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Geochemical Modeling. Bruce Herbert Geology & Geophysics. Introduction. Geochemical models can predict the concentrations (activities) of species present in a system. - PowerPoint PPT Presentation

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Geochemical Modeling

Geochemical ModelingBruce HerbertGeology & GeophysicsEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// IntroductionGeochemical models can predict the concentrations (activities) of species present in a system. From this data, they can be used to deduce and quantify the chemical and biologic processes that affect the fate and transport of pollutants in soils and aquifers. There are two distinct methods that can be used to delineate these processes:Inverse models uses hydrologic and chemical data to deduce the operative geochemical processes in a particular hydrologic systemForward models makes a priori predictions of water chemistry based on assumed geochemical processes.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Hierarchy Model of Geochemical ModelingSpecial considerations:HydrologyMineralogical and other solid phaseEquilibrium and reaction kinetics

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Hydrologic ConsiderationsBasic strategy is to determine geochemical reactions as they occur along a flow pathMatching sampling locations to the questions being asked is an important component of geochemical modelingThe residence time of water in a system controls the amount of time rock-water reactions have to attain equilibriumVarious possible groundwater flow paths and placement of wells at a leaky landfill.

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Hydrologic ConsiderationsVarious possible groundwater flow paths and placement of wells at a leaky landfill.

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Mineralogical and Other Solid Phase ConsiderationsWhat minerals are present; how abundant are they?How does mineral abundance vary spatially?How does mineral composition, including elemental substitutions, ion exchange and crystallinity, vary spatially?How does mineralogy vary with respect to flow?Any evidence for secondary minerals?What type and quantities of organic matter exist?A detailed knowledge of the minerals and natural organic that comprise the solid matrix is needed to determine solid phase reactionsEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Equilibrium and Reaction Kinetics ConsiderationsThermodynamics is a system of consistent chemical equations used to describe the state of a system. In particular it describes the energy differences between different statesUse of thermodynamics to describe geochemical processes is based on the assumption of equilibriumThermodynamics can give information of what reactions may occur (thermodynamically feasible) but not their rateKinetics describes the rates of reactionTypically, water velocity is compared to the rate of reaction. This determines if the system reaches equilibriumEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// ThermodynamicsTo achieve equilibrium, an infinite amount of time must passMany reactions are fast. Ignoring the slow reactions results in a partial equilibrium modelTo include descriptions of slower reactions would result in a description of pseudo-equilibriumThermodynamics describes changes in macroscopic properties of a systemThermodynamics describes the differences in energy between two different states of a system. These differences can be used to predict the final equilibrium state of a system.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic SystemsA thermodynamic system is a macroscopic region of solid, liquid and gaseous matter with electrical and gravitational gradients, enclosed by a bounding surface called the thermodynamic wall. The wall separates the system from its reservoirs. A reservoir is a large thermodynamic system whose intensive properties are unaffected by matter and heat flowing into and out of the reservoir.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: State VariablesState: the set of numerical values of the system's properties. The values of these properties define the energy of the system at a particular state.System properties can either be extensive or intensiveExtensive Properties depend on the amount of matter present: mass, volume, energy, and entropyIntensive Properties are independent of the amount of matter: bulk density, pressure, temperature, and concentration

A thermodynamic system is described or defined in terms of the values of a set of parameters called state variables.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: State Variables

Other state variables may also have to be defined including magnetic, gravitational, and surface propertiesEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: ProcessesA reversible process occurs if the system passes through equilibrium states.Changes that take place in a system spontaneously and of their own accord are natural processes. Because every process is accompanied by a loss of heat to the universe (entropy), natural processes can never be reversed to return the system to its (exact) original condition. Because of this characteristic of natural processes, they are termed irreversible processes.An infinitesimal process occurs if the system's properties change extremely small amounts.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: ProcessesThermodynamic processes take place when thermodynamic properties change. This results in a change in state of the system. A reversible process occurs if the system passes through equilibrium states.Changes that take place in a system spontaneously and of their own accord are natural processes. Because every process is accompanied by a loss of heat to the universe (entropy, see below), natural processes can never be reversed to return the system to its (exact) original condition. Because of this characteristic of natural processes, they are termed irreversible processes.An infinitesimal process occurs if the system's properties change infinitesimally.

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: PhasesThermodynamic systems are defined in terms of their phases (solids, liquids and gases)Given a mass of soil or aquifer material the homogeneity and heterogeneity is defined:The material is uniform if its properties exhibit no spatial variabilityThe material is constant if no temporal variabilityIf intensive properties are uniform and phases are not differentiated, then system is homogeneousEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: PhasesThermodynamic systems are defined in terms of their phases (solids, liquids and gases)Environmental chemistry uses homogeneous mixtures or solutions: uniform intensive properties with no differentiation between phases.Soil air: phases (O2, N2, Ar, CO2) are mixed uniformlyGroundwaters: aqueous mixture of water and dissolved solids.Solid phase mixtures: solid solutions of specific minerals, natural organic matterEnvironmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Thermodynamic Descriptions of Systems: ExchangeExchange describes the exchange of matter and energy between system and surroundings

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Kinetic Descriptions of SystemsClassic geochemical descriptions emphasize equilibriumThree reasons to consider time dependent process:Many reactions are slowNonequilibrium conditions can exist due to the physical transport of gases, solution or soluteskinetic data can provide information about reaction mechanismsResidence times of natural waters along with rates of selected reactions

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Kinetic Descriptions of SystemsSoil solution and groundwaters may not always be well mixed due to time-dependence of diffusionThis creates concentration variations within small volumesThis may induce a time-dependent behaviorTime-dependent behavior can also exist due to spatial variations in steady-state conditions where advection-diffusion is balanced by reaction

Kinetics can describe both chemical reaction rates and physical processes.Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Soil Solution Partitioned between Macropore and Micropore(Left) Diagram of soil solution partitioned between macropore and micropore. Graph of solute concentration (C) as a function of position (x). Taken from (Right) A species present in bulk solution is consumed by a chemical reaction on the available surface. (a) Surface chemical reaction is fast relative to molecular transport. (b) Mixed chemical and transport rate control. (c) Transport is fast relative to surface chemical reaction. Taken from Stone and Morgan (1990) and Skopp (1986).

Environmental Geochemistry at Texas A&M Universityhttp://environmentalgeochemistry.pbworks.com// Internal Energy and Thermodynamic PotentialsInternal Energy, U, of a system is given by the function

Intensive variables (T, P, u) can be written as partial derivatives of one extensive variable to ano

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