Angelo Sommella

Effectiveness of equilibrium and physical non-equilibrium approaches for interpreting solute transport through undisturbed soil columns

This study observes the dispersion process of chloride through three undisturbed soil columns (sabA, sabB and arg) of different texture under both saturated and unsaturated conditions. Overall, 17 leaching experiments were conducted by means of an equipment designed and built in Portici together with the DITEC of the University of Basilicata. During the experiments, both symmetric and positive asymmetric breakthrough curves of the chloride anion were measured. As expected, manifest asymmetry was obtained especially for soil columns labeled as sabA and arg. In such columns, it was also noted that the degree of asymmetry increased as the flow density increased at the sample surface. The experiments conducted on soil column labeled as sabB resulted in more symmetric BTCs. The experimental data were firstly analysed using the classical two-parameter CD model. The variation of the dispersion coefficient, D, with pore water velocity, upsilon 0, and porous medium characteristics was explored in detail. The application to all experimental data of the four-parameter MIM model, which compartmentalises the pore water in the mobile-stagnant domains provided an insignificant different description of BTCs, even of the asymmetric ones. Furthermore, it resulted in ill-defined parameters, especially those related to the partitioning in the mobile-immobile phases. However, for all the three soils examined the two parameter CD model proved to be very powerful even in interpreting asymmetric BTCs. In any case, it should be emphasised that the agreement of outflow experimental data with a mechanistic model does not unequivocally identify the mechanism of solute transport in the soil. In fact, when we applied the CLT model, which is based on a stochastic approach, the BTCs were comparably well predicted.

Nonreactive solute transport in variously structured soil materials as determined by laboratory-based time domain reflectometry (TDR)

Abstract The possibility of using the time domain reflectometry technique (TDR) is tested for estimating transport parameters of an inert solute (KCl) in the soil. Undisturbed soil samples underwent steady flow processes in the laboratory in near-saturation conditions. Space/time distributions of the chloride ion concentration were analysed with the two-parameter convection–dispersion model (CD). Conventional nonlinear optimisation algorithm was used for the derivation of the parameter values. Applied to soil samples of different textures and structures, the method supplied satisfactory results, which indicates that the straightforward use of TDR may permit the method to be more widely applied, both in the laboratory and in the field. Research is currently going on to study the spatial variability of characteristic parameters of solute transport.

Macrodispersion by diverging radial flows in randomly heterogeneous porous media

Radial flow takes place in a heterogeneous porous formation where the transmissivity T is modelled as a stationary random space function (RSF). The steady flow is driven by a given rate, and the mean velocity is radial. A pulse-like of a tracer is injected in the porous formation, and the thin plume spreads due to the fluctuations of the velocity which results a RSF as well. Transport is characterized by the mean front, and by the second spatial moment of the plume. We are primarily interested in tracer macrodispersion modelling. With the neglect of pore-scale dispersion, macrodispersion coefficients are computed at the second order of approximation, without neglecting the head-gradient fluctuations. Although transport is non-ergodic at the source, it is shown that ergodicity is achieved at small distances from the source. This is due to the fact that close to the source local velocities are quite large, and therefore solute particles become uncorrelated very soon. Under ergodic conditions, we compare macrodispersion mechanism in radial flows with that occurring in mean uniform flows. At short distances the spreading effect is highly enhanced by the large variability of the flow field, whereas at large distances transport exhibits a lesser dispersion due to the reduction of velocities. This supports the explanation provided by Indelman and Dagan (1999) to justify why the macrodispersivity is found smaller than that pertaining to mean uniform flows. The model is tested against a tracer transport experiment (Fernàndez-Garcia et al., 2004) by comparing the theoretical and experimental breakthrough curves. The accordance with real data, that is achieved without any fitting to concentration values, strengthens the capability of the proposed model to grasp the main features of such an experiment, the approximations as well as experimental uncertainties notwithstanding.

On the Longitudinal Dispersion in Conservative Transport Through Heterogeneous Porous Formations at Finite Peclet Numbers

We consider transport of a conservative solute through an aquifer as determined: i) by the advective velocity, which depends upon the hydraulic conductivity K, and ii) by the local spreading due to the pore-scale dispersion (PSD). The flow is steady, and it takes place in a porous formation where, owing to its erratic spatial variations, the hydraulic log-conductivity Y ≡ ln K is modeled as a stationary Gaussian random field. The relative effect of the above mechanisms i)-ii) is quantified by the Peclet number (Pe) which, in most of the previous studies, was considered infinite (i.e. no PSD) due to the overtake of advective heterogeneities upon the PSD. Here, we aim at generalizing such studies by accounting for the impact of finite Pe on conservative transport. Previous studies on the topic required extensive numerical computations [see, e.g. Fiori, 1996]. In the present note we remove the computational burden by adopting the rational approximate expression of Dagan and Cvetkovic [1993] for the covariance of the velocity field. This allows one to obtain closed form expressions for the quantities characterizing the longitudinal plumes dispersion. Transport can be straightforwardly investigated by dealing with a modified Peclet number (Pe¯) incorporating both the PSD and the aquifers anisotropy. The satisfactory match to Cape Cod field data suggests that the present theoretical results lend themselves as a useful tool to assess the impact of the PSD upon conservative transport through heterogeneous porous formations.

USE OF THE JONASSON’S MODEL TO ESTIMATE THE VAN GENUCHTEN PARAMETERS FROM TEXTURAL DATA OF SOME SOILS IN SOUTHERN ITALY

In the light of potential application of deducing soil water retention from some simply determined physical properties, due consideration was given to physical and empirical models in the literature, based on the transformation of a granulometric curve PSD into a retention curve θ(h). In particular, forecasting po- 5 tentials as proposed by Jonasson’s model were evaluated in 15 soils from Southern Italy. The estimated θ(h) curves provided a good reliability only when the weighing factor Wf was optimised. Even if the results obtained should be further verified in soils having different pedological characteristics, the model, used, may be particularly suitable to determine θ(h) curves, when experimental observations are poor or lack.