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Transport Modeling (transport + modeling)
Selected AbstractsMODFLOW/MT3DMS-Based Reactive Multicomponent Transport ModelingGROUND WATER, Issue 2 2003H. Prommer This paper presents a three-dimensional, MODFLOW/MT3DMS-based reactive multicomponent transport model for saturated porous media. Based on a split-operator technique, the model, referred to as PHT3D, couples the transport simulator MT3DMS and the geochemical modeling code PHREEQC-2. Through the flexible, generic nature of PHREEQC-2, PHT3D can handle a broad range of equilibrium and kinetically controlled reactive processes, including aqueous complexation, mineral precipitation/dissolution, and ion exchange. The diversity of potential applications is demonstrated through simulation of five existing literature benchmarks and a new three-dimensional sample problem. The model might be applied to simulate the geochemical evolution of pristine and contaminated aquifers as well as their cleanup. The latter problem class includes the natural and enhanced attenuation/remediation schemes of a wide range of organic and inorganic contaminants. Processes/reactions not included in the standard PHREEQC-2 database but typical for this type of application (e.g., NAPL dissolution, microbial growth/decay) can be defined and included via the extensible PHREEQC-2 database file. [source] Methodology for the evaluation of cumulative episodic exposure to chemical stressors in aquatic risk assessment,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2000Michael G. Morton Abstract An ecological risk assessment method was developed to evaluate the magnitude, duration, and episodic nature of chemical stressors on aquatic communities. The percent of an ecosystem's species at risk from a designated chemical exposure scenario is generated. In effects assessment, probabilistic extrapolation methods are used to generate estimated safe concentrations (ESCs) for an ecosystem using laboratory toxicity test results. Fate and transport modeling is employed to generate temporal stressor concentration profiles. In risk characterization, area under the curve integration is performed on predicted exposure concentration profiles to calculate a cumulative exposure concentration (CEC) for the exposure event. A correction is made to account for the allowable exposure duration to the stressor ESC. Finally, the CEC is applied to the extrapolation model (curve) of the stressor to predict percent species at risk to the episodic exposure. The method may be used for either prospective or retrospective risk assessments. The results of a retrospective risk assessment performed on the Leadenwah Creek, South Carolina, USA, estuarine community are presented as a case study. The creek experienced periodic episodes of pesticide-contaminated agricultural runoff from 1986 through 1989. Although limited biological data were available for method validation, the risk estimates compared well with the Leadenwah Creek in situ bioassay results. [source] Numerical modeling of hydrothermal zinc silicate and sulfide mineralization in the Vazante deposit, BrazilGEOFLUIDS (ELECTRONIC), Issue 2 2009M. S. APPOLD Abstract The Vazante zinc deposit in central Brazil is currently the world's largest known example of a hypogene nonsulfide (i.e. willemite-dominant) zinc deposit. The mineralization is hypothesized to have formed as a result of mixing between a hot, acidic, reducing, metal-rich brine and a cool, more basic and dilute, metal-poor meteoric fluid. The present study sought to investigate this scenario by quantifying the individual effects of temperature, pH, salinity, and oxidation state on willemite and sphalerite solubility, and modeling their combined effects during mixing through reaction path and reactive transport modeling. Solubility calculations showed that in an initially hot, moderately acidic, reducing, metal-rich ore fluid saturated with respect to silica, willemite solubility is relatively insensitive to changes in temperature and log , but highly sensitive to changes in pH and salinity. In contrast, sphalerite solubility was highly sensitive to changes in temperature and log , as well as salinity, and was less sensitive than willemite to changes in pH. Reaction path models sought to extend these observations by modeling the geochemistry of mixing. The results show that mixing is able to produce most of the major zinc ore and gangue minerals observed in the field, though not necessarily at the same paragenetic stages, and that both compositional and temperature changes from mixing are needed. Reactive transport models were formulated to investigate spatial patterns of mineralization. The results showed that sphalerite deposition was strongly controlled by temperature and concentrated in the regions of greatest temperature change. Willemite deposition was concentrated along the interface between the metal-rich ore fluid and the surrounding meteoric fluid. The more rapid transport of solute than heat, in conjunction with the higher concentration of silica than sulfide in both fluids meant that willemite mineralization developed over a broader region and in greater concentrations compared with sphalerite. [source] Transient Leakance and Infiltration Characteristics during Lake Bank FiltrationGROUND WATER, Issue 1 2009B. Wiese Infiltration capacity of bank filtration systems depends on water extraction and hydraulic resistance of the bed sediments. Lakebed hydraulics may be especially affected by clogging, which is dependent on settlement of fine particles, redox potential, and other factors. In the field, most of these processes are difficult to quantify, and thus, when calculating response to pumping the water flux across the sediment surface is assumed to be linearly dependent on the hydraulic gradient. However, this assumption was not adequate to describe conditions at a bank filtration site located at Lake Tegel, Berlin, Germany. Hence, we first assumed the leakage coefficient (or leakance) is spatially distributed and also temporally variant. Furthermore, observations show that the leakance is considerably higher in shallow than in deeper areas; hence, leakance was assumed to be dependent on the existence and thickness of an unsaturated zone below the lake. The proposed explanation of spatial and temporal variability in leakance involves a hypothesis for redox dependent and reversible biogeochemical clogging, supported by geochemical observations in surface water and ground water. Four leakance approaches are implemented in the ground water flow code MODFLOW2000 and calibrated by inverse modeling using the parameter estimation software PEST. These concepts are evaluated by examining the fit to the hydraulic heads, to infiltration measurements, transport modeling results, and considering the degrees of freedom due to the number of calibration parameters. The leakage concept based on the assumption of the influence of an unsaturated zone on clogging processes best explains the field data. [source] Validation of Numerical Ground Water Models Used to Guide Decision MakingGROUND WATER, Issue 2 2004Ahmed E. Hassan Many sites of ground water contamination rely heavily on complex numerical models of flow and transport to develop closure plans. This complexity has created a need for tools and approaches that can build confidence in model predictions and provide evidence that these predictions are sufficient for decision making. Confidence building is a long-term, iterative process and the author believes that this process should be termed model validation. Model validation is a process, not an end result. That is, the process of model validation cannot ensure acceptable prediction or quality of the model. Rather, it provides an important safeguard against faulty models or inadequately developed and tested models. If model results become the basis for decision making, then the validation process provides evidence that the model is valid for making decisions (not necessarily a true representation of reality). Validation, verification, and confirmation are concepts associated with ground water numerical models that not only do not represent established and generally accepted practices, but there is not even widespread agreement on the meaning of the terms as applied to models. This paper presents a review of model validation studies that pertain to ground water flow and transport modeling. Definitions, literature debates, previously proposed validation strategies, and conferences and symposia that focused on subsurface model validation are reviewed and discussed. The review is general and focuses on site-specific, predictive ground water models used for making decisions regarding remediation activities and site closure. The aim is to provide a reasonable starting point for hydrogeologists facing model validation for ground water systems, thus saving a significant amount of time, effort, and cost. This review is also aimed at reviving the issue of model validation in the hydrogeologic community and stimulating the thinking of researchers and practitioners to develop practical and efficient tools for evaluating and refining ground water predictive models. [source] Atmospheric and hydrological transport modelling of SOx emissions in a unique verification contextAICHE JOURNAL, Issue 3 2010B. C. McLellan Abstract In this work, we developed a conceptual model incorporating atmospheric transport and hydrological removal of sulfur compounds from a single isolated source. A process engineering approach with conceptual tanks, reactors, pipes, and valves is used for environmental transport modeling. The work includes verification of the model using current data and historical soil sulfur data from a study 23 yrs earlier, collected from sites in a forest and within 20 km from an isolated coal-fired power plant. This verification opportunity is unique in that the power plant is the single major pollutant source within the airshed. In the conceptual process engineering model, environmental relationships with local soil conditions and climate are modeled. The model is validated for three sampling sites, and a sensitivity analysis shows that rainfall has the greatest variance among several other parameters, including sulfur emissions, dry deposition rate, runoff factor, permeability factor, and airshed dimensions. The model is shown to be suitable for a location-specific sustainability metrics application, but it has limitations that further research could improve on including the incorporation of more complexity with the modeling of ground and surface water flows, atmospheric and soil reactions, and vegetation effects. © 2009 American Institute of Chemical Engineers, AIChE J, 2010 [source] Comparison of 85Kr and 3H Apparent Ground-Water Ages for Source Water Vulnerability in the Collyer River Catchment, Maine,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 1 2008William C. Sidle Abstract:, Apparent ground-water ages as determined by the noble gas isotope 85Kr and the water isotope 3H are compared. Refined gas extraction methodology at the wellhead permits efficient collection of Kr for 85Kr isotope enrichment. 85Kr isochrones elucidate areas of much younger ground-water ages than 3H. Declining 3H activities in the catchment prevent its correlation with the youngest measured 85Kr ages. Source water for most drinking water supplies in the Collyer River catchment is recharged within 40 years BP (2004). Mean-age (,) transport modeling suggests uncertainty of ground-water ages is greatest in the central basin area. [source] A deterministic approach to evaluate and implement monitored natural attenuation for chlorinated solventsREMEDIATION, Issue 4 2007Michael J. Truex A US EPA directive and related technical protocol outline the information needed to determine if monitored natural attenuation (MNA) for chlorinated solvents is a suitable remedy for a site. For some sites, conditions such as complex hydrology or perturbation of the contaminant plume caused by an existing remediation technology (e.g., pump-and-treat) make evaluation of MNA using only field data difficult. In these cases, a deterministic approach using reactive transport modeling can provide a technical basis to estimate how the plume will change and whether it can be expected to stabilize in the future and meet remediation goals. This type of approach was applied at the Petro-Processors Inc. Brooklawn site near Baton Rouge, Louisiana, to evaluate and implement MNA. This site consists of a multicomponent nonaqueous-phase source area creating a dissolved groundwater contamination plume in alluvial material near the Mississippi River. The hydraulic gradient of the groundwater varies seasonally with changes in the river stage. Due to the transient nature of the hydraulic gradient and the impact of a hydraulic containment system operated at the site for six years, direct field measurements could not be used to estimate natural attenuation processes. Reactive transport of contaminants were modeled using the RT3D code to estimate whether MNA has the potential to meet the site-specific remediation goals and the requirements of the US EPA Office of Solid Waste and Emergency Response Directive 9200.4-17P. Modeling results were incorporated into the long-term monitoring plan as a basis for evaluating the effectiveness of the MNA remedy. As part of the long-term monitoring plan, monitoring data will be compared to predictive simulation results to evaluate whether the plume is changing over time as predicted and can be expected to stabilize and meet remediation goals. This deterministic approach was used to support acceptance of MNA as a remedy. © 2007 Wiley Periodicals, Inc. [source] | |||||||||||||