1,721,002 research outputs found
Simulazione mediante modello numerico di un sistema acquifero multistrato che alimenta una centrale di pompaggio dell'Acquedotto di Milano
No full textIdentification of thermal conductivities by temperature gradient profiles: one-dimensional steady state flow
No full textThe comparison model method (CMM) is applied to the identification of spatially varying thermal conductivity in a one-dimensional domain. This method deals with the discretized steady-state heat equation written at the nodes of a lattice, a lattice which models a stack of plane parallel layers. The required data are temperature gradient and heat source (or sink) values. The unknowns of this inverse problem are not nodal values but internode thermal conductivities, which appear in the node heat balance equation. The main advantage of the CMM is its simple algebraic formulation. Its implementation in the field by means of a pocket calculator allows both a consistency check on the data being collected and estimates of the unknown values of conductivity. -from Author
The hydrogeological role of an aquitard in preventing drinkable water well contamination : a case study
No full textGroundwater pollution has become a worrisome phenomenon, mainly for aquifes underlying industrialized areas. In order to evaluate the risk of pollution, a model of the aquifer is needed. Herewith, we describe a quasi-tridimensional model, which we applied to a multilayered aquifer where a phreatic aquifer was coupled to a confined one by means of an aquitard. This hydrogeological scheme is often met in practice and, therefore, models a number of situations. Moreover, aquitards play an important role in the management of natural resources of this kind. The model we adopted contains some approximations: the flow within the aquifers is assumed to be horizontal, whereas leakage is assumed vertical. The effect of some wells drilled in these aquifers is also taken into account. In order to evaluate the leakage fluxes that correspond to different exploitation conditions, we numerically solve a system of quasilinear and time-dependent partial differential equations. This model has been calibrated by the hydrological data from a water supply station of the Milan Water Works, where water is polluted by some halocarbons. Our simulations account for several experimental facts, both from the hydrogeological and hydrogeochemical viewpoints. Maxima of computed downward leakage rates are found to correspond with measured pollutant concentration maxima. Other results show how the aquitard can help in minimizing the contamination of drinkable water
Comments on "Steady- and transient-state inversion in hydrogeology by successive flux estimation" by P. Pasquier and D. Marcotte
No full textPasquier and Marcotte [Pasquier P, Marcotte D. Steady- and transient-state inversion in hydrogeology by successive flux estimation. Adv Wat Res 2006;29:1934-52] propose some modifications to the Comparison Model Method (CMM), in order to apply it to transient 3D ground water flow data for conductivity identification. We present some remarks on that paper to improve the comprehension of the basic features of the CMM and of the real value of the novelties introduced by Pasquier and Marcotte
Realizzazione di un modello preliminare del flusso idrico nel sistema acquifero della Provincia di Cremona. Relazione illustrativa della definizione delle scale spaziale e temporale del modello di simulazione
No full textA new method for the identification of distributed transmissivities
No full textFor two-dimensional groundwater flow in an isotropic confined aquifer, it has been shown elsewhere that two independent steady state sets of data, ie, piezometric heads and source terms corresponding to different steady state flow conditions, and the value of transmissivity at one point suffice to determine transmissivity uniquely in a connected domain. The data are independent if the hydraulic gradients are not parallel anywhere over the domain. The applicability of this technique to real cases is tested with two synthetic examples. The fit is good. The relative error for the identified internode transmissivities is very low when error-free data are used, and it varies by an amount approximately constant over the entire aquifer when an error on the initial value of transmissivity is introduced. The errors on the piezometric heads bear more relevance, but nonetheless, the affected results are still good. -from Author
Some lessons from modelling ground water flow in the metropolitan area of Milano (Italy) at different scales
No full textThe importance of observations on fluxes to constrain ground water model calibration
No full textThe aquifer system in the alluvial basin bordered by Adda, Po and Oglio rivers (Northern Italy) is characterised by a dual flow regime.
In shallow sediments, which constitute a phreatic aquifer with high conductivity, great fluxes are driven by the interaction between
ground water and the network of surface water, by the infiltration of rain and irrigation water, and by the fluxes drained from depression
springs and river valley terraces. The underlying semiconfined aquifers are characterised by minor fluxes driven by water abstraction
from wells of the public Water Works. Since most of the ground water flow occurs in the phreatic aquifer, an equivalent single layer
2D steady state flow model has been calibrated. The identification of the transmissivity field at the scale of the model has been obtained
by solving an inverse problem with the comparison model method which requires an initial configuration, i.e., reference head, initial
transmissivity field, source terms. Most of the head and source data are related to the phreatic aquifer, but most of the estimates of transmissivity
are obtained with field tests conducted in deep wells pumping from the semiconfined aquifers, so that this kind of prior information
cannot be used directly for model calibration. The inverse problem is underdetermined and a unique solution is not available.
Furthermore information on surface hydrology is poor. Therefore many tests with different hypotheses about the initial configuration
have been performed and some of them have been selected and used to initialise the automatic inversion procedure
Is the forward problem of ground water hydrology always well posed?
No full textComplex aquifer systems are often modeled with quasi-three-dimensional models. which consider two-dimensional horizontal flow in the aquifers and one-dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically, The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions
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