Tracer test modeling for characterizing heterogeneity and local-scale residence time distribution in an artificial recharge site

Artificial recharge of aquifers is a technique for improving water quality and increasing groundwater resources. Understanding the fate of a potential contaminant requires knowledge of the residence time distribution (RTD) of the recharged water in the aquifer beneath. A simple way to obtain the RTDs is to perform a tracer test. We performed a pulse injection tracer test in an artificial recharge system through an infiltration basin to obtain the breakthrough curves, which directly yield the RTDs. The RTDs turned out to be very broad and we used a numerical model to interpret them, to characterize heterogeneity, and to extend the model to other flow conditions. The model comprised nine layers at the site scaled to emulate the layering of aquifer deposits. Two types of hypotheses were considered: homogeneous (all flow and transport parameters identical for every layer) and heterogeneous (diverse parameters for each layer). The parameters were calibrated against the head and concentration data in both model types, which were validated quite satisfactorily against 1,1,2-Trichloroethane and electrical conductivity data collected over a long period of time with highly varying flow conditions. We found that the broad RTDs can be attributed to the complex flow structure generated under the basin due to three-dimensionality and time fluctuations (the homogeneous model produced broad RTDs) and the heterogeneity of the media (the heterogeneous model yielded much better fits). We conclude that heterogeneity must be acknowledged to properly assess mixing and broad RTDs, which are required to explain the water quality improvement of artificial recharge basins.Peer ReviewedPostprint (published version

Quantifying chemical reactions by using mixing analysis

This work is motivated by a sound understanding of the chemical processes that affect the organic pollutants in an urban aquifer. We propose an approach to quantify such processes using mixing calculations. The methodology consists of the following steps: (1) identification of the recharge sources (end-members) and selection of the species (conservative and non-conservative) to be used, (2) identification of the chemical processes and (3) evaluation of mixing ratios including the chemical processes. This methodology has been applied in the Besos River Delta (NE Barcelona, Spain), where the River Besos is the main aquifer recharge source. A total number of 51 groundwater samples were collected from July 2007 to May 2010 during four field campaigns. Three river end-members were necessary to explain the temporal variability of the River Besos: one river end-member is from the wet periods (W1) and two are from dry periods (D1 and D2). This methodology has proved to be useful not only to compute the mixing ratios but also to quantify processes such as calcite and magnesite dissolution, aerobic respiration and denitrification undergone at each observation point

Optical and magneto-optical studies of wide-bandgap semiconductors

The nature of excitonic localisation in binary, ternary and quaternary wide-bandgap II-VI and III-V semiconductor alloys has been investigated using photoluminescence, resonant vibrational Raman scattering and spin-flip Raman scattering spectroscopies. Despite the importance of wide-bandgap alloys in optoelectronic applications, details of their microstructure, doping and bandstructure remain incompletely understood. In this study doping and compensation issues have been examined in the binary alloy ZnSe and the magnetic field dependent behaviour of electrons and holes bound to neutral impurity centres has been established. In ternary alloys the sensitivity of the hole to strain was exploited to identify unambiguously simultaneously observed electron and hole signals. Modelling the anisotropic hole dependence on the magnetic field direction enabled the residual strain in lattice-matched epilayers to be precisely determined, together with impurity related bandstructure parameters. The k.p approximation was applied successfully to the alloy systems and employed throughout the study in the interpretation of the spectroscopic results, thus providing an important test of the bandstructure theory. The predicted compositional dependence of the electron gyromagnetic ratio was satisfactory, enabling discrimination between different sets of bandstructure parameters published in the literature. However, as with other semiconductors the hole dependence proved more problematic. Magneto-optical measurements revealed evidence of excitons weakly localised by potential fluctuations. Modelling these results with a suitable Hamiltonian enabled the electron-hole exchange interaction to be determined. Significantly, analogous results were obtained in both ZnMgSSe and AlGaInP quaternary alloy systems, inferring that the behaviour is a general feature of disordered alloys. Further, in MBE-grown AlGaInP the exchange splitting was found to be linearly dependent on the degree of localisation, implying the existence of an excitonic mobility edge. A simple model accounting for this linear dependence required a two-dimensional confining potential; this may therefore be related to excitonic localisation at disc-like antiphase boundaries previously reported for AlGaInP, although further work is required to investigate this possibility. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN041422 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A sequence stratigraphic based geological model for constraining hydrogeological modeling in the urbanized area of the Quaternary Besòs delta (NW Mediterranean coast, Spain)

The Quaternary Besòs delta is located on the Mediterranean coast in NE Spain. The Besòs Delta Complex includes 3 aquifers constituted by 3 sandy and gravelly bodies, separated by lutitic units. These aquifers supply water for domestic and industrial use in this area. Management of groundwater has been problematic in the Besòs delta since the 1960s, and continues to pose major problems for subsurface engineering works in this highly urbanized region. This study seeks to demonstrate the advantages of detailed geological characterization and modeling for designing and constructing a hydrogeological model. Available information of the subsurface was compiled, integrated and homogenized in a geospatial database. The interpretation of these data enabled us to delimit geological units by means of a sequence stratigraphic subdivision. A three-dimensional facies belt-based model of the Besòs delta was built on the basis of this geological characterization. This model was used to constrain the distribution of hydraulic parameters and thus to obtain a consistent hydrogeological model of the delta, which was calibrated by data of water management and production over the last hundred years. The resulting hydrogeological model yielded new insights into water front displacements in the aquifer during the time-span considered, improving predictions in an attempt to optimize aquifer management.Peer Reviewe

GIS-Based Software Platform for Managing Hydrogeochemical Data

A GIS-based software platform was developed to arrange all the available hydrogeochemical data into a comprehensive structure and provide support for its proper storage, management, analysis and interpretation. This platform is composed of a geospatial database and a set of analytical instruments integrated in a graphical user interface that coordinates its activities with several software. The geospatial database was specifically developed to store and manage organic and inorganic chemical records, as well as other physical parameters. The analytical tools cover a great range of methodologies for querying, comparing and interpreting groundwater quality parameters. This tools enable us to obtain automatically several calculations such as charge balance error and ionic ratios as well as calculations of various common hydrogeochemical diagrams (e.g. Schöeller-Berkaloff, Piper, Stiff) to which the spatial components are added. Moreover, it allows performing a complete statistical analysis of the data (e.g. generation of correlation matrix and bivariate analysis). Finally, this platform allows handling relevant auxiliary information in an efficient way, and it is coupled to a number of technologies such as hydrogeochemical modelling or geostatistical analysis. The software platform was used in a case study involving several urban aquifers located into the metropolitan area of Barcelona (Spain) to illustrate its performance.Fil: Velasco, Violeta. No especifíca;Fil: Vazquez Suñe, Enric. No especifíca;Fil: García Alcaraz, María del Mar. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Serrano Juan, Alejandro. No especifíca;Fil: Tubau, Isabel. No especifíca;Fil: Sanchez Vila, Xavier. No especifíca;Fil: Fernandez Garcia, Daniel. No especifíca;Fil: Garrido, Teresa. No especifíca;Fil: Fraile, Jpsep. No especifíca

Tracer test modeling for characterizing heterogeneity and local-scale residence time distribution in an artificial recharge site

Artificial recharge of aquifers is a technique for improving water quality and increasing groundwater resources. Understanding the fate of a potential contaminant requires knowledge of the residence time distribution (RTD) of the recharged water in the aquifer beneath. A simple way to obtain the RTDs is to perform a tracer test. We performed a pulse injection tracer test in an artificial recharge system through an infiltration basin to obtain the breakthrough curves, which directly yield the RTDs. The RTDs turned out to be very broad and we used a numerical model to interpret them, to characterize heterogeneity, and to extend the model to other flow conditions. The model comprised nine layers at the site scaled to emulate the layering of aquifer deposits. Two types of hypotheses were considered: homogeneous (all flow and transport parameters identical for every layer) and heterogeneous (diverse parameters for each layer). The parameters were calibrated against the head and concentration data in both model types, which were validated quite satisfactorily against 1,1,2-Trichloroethane and electrical conductivity data collected over a long period of time with highly varying flow conditions. We found that the broad RTDs can be attributed to the complex flow structure generated under the basin due to three-dimensionality and time fluctuations (the homogeneous model produced broad RTDs) and the heterogeneity of the media (the heterogeneous model yielded much better fits). We conclude that heterogeneity must be acknowledged to properly assess mixing and broad RTDs, which are required to explain the water quality improvement of artificial recharge basins.Peer Reviewe

GIS-based software platform for managing hydrogeochemical data

In over three decades, The Handbook of Environmental Chemistry has established itself as the premier reference source, providing sound and solid knowledge about environmental topics from a chemical perspective. Written by leading experts with practical experience in the field, the series continues to be essential reading for environmental scientists as well as for environmental managers and decision-makers in industry, government, agencies and public-interest groups.Peer Reviewe

GIS-based software platform for managing hydrogeochemical data

In over three decades, The Handbook of Environmental Chemistry has established itself as the premier reference source, providing sound and solid knowledge about environmental topics from a chemical perspective. Written by leading experts with practical experience in the field, the series continues to be essential reading for environmental scientists as well as for environmental managers and decision-makers in industry, government, agencies and public-interest groups.Peer ReviewedPostprint (published version

GIS-based software platform for managing hydrogeochemical data

In over three decades, The Handbook of Environmental Chemistry has established itself as the premier reference source, providing sound and solid knowledge about environmental topics from a chemical perspective. Written by leading experts with practical experience in the field, the series continues to be essential reading for environmental scientists as well as for environmental managers and decision-makers in industry, government, agencies and public-interest groups.Peer Reviewe