Showing papers by "Massimo Rolle published in 2019"

Model-Based Interpretation of Groundwater Arsenic Mobility during in Situ Reductive Transformation of Ferrihydrite

Abstract: Arsenic (As) release and mobility in groundwater is coupled to the iron (Fe) cycling and the associated transformation of Fe-oxides present in sediments. Recent in situ experiments have provided observations on arsenic mobilization and co-occurring reductive mineral transformation when placing As-loaded ferrihydrite-coated sand for 80 days in wells of an As-contaminated aquifer of Northern China. However, the complex temporal change in solid-associated arsenic and the multiple geochemical processes occurring when the flowing groundwater contacts the As-loaded ferrihydrite-coated sand hamper a detailed evaluation of the experimental data set. In this study, we develop a modeling approach that allows a quantitative interpretation of arsenic release and ferrihydrite transformation observed during the in situ experiments. The model accounts for the interplay of abiotic and biotic geochemical processes (i.e., surface complexation, reductive dissolution, formation of secondary iron minerals, and arsenic sequestration into the newly formed minerals) involved in the transformation of Fe-oxides and controlling arsenic mobility. The results show the capability of the proposed approach to reproduce the temporal trends of solid arsenic and ferrihydrite concentrations, as well as the spatial variability of mineral transformation, observed in different wells using a common set of surface complexation parameters and kinetic rate constants. The simulation outcomes allowed us to disentangle the specific contribution of the different mechanisms controlling the release of arsenic. It was possible to identify an initial rapid but minor release of As (13-23% of the initial surface concentration) due to desorption from ferrihydrite, as well as the reduction of adsorbed As(V) to As(III) upon contact with the flowing anoxic groundwater. Successively, reductive dissolution of ferrihydrite caused the decrease of the amount of the Fe mineral phase and led to a major depletion of solid-associated arsenic. The produced Fe(II) catalyzed the ferrihydrite conversion into more crystalline Fe(III) oxides (i.e., lepidocrocite and goethite) through Ostwald ripening, and resulted in the formation of siderite and mackinawite upon reaction with carbonates and sulfides naturally present in the groundwater. The model results also showed that, whereas the decrease in surface sites during reductive dissolution of ferrihydrite promoted arsenic mobilization, the mineral transformation limited As release through its sequestration into the newly formed secondary mineral phases.

Multicomponent Ionic Transport Modeling in Physically and Electrostatically Heterogeneous Porous Media With PhreeqcRM Coupling for Geochemical Reactions

Abstract: Low-permeability aquitards, such as clay layers and inclusions, are of utmost importance for contaminant transport in groundwater systems. Although most dissolved species, contaminants, and clay surfaces are charged, the role of electrostatic interactions in subsurface flow-through systems has not been extensively investigated. This study presents a two-dimensional multicomponent reactive transport investigation of diffusive/dispersive and electrostatic processes in homogeneous and heterogeneous clay systems. The proposed approach is based on multiple continua and is capable to accurately describe charge interactions during ionic transport in the free water, diffuse layer, and interlayer water of charged porous media. The diffuse layer composition is simulated by considering a mean electrostatic potential following Donnan approach, whereas the interlayer composition is calculated by adopting the Gaines-Thomas convention. Diffusive/dispersive fluxes within each sub-continuum (free water, diffuse layer, and interlayer) are calculated solving the Nernst-Planck equation while maintaining a net zero-charge flux. Furthermore, the

Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa

Abstract: The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep ( 10 μg/L) stemming from the oxidation of sulphide minerals (pyrite, arsenopyrite) in mineralised zones. These aquifers are still little investigated, especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry. To gain insights on the temporal aspects of As contamination, we have used isotope tracers (noble gases, 3 H, stable water isotopes ( 2 H, 18 O)) and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso. Results revealed a great variability in groundwater properties (e.g. redox conditions, As concentrations, water level, residence time) over spatial scales of only a few hundred metres, characteristic of the highly heterogeneous fractured underground. Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters. Arsenic concentrations are relatively stable over the course of the year, with little effect seen by the monsoon. Groundwater residence time does not seem to have an influence on As concentrations, as elevated As can be found both in groundwater with short ( 10 3 a) residence times as indicated by 3 He/ 4 He ratios spanning three orders of magnitude. These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions. The existence of very old water portions with residence times >10 3 years already at depths of

A Large Undisturbed Column Method to Study Flow and Transport in Macropores and Fractured Media

Abstract: Intact soil columns can bridge the gap between field studies and idealized laboratory investigations of flow and transport in macropores and fractured media. However, the value of intact column studies is often hampered by shortcomings such as lack of column intactness, small column size, and column rim flow, which can cause serious artifacts and hamper system understanding. The flexible-wall pressurized large undisturbed column (LUC) method overcomes these limitations and is a valuable approach to analyze fluid flow and solute transport in macroporous and fractured geological formations. The method investigates subsurface processes in complex media, mimicking in situ conditions and facilitating the control of system boundary conditions including effective stress. In recent years, considerable experience has been gained through different applications of the LUC approach. Modeling tools have also been developed for a detailed interpretation of flow and transport processes in LUC systems. This paper describes the steps of the LUC method from column excavation in the field to experimental setup in the laboratory. The description encompasses the key features of the sampling of LUCs in field excavations, the laboratory setup, the procedure for hydraulic and transport experiments, as well as practical challenges and potential issues during operation of an LUC system. Application examples with a fully three-dimensional numerical model of LUC tracer experiments are also presented to illustrate the quantitative interpretation of transport processes in macroporous clayey tills.