1,720,975 research outputs found
Characterization of a coastal fractured karstic aquifer by means of sequential indicator simulation algorithm
No full textThe characterization and the description of phenomena that involve fractured and karstic aquifers remains an open issue that calls the attention of numerous researchers belonging to different disciplinary fields as far as the aspects linked both to shallow and to subsoil phenomena are concerned. The geologic knowledge of these phenomena proves to be of high importance especially if considered in relationship with water resource exploitation, with the problems linked to contamination and the ones lined to urban and industrial development of the territory. The examined area is located in the city centre of Bari, inside a geomorphologically depressed zone named "lago di Marisabella" and is heavily contaminated by hydrocarbons. Nowadays thi s area is object of an intense scientific panel discussion because of issues linked to groundwater remediation as well as to the realization of a fill in the bend of Marisabella, located downstream to the examined area. The realization of the landfill has required the development of a detailed local scale hydrogeologic model in order to evaluate the effects on groundwater flow and contaminant propagation. The present work emphasizes the importance of the realization of a detailed geological model of the aquifer that supports the hydrogeologic model in that it permits to widen considerably the comprehension of fluid flow and solute transport phenomena; it can allow consequently to implement conceptually plausible numerical models that make use of adequate state variables (permeability, porosity, solution features) and boundary conditions adherent to reality. This permits to optimize any anthropic intervention that can involve this environmental matrix
Analysis of gravel back-filled borehole heat exchanger in karst fractured limestone aquifer at local scale
Full text linkIn designing and sizing of borehole thermal energy system, natural groundwater movement and temperature driven flow have a great importance on the borehole heat exchanger efficiency. The efficiency of double U – tube arrangement in gravel – backfilled borehole installed in a fractured limestone aquifer has been analyzed by means of three – dimensional numerical simulations. The numerical model is representative of 1 m deep of gravel back – filled borehole surrounded by the fractured aquifer. Several simulations have been carried out in order to evaluate the effect of aquifer parameters and boundary conditions on heat exchange efficiency by varying the mean temperature within the double U - tube. The fractured limestone aquifer of the industrial area of Bari (Italy) has been chosen as field site in order to identify the aquifer parameter range and the respective combinations.
The results highlight that borehole thermal energy system efficiency is strictly dependent on aquifer transmissivity and groundwater Darcian velocity. The conducted analysis shows that, under lower Darcian groundwater flow and lower aquifer transmissivity, heat transfer efficiency increases at least by 25% compared to stagnant water, whereas heat transfer in the aquifer is governed by heat conduction. The increase of aquifer transmissivity induces the thermosiphon effect enhancing heat transfer processes both in the gravel back-filled borehole and aquifer. At higher values of groundwater Darcian velocity (> 0.1 m/d) advection due to groundwater
flow is not negligible and mixed with free convection enhancing heat transfer further. Based on the results, discussion on the performance and environmental constraint of gravel back – filled borehole at field site has been presented
Kinematic diffusion approach to describe recharge phenomena in unsaturated fractured chalk
Full text linkWhen dealing with groundwater resources, a better knowledge of the hydrological processes governing flow in the unsaturated zone would improve the assessment of the natural aquifer recharge and its vulnerability to contamination. In North West Europe groundwater from unconfined chalk aquifers constitutes a major water resource, therefore the need for a good hydrological understanding of the chalk unsaturated zone is essential, as it is the main control for aquifer recharge. In the North Paris Basin, much of the recharge must pass through a regional chalk bed that is composed of a porous matrix with embedded fractures. The case study regards the role of the thick unsaturated zone of the Cretaceous chalk aquifer in Picardy (North of France) that controls the hydraulic response to rainfall. In order to describe the flow rate that reaches the water table, the kinematic diffusion theory has been applied that treats the unsaturated water flow equation as a wave equation composed of diffusive and gravitational components. The kinematic diffusion model has proved to be a convenient method to study groundwater recharge processes in that it was able to provide a satisfactory fitting both for rising and falling periods of water table fluctuation. It has also proved to give an answer to the question whether unsaturated flow can be described using the theory of kinematic waves. The answer to the question depends principally on the status of soil moisture. For higher values of hydraulic Peclet number (increasing saturation), the pressure wave velocities dominate and the preferential flow paths is provided by the shallow fractures in the vadose zone. With decreasing values of hydraulic Peclet number (increasing water tension), rapid wave velocities are mostly due to the diffusion of the flow wave. Diffusive phenomena are provided by matrix and fracture-matrix interaction. The use of a kinematic wave in this context constitutes a good simplified approach especially in cases when there is a lack of information concerning the hydraulic properties of the fractures/macropores close to saturation
The Evaluation Of Deformability Modulus By Rock Mass Classification Systems: A Comparative Study
No full textThe systems of classification of rock mass give, apart from useful qualitative and quantitative indications about the problems connected to the excavation of tunnels and the stability of slopes, also qualitative indications about the strength envelope and about deformability. This paper means to dwell upon the evaluation of deformability of rock masses by passing through the relations available in literature and by discussing the fundamental parameters on which these relations are based, which need to be verified with adequate in situ - tests. An application of some of literature relations to the Bari Limestone rock mass has been developed, on the basis of the achievements obtained through the use of the methods of classification of Bieniawski and Barton and by comparing the results obtained with the ones obtained from in situ tests.
Key Words: Deformability modulus, Rock classification system
Numerical model of the behavior of chlorinated ethenes in a fractured, karstic limestone aquifer
Full text linkCharacterizing the transport and degradation of chlorinated ethenes in fractured aquifers, as well as the assessment of cleanup times, poses an extreme technical challenge. In the presented study, a method to analyze reactive transport and reductive dechlorination of chlorinated solvents in fractured aquifers is developed. A rough-walled parallel-plate model of nonlinear flow behavior is coupled with random-walk particle tracking, incorporating particle exchange between the mobile and stagnant zones, adsorption processes, and reductive dechlorination reaction pathways. The developed methodology, considering reductive dechlorination processes in a Lagrangian framework, is able to simulate the motion of particles affected by first-order network reactions, so that particles move according to their chemical state, affecting physical transport processes (advection, dispersion, mass-transfer exchange between mobile and stagnant zones). The developed model is applied to a case study of groundwater contamination in the industrial area of Bari and Modugno (Italy), where the limestone aquifer has a fractured, karstic nature. The steady-state distribution of the contamination by chlorinated ethenes from a source at a hot spot is obtained and compared with the observed scenario of contamination, in order to estimate the plausible transport and degradation processes and the mass loading at source. The study represents a valuable tool in deciding the role of natural attenuation as a treatment option, where the natural attenuation capacity of groundwater can be integrated with engineering methods in order to obtain site remediation
Numerical modeling of flow and transport in the Bari industrial area by means of rough walled parallel plate and random walk models
Full text linkModeling fluid flow and solute transport dynamics in fractured karst aquifers is one of the most challenging tasks in hydrogeology.The present study investigates the hotspots of groundwater contamination in the industrial area of Modugno (Bari southern Italy), where the limestone aquifer has a fractured and karstic nature. A rough walled parallel plate model coupled with a geostatistical analysis to infer the values of the equivalent aperture has been implemented and calibrated on the basis of piezometric data. Using the random walk theory, the steady-state distribution of hypothetical contamination with the source at the hotspot has been carried out, reproducing a pollution scenario which is compatible with the observed one. From an analysis of the flow and transport pattern it is possible to infer that the anticline affecting the Calcare di Bari formation in direction ENE-WSW influences the direction of flow as well as the propagation of the contaminant.The results also show that the presence of nonlinear flow influences advection, in that it leads to a delay in solute transport with respect to the linear flow assumption. This is due to the non-constant distribution of solutes according to different pathways for fractured media which is related to the flow rate
The application of multivariate geostatistical techniques for the study of natural attenuation processes of chlorinated compounds
No full textModeling spatial variability of environmental parameters through the application of modem techniques of geostatistics has permitted to acquire elements apt to evaluate the possibility of using natural attenuation as a remediation technique for groundwater contaminated by chlorinated compounds. Moreover, a precise delineation of hazardous areas in a polluted site is strongly based on accurate predictions of contaminant concentrations, a task that is complicated by the presence of censored data and highly positively skewed distributions.
The present work proposes a geostatistical approach that includes different methods for the study and the control of the ongoing activity of natural attenuation in an abandoned industrial area. The considered variables in this application are the values of concentration in groundwater of some aliphatic chlorinated carcinogenic compounds (PCE and TCE), measured in numerous piezometers, which show highly skewed distributions, characterized by a considerable number of values below the detection limit of the instrument.
Indicator Kriging has been applied and the areas with 90% probability of exceeding the detection limit have been considered, inside which the variables were transformed through gaussian anamorphosis. The application of geostatistics has put into evidence the existence of a correlation between them not only from the chemical point of view, but also from the point of view of their spatial distribution: the shared presence of "hot spot" areas might be imputed to the existence of localized hazard sources. In those areas it is possible to detect coherence between the concentrations of the contaminants in study and of the parameters that control the phenomenon, which might be considered a clue that the degrading chemical activity is in progress.
In the areas characterized by higher uncertainty for both variables (PCE and TCE), it is possible to presume that the phenomenon of attenuation is in progress, because it is in those areas that the transformations from PCE to TCE may happen in function of the parameters that control the phenomenon
The application of multivariate geostatistical techniques for the study of natural attenuation processes of chlorinated compounds
No full textLaboratory experimental investigation of heat transport in fractured media
Full text linkLow enthalpy geothermal energy is a renewable resource that is still underexploited nowadays in relation to
its potential for development in society worldwide. Most of its applications have already been investigated, such as heating and cooling of private and public buildings, road defrosting, cooling of industrial processes, food drying systems or desalination. Geothermal power development is a long, risky and expensive
process. It basically consists of successive development stages aimed at locating the resources (exploration), confirming the power generating capacity of the reservoir (confirmation) and building the power plant and associated structures (site development). Different factors intervene in influencing the length, difficulty and materials required for these phases, thereby affecting their cost. One of the major limitations related to the installation of low enthalpy geothermal power plants regards the initial development steps that are risky and the upfront capital costs that are huge. Most of the total cost of geothermal power is related to the
reimbursement of invested capital and associated returns. In order to increase the optimal efficiency of installations which use groundwater as a geothermal resource, flow and heat transport dynamics in aquifers need to be well characterized. Especially in fractured rock aquifers these processes represent critical elements that are not well known. Therefore there is a tendency to oversize geothermal plants. In the literature there are very few studies on heat transport, especially on fractured media. This study is aimed at deepening the understanding of this topic through heat transport experiments in fractured networks and their interpretation. Heat transfer tests have been carried out on the experimental apparatus previously employed to perform flow and tracer transport experiments, which has been modified in order to analyze heat transport dynamics in a network of fractures. In order to model the obtained thermal breakthrough curves, the Explicit Network Model (ENM) has been used, which is based on an adaptation of Tang’s solution for the transport of the solutes in a semi-infinite single fracture embedded in a porous matrix. Parameter estimation, time moment analysis, tailing character and other dimensionless parameters have permitted a better understanding of the dynamics of heat transport and the efficiency of heat exchange between the fractures and the matrix. The results have been compared with the previous experimental studies on solute transport
Application of modeling for optimal localization of environmental monitoring sensors
No full textThe control of environmental phenomena is based on the existence of quantifiable and measurable parameters representative of the phenomena themselves. In the case of studies concerning contaminated aquifers, after an initial step of characterization it is necessary to implement groundwater hydrogeochemical monitoring phases. In the field of groundwater pollution long-term monitoring realized by means of samples withdrawal and subsequent analysis is labor-intensive, time-consuming and dependent on remote laboratories for sample analyses. Therefore, a high portion of the costs connected with long- term monitoring is associated with the sampling procedures, samples transportation and laboratory analyses. In this context, the utilization of sensors that provide real time measurements would significantly reduce site characterization times and costs and provide more complete and continuous data-sets. However, within the use of sensors in the environmental monitoring field, one of the most relevant problems is linked, in addition to the requisites of the sensor itself, to the correct individuation of the sensor's position in relation to the phenomenon to monitor. In the present paper are reported some case studies in which the precision of data measured by sensors may be invalidated by an incorrect localization of the monitoring points. In this field it proves to be fundamental to implement models able to simulate the real conditions of the system to be subjected to monitoring. The above mentioned models include both deterministic geostatistical techniques aimed at reconstructing the present situation, and also numerical simulations fmalized at making prediction on spatial and temporal trends of the studied phenomena. On the basis of these simulations it is possible to optimize the positioning of the sensor in such a way as to guarantee the efficiency of data detection
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