i.e. the variance of the conductivity, and not only of the average value. The last issues considered in the paper are the solutions of some inverse problems: the location of the pollu-tant sources and the restoring of the pollutant release history. These two are of crucial importance as a pre-liminary step to the contaminate sites remediation actions: the identification of the source allows the re-moval of it and then make more reliable a remediation. The determination of the pollutant release history assumes remarkable practical importance in the case in which the principle "Polluter pays" has to be applied. In fact, it often happens that the most serious epi-sodes of groundwater pollution originate from industrial areas in which various companies have operated in the past. In these cases, the position of the source is practically known, but it is fundamental to deter-mine the time in which these releases were performed. This is an inverse problem in which concentrations in a good number of points, the characteristics of the aquifer, the coefficients of transport, the position of the source of pollutant must be known, and it is of interest to recover the contaminant releases history in order to identify the guilty company to which request the coverage of the remediation costs.
|Titolo:||TRASPORTO DI CONTAMINANTI NELLE FALDE ACQUIFERE|
TANDA, Maria Giovanna [Writing – Original Draft Preparation] (Corresponding)
|Data di pubblicazione:||2019|
|Appare nelle tipologie:||4.1b Atto convegno Volume|