Fate-Transport-Toxicity Model

Kassim TA, Williamson KJ (2000) Environmental Fate-Transport-Toxicity modeling of highway construction and repair materials on surface and ground waters The project approaches. In Maione U, Lehto BM, Monti R (eds) New trends in water and environmental engineering for safety and life eco-compatible solution for aquatic environments. Balkema, Rotterdam/Brookfield, July 3-7, Capri-ltaly, pp 678-691... 

If significant toxicity is found at one site, then the relationship of toxicity to exposure must be characterized. This may be done by analyzing the relation between toxicity and concentration of chemicals in the media. When sources of toxicants have been identified and tests have been performed on dilution series, the transport and fate of toxicity can be modeled like that of individual chemicals (Suter, 1996). Nevertheless, combined toxic effects of pollutant mixtures are not considered with this approach. 

Due to the certainty that nanomaterials will be exposed to the environment, it is imperative to understand the subsequent fate, transport and transformation that will determine their impact on the environment. A model of the movement of nanoparticles through the air, soil, and water is shown in Figure 21.7 (11). The size-dependent properties of nanomaterials can influence how they will be transported and transformed in the environment therefore, the behavior of larger-sized materials cannot be used to predict nanomaterial behavior. The type of nanomaterial exposed and the medium it enters are important factors for determining the fate and toxicity of the nanomaterial. These factors will also be important when considering measures for exposure control, waste treatment, or removal of nanomaterials from the environment or biological systems. 

UTM-TOX. The Unified Transport Model for Toxicants (UTM-TOX, 52) was developed at Oak Ridge National Laboratory (ORNL) on the base of the ORNL Unified Transport Model (UTM), itself under development from the early 1970 s. UTM-TOX includes air, water, and terrestrial submodels. The aquatic fate submodel includes volatilization, hydrolysis, biolysis, photolysis, and sorption equilibria. Sorbed phases are assumed unreactive in the 1982 version. 

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

An application of transport and compartment-type models to hazard analysis is described in the paper by Honeycutt and Ballantine (19). The compound CGA-72662 running off from agricultural areas into surface waters was modeled in order to set safe application procedures consistent with the protection of aquatic environments. Patterson, et al (2 0) have adapted the UTM model to a software package that is generally applicable to fate assessments of toxic substances in air, water, soil and biota. Their work, now in working draft form, is being used by Dr. William Wood and Dr. Joan Lefler in the Office of Toxic Substances of the U.S. Environmental Protection Agency. 

Phase I focused on a broad screening of common C R material to identify the extent of the problem and to guide the succeeding phases. Phase I resulted in a comprehensive list of commonly used C R materials, their toxicity assessment, and a preliminary description of toxicity assessment protocol, and fate and transport model. 

Hites RA, Lopez-Avila V. 1980. Sedimentary accumulation of industrial organic compounds discharged into a river system. In Baker Ra, ed. Contaminants and sediments. Vo1. 1. Fate and transport case studies, modeling, toxicity. Ann Arbor, Ml Ann Arbor Sci., 53-66. 

The chemical industry has historically been the top TRl emitter, but by figuring in toxicity and exposure, the baton has been passed to a new leader—the primary metals industrial sector. The model does not provide chemical-specific quantitative risk assessments. Instead, EPA developed toxicity weights for each chemical. Those are combined with exposure, fate, and transport information to generate an indicator value for the health impact of emissions from a particular factory (Johnson, 1999). 

The accumulation of hydrophobic contaminants in phytoplankton plays a significant role in the transport and fate of these potentially toxic compounds. However, the limited amount of available field data indicate that partitioning models fail to adequately predict the distribution of these compounds in the water column. Several hypotheses have been proposed to explain these differences. In this chapter we propose additional explanations for these differences. We hypothesize that assumptions in the partitioning model about the rate of uptake, mechanism of uptake, and effect of phytoplankton growth also contribute to these deviations. 

A thorough analysis of atmospheric transport and deposition to the Great Lakes has been carried out using the HYSPLIT model developed by the US National Atmospheric and Oceanic Administration (NOAA) [28,29]. An emissions inventory of PCDD/Fs for North America in 1996 was used as input to the model. Factors considered in the fate and distribution were meteorological data, vapor-particle partitioning, aerosol characteristics, reaction with hydroxyl radicals, photolysis, and dry and wet deposition. The model was generally satisfactory at estimating fluxes, except for HpCDD and OCDD, which appeared to be underestimated by about a factor of four. The model output was summarized as 2378-TeCDD toxic equivalent concentrations (TEQs) based on the WHO mammalian 2378-TeCDD toxic equivalent factors (TEFs) [30]. Since HpCDD and OCDD were estimated to contribute only 2% of TEQs, the model was considered to be valid for the purpose intended. 

The explanation of the pharmacokinetics or toxicokinetics involved in absorption, distribution, and elimination processes is a highly specialized branch of toxicology, and is beyond the scope of this chapter. However, here we introduce a few basic concepts that are related to the several transport rate processes that we described earlier in this chapter. Toxicokinetics is an extension of pharmacokinetics in that these studies are conducted at higher doses than pharmacokinetic studies and the principles of pharmacokinetics are applied to xenobiotics. In addition these studies are essential to provide information on the fate of the xenobiotic following exposure by a define route. This information is essential if one is to adequately interpret the dose-response relationship in the risk assessment process. In recent years these toxicokinetic data from laboratory animals have started to be utilized in physiologically based pharmacokinetic (PBPK) models to help extrapolations to low-dose exposures in humans. The ultimate aim in all of these analyses is to provide an estimate of tissue concentrations at the target site associated with the toxicity. 

In fact, physiologically based pharmacokinetic models are similar to environmental fate models. In both cases we divide a complicated system into simpler compartments, estimate the rate of transfer between the compartments, and estimate the rate of transformation within each compartment. The obvious difference is that environmental systems are inherently much more complex because they have more routes of entry, more compartments, more variables (each with a greater range of values), and a lack of control over these variables for systematic study. The discussion that follows is a general overview of the transport and transformation of toxicants in the environment in the context of developing qualitative and quantitative models of these processes. 

Steen, W.C., Paris, D.F., Baughman, G.L. (1982) Effects of sediment sorption on microbial degradation of toxic substances. In Contaminants and Sediments Fate and Transport, Case Studies, Modeling, Toxicity. Vol. 1, Baker, R.A., Editor, p. 477, Ann Arbor Science, Ann Arbor, Michigan. 

Di Toro DM, Paquin PR, Santore R, Wu KB. 1997. Chemistry of silver bioavailability a model of acute silver toxicity to fish. 5th International Argentum Conference on Transport, Fate and Effects of Silver in the Environment, Hamilton, Ontario, Canada, p 191-204. 

Environmental effects can be examined using studies on the toxicity, persistence and bioaccumulation for the substance in representative studies in individual species, in microcosms and in observations during field trials. Modelling of the transport and fate of the substance is also helpful. Surveys assist in providing baseline data on habitats and communities present. One aim of this exercise will be to determine how tolerant the environment in question will be at accepting the substance before some form of environmental degradation occurs. 

---