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CalTOX

Submitted by arasikai on Wed, 2010-01-20 10:18
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CalTOX is a set of spreadsheet models and spreadsheet data sets to assist in assessing human exposures from continuous releases to multiple environmental media, i.e. air, soil, and water. The modeling components of CalTOX include a multimedia transport and transformation model, multi-pathway exposure scenario models, and add-ins to quantify and evaluate uncertainty and variability. All parameter values used as inputs to CalTOX are distributions, described in terms of mean values and a coefficient of variation, rather than as point estimates or plausible upper values such as most other

models employ. This probabilistic approach allows both sensitivity and uncertainty analyses to be directly incorporated into the model operation.

Model description
Purpose: 

The multimedia transport and transformation model is a dynamic model that can be used to assess time-varying concentrations of contaminants that are placed in soil layers at a time-zero concentration or contaminants released continuously to air, soil, or water. This model assists the user in examining how chemical and landscape properties and exposure factors impact both the ultimate route and quantity of human contact.

Multimedia, multiple pathway exposure models are used in the CalTOX model to

estimate average daily potential doses within a human population linked to a

pollutant source. The exposure models encompass twenty-three exposure pathways. The exposure assessment process consists of relating contaminant concentrations in the multimedia model compartments to contaminant concentrations in the media with which a human population has contact (personal air, tap water, foods, household dusts soils, etc.). The average daily dose is the product of the exposure concentrations in these contact media and an intake or uptake factor that relates the concentrations to the potential dose within the population.

Boundaries: 

Applications Limitations

As is the case with any model, CalTOX was designed for use in a limited range of spatial scales, time scales, geographic conditions, and chemical classes. These technical limitations apply to version 4.0 of CalTOX. Future versions of the model may be modified to overcome these limitations.

 

Chemical Limitations

CalTOX is designed primarily for non-ionic organic chemicals and inorganic chemicals with linear and reversible distribution coefficients in soil and sediments. CalTOX was August 2002 4 University of California also designed to model emissions that can be treated as uniform over a region. CalTOX has not been designed or tested for surfactants or volatile metals. It can be used for partially ionized organic chemicals only when great care is exercised to adjust the partition coefficients to confirm that they are appropriate for the pH of the landscape under consideration. The model is further limited to chemicals for which one has reliable values for physical and chemical parameters.

Time Scale Limitations

The CalTOX transport model is intended for application over long time scales, months to years. It should be used cautiously for time periods less than one month and then only when properly time-averaged landscape properties are employed. When this is not the case, CalTOX can be used, but some discount of model reliability must be considered.

Landscape Limitations

CalTOX should not be used for landscapes in which water occupies more than 10% of the land surface area. If an area is too small, it is flagged by an error message in the program.

Input: 

Input data is required for

  • Chemical properties (molecular weight; octanol-water partition coefficient; melting point; solubility, Henry’s law constant or vapor pressure; diffusion coefficients in pure air and water; and intermedia distribution coefficients,

    such as Kd and Koc; media-specific transformation rates)
  • Landscape properties (meteorological data, hydrological data, and soil properties)
  • Exposure factors for the population (exposure duration and averaging time, anatomical and dietary properties, food consumption patterns, activity patterns and exposure times, household parameters, other human factors such as soil ingestion and breast milk intake, and parameters associated with food crops and food producing animals. In addition, the calculation of intermedia transfer factors requires that a number of partition factors be available)
  • Information of toxicity

Input information can be user-defined or the data provided by the system. Input data is fed as spreadsheet format to the system.

Output: 

Results as distributions (when applicable) are provided for

  • Exposure
  • Potential dose
  • Exposure media concentration
  • Cumulative dose
Description of processes modelled and of technical details: 

Multimedia, multiple pathway exposure models are used in the CalTOX model to estimate average daily potential doses within a human population linked to a pollutant source. The exposure models encompass twenty-three exposure pathways. The exposure assessment process consists of relating contaminant concentrations in the multimedia model compartments to contaminant concentrations in the media with which a human population has contact (personal air, tap water, foods, household dusts soils, etc.). The average daily dose is the product of the exposure concentrations in these contact media and an intake or uptake factor that relates the concentrations to the potential dose within the population.

CalTOX has been designed to provide users with the options of: (1) making point estimates of average exposure during an exposure duration, (2) applying a sensitivity analyses to these point estimates, (3) using a Monte Carlo add-in to make stochastic estimates of the distribution of exposure uncertainty/variability, and (4) applying a

sensitivity analysis to the results of the Monte Carlo analysis to determine the variance contributions of model inputs to the overall variance in the estimated distribution of exposure and risk.

Excel Spreadsheet Program

CalTOX is designed to work with either Microsoft Excel 97 or Microsoft Excel 2000. We have developed macros that allow you to load data, operate the CalTOX worksheet, and view the results of a CalTOX calculation with a minimum understanding of Excel. Nonetheless, those with a more in-depth understanding of Excel will be able to master the operations of CalTOX more rapidly and be more comfortable making their own modifications to the program execution.



Fugacity Mass-Balance Models

CalTOX includes an eight-compartment regional and dynamic multimedia fugacity model. For all chemicals, fugacity and fugacity capacities are used to represent mass potential and mass storage within compartments. The eight compartments used in CalTOX are (1) air, (2) ground-surface soil, (3) plant leaves, (4) plant leaf surfaces, (5) root-zone soil, (6) the vadose-zone soil below the root zone, (7) surface water, and (8) sediments. Contaminant concentrations in ground water are currently based on a simple mass balance. CalTOX accounts systematically for gains and losses in each compartment and for the whole system in concert. Understanding the results of the CalTOX spreadsheet requires a basic familiarity with this model, which is described in the CalTOX reports (DTSC, 1993a, 1993b, 1993c; McKone at al., 2002).

Probabilistic Add-in Program

CalTOX was designed to have both sensitivity and uncertainty analyses incorporated directly into the model operation. Parameter values suggested for use in CalTOX are described in terms of mean values and coefficients of variation in place of plausible upper values. Model results are described in terms of the confidence intervals associated with model predictions. CalTOX has its own routine for carrying out a sensitivity analysis on point estimates of exposure and risk. CalTOX is designed to integrate with the add-in program Crystal Ball, which can be used to carry out sensitivity and uncertainty analyses with the results produced by CalTOX. In order to

use Crystal Ball, the user must be familiar both with the operation of Crystal Ball and with the techniques of uncertainty/sensitivity analysis. The current version of CalTOX has been set up for use with Crystal Ball version 4.0, which is available as an add-in program for both Excel 97 or 2000.