Erio Pasqualini

Retention of heavy metals in conventional and factory-prehydrated GCLs materials

A conventional Geosynthetic Clay Liner (C-GCL) and a dense prehydrated (DPH) GCL were permeated with an acidic solution (pH=2) containing relatively high concentrations (0.025 M) of Pb, Zn, and Cu as nitrate salts. Both GCLs were permeated with distilled water prior to exposure to the metal solution. The DPH-GCL preserved hydraulic conductivity about one order of magnitude lower than the C-GCL. Breakthrough of metals occurred within three months of permeation for the C-GCL whereas it was not complete after more than one year of permeation for the DPH- GCL material. Cation exchange and precipitation of metals were the likely retention mechanisms.

Effects of Lime Stabilization on Hydraulic Behavior of Finnish Soft Sensitive Clays

The note presents the results of an experimental research study on the hydraulic behavior of Finnish soft clays treated by quicklime. The effect of a wide range of water contents and of curing time on hydraulic characteristics of the treated soil was studied. 7% of quicklime was identified as the optimum lime amount by means of initial consumption of lime tests and by evaluating the drying capability of different lime amounts. Soil-lime samples were prepared by wet mixing and Standard Proctor compaction. Hydraulic conductivity tests are carried out in flexible wall permeameters both on treated and untreated soil. A general increase in hydraulic conductivity due to the addition of lime is observed (2 order of magnitude at optimum conditions). For the treated specimens, a decrease of two orders of magnitude in permeability values was observed, increasing the water content of the soil. Reduction of permeability due to curing time is within 1 order of magnitude. Lime addition reduced water sensitivity and improves the draining capability of the soil.

About the state parameters of active clays

The common use of active clays, such as bentonites, as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical models that take into account the physical, chemical and mechanical coupled phenomena. The elementary particle (platelet) of a bentonite is characterized by a high total specific surface (S) and a permanent or fixed negative electric surface charge (sigma) that interacts with the ions in solution at different concentration (cs) in the pore water. This interaction is the primary factor that determines the final fabric of the solid skeleton and, consequently, most of the bentonite properties in terms of both mechanical behavior and solute transport. The tactoid formation or flocculation is the primary phenomenon that influences bentonite behavior and can be quantified with appropriate state parameters, such as the effective specific surface, Seff, the average number of platelets per tactoid, Nl,AV, and the electric fixed-charge concentration of the solid skeleton, csk,0. On the basis of the aforementioned state parameters, a theoretical hydro-chemico-mechanical framework has been developed. The validity of this framework has been tested, within the present paper, by comparison of its predictions with some of the available experimental results on bentonites with more than 70% montmorillonite content, in terms of hydraulic conductivity, swelling pressure and osmotic efficiency versus void ratio and ion concentrations of the pore solution.

Modeling contaminant leaching and transport to groundwater in Tier 2 risk assessment procedures of contaminated sites

Abstract The current approach to the management of contaminated sites includes a risk assessment procedure as a crucial step. In some jurisdictions (e.g., Italy), groundwater must be considered as a target of contamination. In other cases (e.g., USA), risks from exposure to contaminated groundwater (e.g., through the ingestion of contaminated water) must be taken into account. If the contaminant source is located in the vadose zone, the risk assessment requires modeling of contaminants leaching and subsequent transport to groundwater. In Tier 2 risk assessments, analytical transport models are used for this purpose. This paper compares steady state and a transient approach for modeling the leaching and migration of contaminants to groundwater. The steady-state model that was used considers soil–water partitioning, soil attenuation during transport and dilution with groundwater in a defined mixing zone. The transient model that was used considers source depletion due to leaching and volatilization, one-dimensional solute transport and a novel approach to the modeling of dilution with groundwater. Comparative simulations were performed with regard to a given site geometry and to index substances representing different contaminant classes: aromatic solvents, chlorinated solvents, and heavy metals. The results of the study indicate that, although steady-state models are simple and easy-to-use, taking into account the time variable is essential for a reasonable simulation of contaminant transport to groundwater, particularly for contaminants with high partition coefficients migrating through fine-grained soils.