Chemical substances modeling fate

The quantitative water air sediment interaction (Qwasi) model was developed in 1983 in order to perform a mathematical model which describes the behavior of the contaminants in the water. Since there are many situations in which chemical substances (such as PCBs, pesticides, mercury, etc.) are discharged into a river or a lake resulting in contamination of water, sediment and biota, it is interesting to implement a model to assess the fate of these substances in the aquatic compartment [34]. 

The ability to predict the behavior of a chemical substance in a biological or environmental system largely depends on knowledge of the physical-chemical properties and reactivity of that compound or closely related compounds. Chemical properties frequently used in environmental assessment include melting/boiling temperature, vapor pressure, various partition coefficients, water solubility, Henry s Law constant, sorption coefficient, bioconcentration factor, and diffusion properties. Reactivities by processes such as biodegradation, hydrolysis, photolysis, and oxidation/reduction are also critical determinants of environmental fate and such information may be needed for modeling. Unfortunately, measured values often are not available and, even if they are, the reported values may be inconsistent or of doubtful validity. In this situation it may be appropriate or even essential to use estimation methods. 

These models require accurate data on physico-chemical properties of organic substances, which is the subject of Dr. Mackay s other interest, namely their measurement and correlation. This includes the compilation and critical review of these properties and their quantitative structure property relationships. He is co-author of the five-volume Illustrated Handbook of Physical Chemical Properties and Environmental Fate of Organic Chemicals, which documents data reported in the literature, and is also available in CD-ROM format from CRC Press. Dr. Mackay s hope is that a combination of the information reported in these handbooks, and the estimated data as described in the present volume, can provide a sound basis for assessment of the large and growing number of chemical substances of environmental concern. 

The EPA uses QSARs to predict a large number of ecological effects, as well as for environmental fate within the PMN process. The EPA s website (www.epa.gov) provides a valuable source of further information on all these predictive methods, as well as a database and aquatic toxicity values and detailed information on how the models have been validated. Many of the predictive models have been brought together into the EPISUITE software (see Table 19.2 for a listing of the models available). This includes the OPPT s models used for the prediction of physical and chemical properties for new chemical substances. The EPISUITE software is downloadable free of charge (www.epa.gov/oppt/exposure/docs/episuitedl.htm). This provides not only an excellent resource for the development of QSARs, but also a transparent mechanism for the assessment of PMNs. 

Due to the difficulties of getting analytical solutions, many numerical methods were developed to simulate the solute transport and retention processes in the soil. Deane et al. (1999) analyzed the transport and fate of hydrophobic organic chemicals (HOCs) in consolidated sediments and saturated soils. Walter et al. (1994) developed a model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. Islam et al. (1999) presented a modeling... 

The number of chemical substances that are in use and constitute a potential risk to the environment exceeds today 100.000 (EEA 1998). Even with the proposed new system for registration, evaluation and authorisation of chemicals, REACH, the number of chemicals that will be included in this scheme will be approx. 30.000 (COM 2001 COM 2003). It is obvious that it is not practically possible experimentally to generate all necessary input for the risk assessment of these compounds. Information concerning the fate and effects of these substances in the environment is needed and may be obtained through modelling, e.g., by comparison with structurally re-... 

In the hazard assessment of chemicals released into the environment, it is important to evaluate the environmental concentrations of chemicals. Predicted environmental concentrations (PEC) are compared with toxicity data of environmental organisms. Thus, environmental fate models are becoming a valuable tool for the assessment of the potential hazard of chemicals, especially new chemical substances. 

Information about persistence is essential for the environment risk assessment of chemical substances. Persistence is needed as input for all predictive approaches, from simple leaching indexes to more complex models. Nevertheless, the availability of reliable persistence data is, at present, the weakest link in the prediction of the environmental fate of chemicals. 

Multiple processes can affect the fate and transport of a chemical substance, each of which can depend not only on the physicochemical properties of the substance but also on the environment around it. In general terms, the processes include changes in state, biodegradation and bioaccumulation, and chemical reactions advective transport can move a substance with wind or water within a localized area or even globally. We look at these processes individually before exploring through examining models and specific examples how the processes combine to determine a chemical s fate and transport in the environment. 

Although most multimedia environmental models generally reflect the same fate and transport mechanisms, they do so at different scales. Some models, such as EPISuite, describe a "generic" environment. Other models are specific to certain regions or else simulate the global transport of chemical substances [66. ... 

As even this brief summary shows, evaluating the fate of a chemical substance during its life cycle requires that the assessor consider many steps that can affect the fate and transport of a chemical substance. Each step, captured in one or more mathematical relationships, requires the modeler to make numerous assumptions to simulate the range of real-world conditions. 

These estimated parameters can be used to model the behavior of a new chemical substance. The inaccuracies inherent in deriving physical/chemical properties or fate and transport behavior from SAR, rafher fhan from reliable experimental data, will obviously affect the accuracy of exposure predictions. 

His interests have included the fate and effects of oil spills, especially under cold climate and arctic conditions, environmentally relevant physical chemical properties of organic chemicals and the development and validation of models of chemical fate in the environment. He has introduced the concept of fugacity to environmental modeling using Mackay-type models and has a particular interest in human exposure to chemical substances, bioaccumulation processes, that is, uptake of chemicals by a variety of organisms, and chemical transport to and fate in cold climates. 

Besides the fugacity models, the environmental science literature reports the use of models based on Markov chain principle to evaluate the environmental fate of chemicals in multimedia environment. Markov chain is a random process, and its theory lies in using transition matrix to describe the transition of a substance among different states [39,40]. If the substance has all together n different kinds of states,... 

The research published in this book uses the presently most comprehensive multicompartment model, the first which comprises a coupled atmosphere-ocean general circulation model (GCM). GCMs are the state-of-the-art tools used in climate research. The study is on the marine and total environmental distribution and fate of two chemicals, an obsolete pesticide (DDT) and an emerging contaminant (perflu-orinated compound) and contains the first description of a whole historic cycle of an anthropogenic substance, i.e. from the introduction into the environment until its fading beyond phase-out. 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

Possible fate in the environment. An industrial chemical that has been released into the environment will exist in differing concentrations in the various environmental compartments. The concentrations of a substance in air, water, soil and other media following release can be modelled using the concept of fugacity.2 At its simplest, this involves only the use of standard physico-chemical data to estimate the partitioning between the various media. 

SimpleBox was created as a research tool in environmental risk assessment. Simple-Box (Brandes et al. 1996) is implemented in the regulatory European Union System for the Evaluation of Substances (EUSES) models (Vermeire et al. 1997) that are used for risk assessment of new and existing chemicals. Dedicated SimpleBox 1.0 applications have been used for integrating environmental quality criteria for air, water, and soil in The Netherlands. Spreadsheet versions of SimpleBox 2.0 are used for multi-media chemical fate modeling by scientists at universities and research institutes in various countries. SimpleBox models exposure concentrations in the environmental media. In addition to exposure concentrations, SimpleBox provides output at the level of toxic pressure on ecosystems by calculating potentially affected fractions (PAF) on the basis of species sensitivity distribution (SSD) calculus (see Chapter 4). 

Structural descriptors are implemented extensively in the development of prediction models for a large number of endpoints related to the fate and toxicity of organic chemicals in the environment and to human health. The TIs were reported for the modeling of properties of the pure substances such as boiling point (Basak et al., 1996), vapor pressure (Liang and Gallagher, 1998) and water... 

Exposure assessment, however, is a highly complex process having different levels of uncertainties, with qualitative and quantitative consequences. Exposure assessors must consider many different types of sources of exposures, the physical, chemical and biological characteristics of substances that influence their fate and transport in the environment and their uptake, individual mobility and behaviours, and different exposure routes and pathways, among others. These complexities make it important to begin with a clear definition of the conceptual model and a focus on how uncertainty and variability play out as one builds from the conceptual model towards the mathematical/statistical model. 

Laws regulating toxic substances in various countries are designed to assess and control risk of chemicals to man and his environment. Science can contribute in two areas to this assessment firstly in the area of toxicology and secondly in the area of chemical exposure. The available concentration ( environmental exposure concentration ) depends on the fate of chemical compounds in the environment and thus their distribution and reaction behaviour in the environment. One very important contribution of Environmental Chemistry to the above mentioned toxic substances laws is to develop laboratory test methods, or mathematical correlations and models that predict the environ-... 

It is also necessary to understand the fate of the chemical in the in vitro system used, be it simple or complex. The nominal in vitro dose applied is not informative enough on cell exposure substances can attach to assay system walls or culture medium proteins, they can be degraded passively or actively, evaporate, penetrate cells differently from in vivo, etc. Knowing that, it is clearly more opportune to use intracellular concentration as a basis for extrapolation, rather than plasma or even unbound plasma concentration. Therefore, a computational model of in vitro PK able to estimate intracellular concentrations, together with the relevant data, is also needed, as described in a specific section below. 

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