Modeling fate and transport

Caruso BS, Cox LTJ, Runkel RE, Velleux ML, Bencala KE, Nordstrom DK, Julien PY, Butler BA, Alpers CN, Marion A, Smith KS (2008) Metals fate and transport modelling in streams and watersheds state of the science and SEPA workshop review. Hydrol Process 22 4011... 

While these objectives for method sensitivity may seem ambitious, experience has shown that data from such studies are much more usable for supporting fate and transport models (development and/or validation efforts) that may have to be used when more precise and geographically detailed probabilistic risk assessments become necessary. 

Crockett, A.B. Hern, S.C. Kinney, W.L. Flatman, G.T. "Guidelines for Field Testing Aquatic Fate and Transport Models Interim Report" U.S. Environ. Prot. Agency, Environ. Monitoring Systems Lab. Las Vegas, Nevada, 1982 p. 174 + Appendices. 

The goal of this paper Is to present the current status of model validation and field testing of chemical fate and transport models other papers in this symposium discuss the state-of-the-art of modeling specific processes, environments, and multimedia problems. The process of model validation, and its various components, is described considerations in field testing, where model results are compared to field observations, are discussedp an assessment of the current extent of field testing for various processes and media is presented and future field testing and data needs are enumerated. 

Comparisons between observed data and model predictions must be made on a consistent basis, i.e., apples with apples and oranges with oranges. Since models provide a continuous timeseries, any type of statistic can be produced such as daily maximums, minimums, averages, medians, etc. However, observed data are usually collected on infrequent intervals so only certain statistics can be reliably estimated. Validation of aquatic chemical fate and transport models is often performed by comparing both simulated and observed concentration values and total chemical loadings obtained from multiplying the flow and the concentration values. Whereas the model supplies flow and concentration values in each time step, the calculated observed loads are usually based on values interpolated between actual flow and sample measurements. The frequency of sample collection will affect the validity of the resulting calculated load. Thus, the model user needs to be aware of how observed chemical loads are calculated in order to assess the veracity of the values. 

Exposure assessment using monitoring data or fate and transport models calculate the predicted environmental concentration (PEC) in each environmental compartment. More information can be obtained from Suciu et al. [4]. 

To construct models of this sort, we combine reaction analysis with transport modeling, the description of the movement of chemical species within flowing groundwater, as discussed in the previous chapter (Chapter 20). The combination is known as reactive transport modeling, or, in contaminant hydrology, fate and transport modeling. 

SRC. 1994b. Fate and transport models Model outputs for the chemical -hexane. Syracuse, NY Syracuse Research Corporation. 

Ollila, R W., 1996, Evaluating Natural Attenuation with Spreadsheet Analytical Fate and Transport Models Ground Water Monitoring Remediation, Fall, Vol. 16, No. 4, pp. 69-75. 

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. 

The results from this case study can be used as input to the general comprehensive RRR fate and transport model (i. e., which includes volatilization, photolysis, biodegradation, and sorption/desorption modules) in order to predict organic leachate-generated chemical loads and concentrations at highway boundary or landfill sites. 

Fate and transport modeling was nsed to estimate the concentration of the insecticide in insect tissne consnmed by birds. The details of this modeling effort, which we omit here, are rather complex and involve characteristics of the field application of the insecticide, local weather, mnltiple pathways of exposure to insects, sequestration of insecticide by mortality of insects, and integration over 0- to 20-g pools of insect tissne that wonld compose a bird s daily diet. The model of the pesticide s fate and transport made a prediction abont the concentration variable, which is characterized by the p-box shown in the lower left graph of Figure 6.14. This p-box synthesizes all of the knowledge and nncertainty captured in the modeling effort. The model predicts the distribntion fnnction for concentrations, whatever it is, snrely lies within the bonnds shown. 

Physical and Chemical Properties. The physical and chemical properties of carbon tetrachloride have been well-studied, and reliable values for key parameters are available for use in environmental fate and transport models. On this basis, it does not appear that further studies of the physical- chemical properties of carbon tetrachloride are essential. 

Kopf, A. Fate and Transport Modeling of PCBs in Lake Hartwell. Department of Environmental Systems Engineering, Clemson University, Clemson, SC, unpublished manuscript. 

Holt, M.S. 1992. Microcomputer program for the estimation of properties for use in environmental fate and transport models. Master s thesis. Utah State University, Logan, UT. 

Thibodeaux LJ, Lipsly D. 1985. A fate and transport model for 2,3,7,8-tetrachlorodibenzo-p-dioxin in fly ash on soil and urban surfaces. Hazard Waste Hazard Mater 2 225-235. 

Jury, W.A., Ghodrati, M. (1989) Overview of organic chemical environmental fate and transport modeling approaches. In Reactions and Movement of Organic Chemicals in Soils. SSSA Special Publication No. 22, Sawhney, B.L., Brown, K., Editors, pp. 271-304, Soil Science Soceity of America and Society of Agronomy, Madison, Wisconsin. 

Epidemiology Clinical Studies Animal Studies Cell/Tissue Experiments Exposure Monitoring Fate and Transport Models... 

Exposure assessment qualitatively and quantitatively characterizes the potential for exposure to occur in particular circumstances and includes an estimate of dose when possible. The assessment includes an estimation or measurement of chemical concentration in the contact media (e.g., soil, water, air, a particular food crop, a consumer product), an estimation of the length of time over which contact will occur, characterization of potential routes of exposure (inhalation, ingestion, and skin contact), and the likelihood for a chemical to be absorbed through those routes. In certain circumstances, direct measurements or fate and transport modeling may be used to estimate chemical concentrations in ambient media. For certain assessments, a quantitative estimate of the total dose of a chemical over a particular time frame and in the given circumstances is made. 

Exposure assessment includes both qualitative and quantitative evaluations of the potential for exposure to site-related chemicals to occur. Assessments commonly address both current and likely future uses of the property (e.g., residential, commercial, industrial, and agricultural). Typically, a conceptual model is developed that summarizes how site-related chemicals may contact receptors (e.g., humans, wildlife, and ecological). The model includes identification of chemical sources, impacted media, potential movement through the environment, identification of the appropriate exposure scenarios, and identification of the points at which contact between receptors and site-related chemicals are likely to occur. Chemical concentrations in environmental media may be estimated based on site data and using statistical analyses and/or fate and transport modeling. An estimate of the dose (intake) attributable to contact with environmental media through significant and completed pathways is made for chemicals of concern at... 

Tier 3 is more complex and detailed. It relies on more site-specific data. While Tier 2 may have relied on simple and relatively uncomplicated fate and transport models. Tier 3 will utilize more sophisticated models and will include additional site-specific data. Tier 3 also may rely on site-specific exposure assumptions, if appropriate. Some states allow the... 

Hathaway DL, Andrews CB. 1990. Fate and transport modeling of organic compounds from a gasoline spill. In Petroleum hydrocarbon and organic chemicals in ground water Prevention, detection and restoration. Houston, TX S. S. Papadopulos and Associates, Inc., 563-576. 

In the air phase, under pressures normally found in the environment, fugacity equals the pressure exerted by the chemical s vapor. (At higher pressures, vapors do not exactly obey the ideal gas law, and a correction must be applied this is small enough to ignore for practical purposes of fate and transport modeling in the environment.) By combining the ideal gas law (Eq. [1-29]) and Eq. [1-34], it is evident that the fugacity capacity for air is 1/RT, for all chemicals ... 

MULTIMED Unsaturated zone/groundwater MULTIMED was developed as a multimedia fate and transport model to simulate contaminant migration from waste disposal units. Release to either air or soil, including the unsaturated and the saturated zones, are possible interception of the subsurface contaminant plume by a surface stream are included. 

It has been suggested that a biological cycle exists for selenium (Shrift 1964), but certain components of the cycle remain uncharacterized. The biological transformation of selenide to elemental selenium has not been well described in the literature (see Maier et al. 1988). Further research on the biological selenium cycle might help to identify "hot spots" of selenium in the environment. For example, further investigation of parameters that influence the tendency of selenium to move from one medium to another (e.g., from soil to water) would improve fate and transport modeling efforts. 

---