Contaminant-transport models

The integration of the ventilation model into the thermal building model can be realized on different levels, from simple stack-flow equations to a full integration of a multizone airflow and contaminant transport model. 

Alexander WR, McKinley IG (1994) Constraints on the use of in situ distribution coefficients (Kd) values in contaminant transport modeling. Eclogae Geol Helv 87 321-324 Andrews JN, Wood DF (1972) Mechanism of radon release in rock matrices and entry into groundwaters. Inst Min Metall Trans B81 198-209... 

Zheng, C. and G. D. Bennett, 2002, Applied Contaminant Transport Modeling, 2nd ed. Wiley, New York. 

Zhu, C. Burden, D. S. 2001. Mmeralogical compositions of aquifer matrix as necessary initial conditions in reactive contaminant transport models. Journal of Contaminant Hydrology, 51, 145-161. 

Rabideau, A. J. (1996). Contaminant transport modeling, in Assessment of barrier containment technologies, Rumer, R., and Mitchell, J., eds., NTIS PB96-5083. 

Zheng C, Bennett GD (1995) Applied contaminant transport modeling theory and practice. Van Nostrand Reinhold, New York NY... 

Wagner BJ (1992) Simultaneously parameter estimation and contaminant source characterization for coupled groundwater flow and contaminant transport modeling. J Hydrol 135 275-303... 

The contaminant transport model, Eq. (28), was solved using the backwards in time alternating direction implicit (ADI) finite difference scheme subject to a zero dispersive flux boundary condition applied to all outer boundaries of the numerical domain with the exception of the NAPL-water interface where concentrations were kept constant at the 1,1,2-TCA solubility limit Cs. The ground-water model, Eq. (31), was solved using an implicit finite difference scheme subject to constant head boundaries on the left and right of the numerical domain, and no-flux boundary conditions for the top and bottom boundaries, corresponding to the confining layer and impermeable bedrock, respectively, as... 

Contaminant transport models are most commonly used for determining the concentrations of contaminants that reach the exposed population. Such models for the atmosphere, surface waters, and the subsurface have developed substantially their use has been described by Anderson and Woessner (1992), Watts (1998), and Zheng and Bennett (2002). 

Figure 4 compares several of these models with respect to the nature of the constants that each uses. The simplest model (linear sorption or Ai ) is the most empirical model and is widely used in contaminant transport models. values are relatively easy to obtain using the batch methods described above. The Aid model requires a single distribution constant, but the Aid value is conditional with respect to a large number of variables. Thus, even if a batch Aid experiment is carefully carried out to avoid introduction of extraneous effects such as precipitation, the Aid value that is obtained is valid only for the particular conditions of the experiment. As Figure 4 shows, the radionuclide concentration, pH, major and minor element composition, rock mineralogy, particle size and solid-surface-area/solution volume ratio must be specified for each Aid value. 

A)jS, whether sampled from probability distribution functions or calculated by regression equations or surface-complexation models, can be used in many contaminant transport models. Alternate forms of the retardation factor equation that use a (Equation (3)) and are appropriate for porous media, fractured porous media, or discrete fractures have been used to calculate contaminant velocity and discharge (e.g., Erickson, 1983 Neretnieks and Rasmuson, 1984). An alternative approach couples chemical speciation calculations... 

Turner, A. Millward, G.E. (1994) The partitioning of trace metals in a macrotidal estuary. Implications for contaminant transport models. Estuarine, Coastal and Shelf Science 39, 45-58. 

In this paper, procedures for calibrating flow models and contaminant transport models are outlined and some of the difficulties frequently encountered during calibration are discussed. An example of a contaminant transport model applied to a problem involving aldicarb migration in groundwater in Wisconsin is also presented. 

Most flow problems can be readily simplified to two-dimensions and most of the standard methods for treating groundwater supply problems are built around two-dimensional analyses. It is less easy to justify the use of two-dimensional analyses for contaminant transport problems. However, because three-dimensional contaminant transport models are particularly unwieldy, most readily available transport models and most reported applications are two-dimensional in nature. 

The output of a flow model consists of the head distribution in time and space. Darcy s Law is used to convert the head distribution to a velocity distribution suitable for input to a contaminant transport model. In a two-dimensional application, Darcy s Law is used to compute two sets of velocity components ... 

Contaminant Transport Modeling. A major difficulty in the calibration of any two-dimensional contaminant transport model is relating the two-dimensional simulated plume to the real three-dimensional plume. A model based on Equation 4 can simulate two dimensions in cross section or areal view. An areal view was selected for the problem considered here. Use of a two-dimensional areal view model implies that the contaminant is uniformly spread out through the entire saturated thickness of the aquifer. However, in the field the aldicarb plume is only around 10 feet (3m) thick while the aquifer is around 70 feet (21 m) thick. Moreover, the concentration data were collected from wells having 3 ft (0.91 m) well screens and hence are representative of only a small fraction of the total aquifer thickness. It was decided to calibrate the model to concentrations representative of the center of the plume vertically. That is, the model was calibrated to maximum measured concentrations in each well nest. As a result, the loading rate to the model is inflated over probable field values. The model assumes the load to the model is distributed over the full aquifer thickness, when in the field the zone of maximum concentration is probably no more than 3 feet thick. Therefore, the probable loading rate in the field is roughly 3/70 or 4% of that used to calibrate the model. 

Contents indude an introduction to nonpoint pollution modeling, sorbed chemical transport modeling, subsurface contaminant transport modeling, and salt transport in the landscape. 

Although it can be said that if you cannot model a natural system, you don t understand it , it must also be said that even if you can model it, you may still not understand it completely . Given the complexity of many environmental problems, it is almost certain that we . .. will not understand it completely . It follows that any predictions we make based on our model will be inaccurate to some degree. This is certainly true for the contaminant transport models that are currently used to predict concentration distribution in space and time (see discussion in 10.3). 

The hydrologic model quantifies the movement of subsurface water and provides inputs to contaminant transport models. Its usage as a simulation tool allows previewing the contaminant behaviour in the groundwater al well as a quantitative assessment for the concentration of the contamination at a particular exposition point. 

We have argued that molecular-scale understanding of the structure and composition of mineral surface-aqueous solution sorption complexes is vital to development of robust reactive contaminant transport models. This argument suggests that significant error may be expected in predicted environmental behavior in the absence of such knowledge. In an attempt to test this claim, we have used the results of our XAFS studies of Co(II) sorbed by alumina to refine a quasi-thermodynamic uptake model, then used the model to evaluate the sensitivity of predicted Co(II) partition coefficients to the choice of reactions included in the sorption model. 

The approach is very commonly used in transport models because mathematically it is relatively easy to incorporate. Goyette and Lewis [28] highlighted the utihty of values in screening level ground water contaminant transport models of inorganic ions with the caution that experimental conditions such as pH, electrolyte composition and soil type are similar to those being modeled. Viotti et al. [29] used values to model phenol transport in an unsaturated soil. Schroeder and Aziz [30] used this approach to account for PCBs sorption into dredged materials. Buczko et al. [31] used the Freundlich approach to model chromium transport in unsaturated zone. 

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