QWASI model

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 Qwasi model estimate the fate of a chemical in a water system (lake, river, etc.) consisting of water, bottom and suspended sediments, and air. The model is... 

Table 1 Principal characteristics of Qwasi model (based on [34])...

As summary, the principal characteristics of the Qwasi model are listed in Table 1. 

Concerning the results, using the concentration of Pb in the river water calculated with the QWASI model as inputs, significant differences on the concentrations of Pb in the arterial blood of children/adults were not observed when these results were compared with the obtained using the literature values for lead concentration in the river water as inputs (data not showed). This was somehow expected considering that the results of QWASI model were in the range as the... 

Under the applied QWASI model assumptions, the QWASI results are in the range of measured data reported in literature and thus support that the strongest impact to sediment and water concentrations of DeBDE are from direct emission to water as opposed to atmospheric concentrations. This result points out the high importance of DeBDE-leaching from deposited waste material and a lower meaning of the fraction that is transferred to the atmosphere. 

To illustrate the use of Z andD values in a simple multimedia model, we present below a steady-state mass balance for an air-water-sediment system representing a small lake with inflow and outflow. It is an application of the quantitative water air sediment interaction (QWASI) model that is available from the Web site www.trentu.ca/cemc. The chemical is similar in properties to a volatile hydrocarbon such as benzene. Table 3.3 lists the lake properties, the chemical input rates in the inflowing water, its properties as partition coefficients, Z values and D values for all the transport and reaction rates. 

We illustrate these concepts by applying various fugacity models to PCB behavior in evaluative and real lake environments. The evaluative models are similar to those presented earlier (3, 4). The real model has been developed recently to provide a relatively simple fugacity model for real situations such as an already contaminated lake or river, or in assessing the likely impact of new or changed industrial emissions into aquatic environments. This model is called the Quantitative Water Air Sediment Interactive (or QWASI) fugacity model. Mathematical details are given elsewhere (15). 

The QWASI fugacity model contains expressions for the 15 processes detailed in Figure 2. For each process, a D term is calculated as the rate divided by the prevailing fugacity such that the rate becomes Df as described earlier. The D terms are then grouped and mass balance equations derived. 

Figure 2A. Diagram of processes included in the QWASI fugacity model showing D values for a trichlorobiphenyl in a lake similar to Lake Michigan.

Mackay, D. Joy, M. Paterson, S. "AQuantitative Water Air Sediment Interaction (QWASI) Fugacity Model for Describing Chemical Fate in Lakes and Rivers" submitted to Chemosphere, 1983. 

The model is composed by different equations which in all cases can be used in unsubscribed format in a basic language program. An important point to highlight is that Qwasi takes into account both steady and unsteady state solutions for the equations for systems involving contamination of lakes (or rivers). The equations considered by Qwasi involve more than 15 physicochemical processes (such as partitioning, sediment transport, deposition, etc.) to estimate the fate of the studied system. These processes and the main involved variables and parameters are summarized in Fig. 2. 

Mackay D, Diamond M (1989) Application of the QWASI (quantitative water air sediment interaction) fugacity model to the dynamics of organic and inorganic chemicals in lakes. Chemosphere 18 1343-1365... 

Mackay D, Joy M, Paterson S (1983) A quantitative water, air, sediment interaction (Qwasi) fugacity model for describing the fate of chemicals in lakes. Chemosphere 12(7/8) 981-997... 

The characteristics of the applied models have been described in detail in the chapters Environmental Fate Models [50] and A Revision of Current Models for Environmental and Human Health Impact and Risk Assessment for Application to Emerging Chemicals [49] and only a brief overview is given here. Since each model has its own approach (i.e., QWASI is focused on the aquatic system), the combined results are expected to give a wider view with in-depth analyses for different aspects compared to just one model with its special characteristics. 

QWASI, the Quantitative Water, Air Sediment Interaction model by Mackay et al. [14] is a fugacity III model (Version 3.10, 2007) and it describes the fate of chemicals in aquatic systems, depending on direct discharge, inflow in rivers, and atmospheric deposition. Hence, this model addresses the local scale, as does the 2-FUN Tool. 

While QWASI is an easy to use multimedia fate modeling tool, it has been originally designed as a fugacity model. Even though an adaptation to ionic substances exists and it has been applied to lead before, it needs to be recognized that it does not take speciation of metals into account. This adds to the overall uncertainty of results. 

Ling, H., Diamond, M., Mackay, D. (1993) Apphcation of the QWASI fugacity/equivalence model to assess) ng the fate of contaminants in the water and sediments of Hamilton Harhour. Journal of Great Lakes Research, 19 582-602. 

LASs were identified mainly in the dissolved phase in agreement with QWASI fugacily models . Consistently with the predicted short life in the aqnatic enviromnent (2 days), we found very low levels in coastal sediments. The concentrations of LASs found in the present study (Table II) are of the same order of magnitude than those reported by Takada and Ishiwatari (230-1500 pg L ) but one order of magnitud lower than those reported by Marcomini etal. (4.8 0.2 mgL ) " for influent wastewaters. 

The concept behind EXWAT is similar to that of Mackay s model QWASI (Mackay et al. 1983a 1983b). The structure is shown in SCHEME 4. 

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