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CHEMICAL PROCESSES AND MODELING IN ECOSYSTEMS
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Trends in regulatory strategies require EPA to understand better chemical behavior in natural and impacted ecosystems and in biological systems to carry out the increasingly complex array of exposure and risk assessments needed to develop scientifically defensible regulations (GPRA Goal 8.1.1). These trends also require EPA to rely more heavily on predictive modeling. The need for multimedia, multistressor, multipathway assessments (from both the human and ecological perspectives) over broad spatial and temporal scales, places a high priority both on the further elucidation of chemical behavior and on the development of new modeling tools. In response to this need, researchers at NERL-Athens are developing tools for elucidating and modeling the behavior of organic contaminants and nutrients in natural and impacted ecosystems and in complex biological systems. Towards this goal, research is being conducted in four areas, each of which is represented by a group of subtasks: (1) Chemical Behavior Elucidation, (2) Chemical Behavior Parameterization, (3) Ecosystem Characterization, and (4) Process/Reactive Transport Model Development. The integration of these areas is described below.
1. Chemical Behavior Elucidation. Research projects under this subtask focus on elucidating the physical, chemical, enzymatic, biological and geochemical processes controlling the movement and transformation of chemicals and nutrients in environmental and biological systems. The reaction processes being investigated include sorption, hydrolysis, phytotransformation, denitrification, chemical and microbiological redox reactions, and metabolism in biological systems. Laboratory and field experiments are designed to delineate the properties of the chemicals and of the reaction systems that are useful for developing predictive models. Accordingly, the results of these studies provide direction to the research conducted under the Chemical Behavior Parameterization and the Ecosystem Characterization subtasks. Furthermore, the conceptual models derived in this subtask provide the basis for the development of analytical and numerical models described in the Process/Reactive Transport Modeling subtask.
2. Chemical Behavior Parameterization. Research projects under this subtask focus on developing and applying tools for measuring and calculating the species-specific parameters that govern the behavior of chemicals in various media. The SPARC (SPARC Performs Automated Reasoning in Chemistry) computer system, which has been under development for several years, currently calculates a wide array of physical/chemical parameters from molecular structure and basic information about the environment (media, temperature, pressure, pH, etc.). Current and future research under this subtask includes quantifying uncertainty in model outputs, refinement and extension of the SPARC system to additional properties of organic chemicals and to nutrients, and extension of the SPARC system to complex biological matrics. Experimental tools that identify and quantify individual species of organic chemicals and nutrients also are developed and applied under this subtask. Speciation phenomena (ionization, tautomerization, hydration, complexation) are studied for chemicals that are either of high environmental priority or which allow speciation to be modeled with known accuracy in SPARC. Metabolic pathway simulation will be improved by incorporating information on chemical speciation, tautomerizations, and hydrolysis from SPARC and from advanced analytical measurements. Lastly, this subtask research will provide the thermodynamic constants to parameterize the analytical and numerical models developed under the Process/Reactive Transport Model subtask.
3. Ecosystem Characterization. Research projects in this subtask concentrate on characterizing intensive and extensive state variables in the environment that affect the fate and transport of nutrients, contaminants and other chemical species. The chemical- and biological-transformation routes and rates of chemical species in ecosystems are highly sensitive to oxidation state; however, because of analytical difficulties and kinetic constraints, quantitative characterization of this state variable has remained uniquely elusive among intensive state variables. Field and lab studies are underway that seek to elucidate the extent of coupling in oxidation state among multivalent species in solution. Observations of numerous experimental systems have shown that surfaces often catalyze redox reactions and, therefore, characterization of bulk-solution oxidation state will not completely fill the knowledge gap regarding characterization of redox in the environment. Consequently studies also are ongoing in which advanced lab and modeling techniques are being used to elucidate the electrochemical potential of environmental surfaces. In addition to the intensive characteristic of oxidation state, the extensive capacity of environmental systems to be buffered at a given oxidation state also is an important issue. Hence, studies are being conducted in which various selective-extraction and microbial-incubation techniques are being compared for their utility in characterizing the reductive capacity of environmental solids. Additional studies include investigation of the potential for selected mineral surfaces to catalyze hydrolytic reactions, and for dissolution and precipitation of authigenic minerals to buffer the chemical environment of ecosystems.
The research conducted under this subtask will aid in the interpretation of research conducted under the Chemical Behavior Elucidation subtask by improving our capability to define the system to which chemicals are exposed as well as the Chemical Behavior Parameterization and Process/Reactive Transport Model Development subtasks by improving methods for quantifying the chemical state of complex systems.
4. Process/Reactive Transport Model Development. This subtask is concerned with the development of analytical and numerical models to simulate chemical behavior in natural and impacted ecosystems. The models developed in this subtask will include both process models of batch systems and reactive-transport models that couple the effects of chemical and biological reactions with the hydrodynamic processes of advection and dispersion. These models will incorporate process-level representations of the interactions between the chemical(s) of interest and the dissolved species, mineral surfaces and microorganisms encountered in the environment. The modeling efforts in this subtask will address a number of important questions including: 1) How complex does the model have to be to describe chemical behavior?; 2) Are rate constants measured in batch systems representative of the same processes in transport systems?; and 3) If some measure of chemical behavior is of interest, how sensitive is that measure to the parameters of a particular reaction process?
With regard to ecosystem fate, the Process/Reactive Transport Model Development subtask can be regarded as the quantitative integration of the three other subtasks in Task 5549. The models developed in this subtask will be the mathematical implementation of the conceptual models derived in the Chemical Behavior Elucidation subtask. The Chemical Behavior Parameterization subtask will provide the necessary coefficients and rate constants to simulate the relevant transformation, partitioning and speciation reactions. Finally, the Ecosystem Characterization subtask will supply the initial conditions for the models. These initial conditions include important measures of capacity that may control the extent of reaction progress in the ecosystem under consideration.