Contamination distribution

Airborne contaminant movement in the building depends upon the type of heat and contaminant sources, which can be classified as (1) buoyant (e.g., heat) sources, (2) nonbuoyant (diffusion) sources, and (d) dynamic sources.- With the first type of sources, contaminants move in the space primarily due to the heat energy as buoyant plumes over the heated surfaces. The second type of sources is characterized by cimtaminant diffusion in the room in all directions due to the concentration gradient in all directions (e.g., in the case of emission from painted surfaces). The emission rare in this case is significantly affected by the intensity of the ambient air turbulence and air velocity, dhe third type of sources is characterized by contaminant movement in the space with an air jet (e.g., linear jet over the tank with a push-pull ventilation), or particle flow (e.g., from a grinding wheel). In some cases, the above factors influencing contaminant distribution in the room are combined. 

The energy of large and medium-size eddies can be characterized by the turbulent diffusion coefficient. A, m-/s. This parameter is similar to the parameter used by Richardson to describe turbulent diffusion of clouds in the atmosphere. Turbulent diffusion affects heat and mass transfer between different zones in the room, and thus affects temperature and contaminant distribution in the room (e.g., temperature and contaminant stratification along the room height—see Chapter 8). Also, the turbulent diffusion coefficient is used in local exhaust design (Section 7.6). 

Kuz mina, L. V., A. M. Kruglikova, and A., S. Gus kov. 1986. Contaminant distribution in industrial hall with spiral vortex ventilation. In Occupational Safety in Industry Transactions of the AlhUnion Research Institutes for Labor Protection. Profizdat, Moscow. 

Christianson, L. L. 1989. Building Systems Room Air and Air Contaminant Distribution. ASHRAE, Atlanta. 

A similar temperature and contaminant distribution throughout the room is reached with stratification as with a piston. The driving forces of the two strategies are, however, completely different and the distribution of parameters is in practice different. Typical schemes for the vertical distribution of temperature and contaminants are presented in Fig. 8.11. While in the piston strateg) the uniform flow pattern is created by the supply air, in stratification it is caused only by the density differences inside the room, i.e., the room airflows are controlled by the buoyancy forces. As a result, the contaminant removal and temperature effectiveness are more modest than with the piston air conditioning strategy. 

Vertical temperature and contaminant distribution within the stratification strategy, typical schemes. To is supply air temperature, T is the temperature at the floor level, Tj Is room temperature, Cq and Q are contaminant concentrations in supply and room air above the stratification height, and y, is the stratification height... 

Scale-model experiments have been used to study a variety of ventilation problems as air movement in a room, air movement around a building, energy flow in a building, contaminant distribution at an operator s workplace, and smoke movement in a building on fire. The theory is discussed at a general level in the references. 

Schindler DW, Kidd KA, Muir DCG, Lockhart WL. 1995. The effects of ecosystem characteristics on contaminant distribution in northern freshwater lakes. Sci Tot Environ... 

Potentially more significant is the fact that a single ion is used to represent the dissolved form of the contaminant in question, an assumption that can lead to serious error. Cadmium in a model calculated at pH 12, for example, is present primarily as the species Cd(OH)2 almost no free ion Cd++ occurs. Employing the reaction Kd model in terms of Cd++ in this case would predict a contaminant distribution unlike that suggested by the distribution coefficient, applied in the traditional sense. We see the importance of applying a Kd model to systems similar to that for which it was originally determined. 

Only the results for the Cauchy and symmetric contaminated distributions are shown in Figs. 13 and 14, respectively. From these figures, it is clearly shown that the robust approach consistently and successfully performs the data reconciliation, regardless of the distributions of the data. This is a very desirable property in real applications, since in most cases the distribution is unknown or known only approximately. 

Wade TL, Atlas EL, Brooks JM, et al. 1988. NOAA Gulf of Mexico status and trends program Trace organic contaminant distribution in sediments and oysters. Estuaries 11 (3) 171-179. 

Johnson, K.A. Naddy, R.B. Weisskopf, C.P. 1995, Passive sampling devices for rapid determination of soil contaminant distributions. Toxicol. Environ. Chem. 51 31 4. 

Contact potential difference measurements, os-cillatoiy reactions, 39 85 Contact synergy model, 40 183 Contaminant distribution... 

Volatilization of an organic mixture of contaminants, distributed vertically in the subsurface, may induce not only a decrease in the component concentrations but also an enrichment of the deeper layers during the volatilization process. Figure 8.13 shows the actual content of three representative hydrocarbons—m-xylene (C ), n-decane (Cj ,), and hexadcane (Cj )—which originated from the applied kerosene found along a 20 cm soil column, 18 days after application on dry soil. Roughly 30% of the initial content of m-xylene still remained in the soil after 18 days. Furthermore, the content of m-xylene increased somewhat after the third day a similar trend was found for the n-decane distribution. Hexadecane was partially removed from deeper layers and redistributed near the soil surface. 

To describe and quantify these complex dynamics, models are used. Modeling of contaminant transport involves formulation of a conceptual framework and corresponding quantitative relationships that lead to determination of contaminant distributions over space and time. Models also can be used to investigate the relative influence of different physical and (geo)chemical mechanisms on contaminant transport and to assist in designing management and remediation strategies. 

Pore-scale network models (including percolation models), on the other hand, are best suited for analyzing fluid and contaminant distribution and movement within pores and clusters of pores. Such models are particularly effective for captur-... 

Vacuum extraction alone is limited to treating unsaturated soils, and successful remediation is contingent upon factors such as soil properties and the volatility of the contaminants. Ideally, measured soil permeabilities should range between 10 " and 10 cm/sec, and contaminants should have a Henry s constant of 0.001 or higher. Also, sites with complex stratigraphy or contaminant distributions may require pilot demonstrations prior to the full implementation of a vacuum extraction system. 

Johnson, R. L., Brillante, S. M., Isabelle, L. M., Houck, J.E. Pankow, J.F. (1984). Migration of chlorophenolic compounds at the chemical waste disposal site at Alkali Lake, Oregon 2. Contaminant distributions, transport, and retardation. GroundWater, 23, 652-66. 

Two kinds of poison distributions must be distinguished. One distribution is that along the catalyst bed, the other one is within the porous system of the catalyst. It may be reasonably anticipated that under most conditions there will be a gradient of contaminant concentration which decreases in the direction from inlet to outlet also that there will be a decreasing concentration of contaminants from the outer confines of each separate catalyst body inwards into the pore system. The contaminant distribution will, however, differ for different types of catalysts and contaminants. 

As will be discussed later, the particular form of contaminant distribution within the porous layer, as observed by the microprobe, does in some cases correlate with the contaminant effect on catalytic activity. 

Eq. 4 is amenable to solution techniques based on the numerical inversion of Laplace-transformed equations these calculations can be performed rapidly and are therefore suitable for calibration. In Figure 1, typical soil/bentonite column predictions are shown to highlight the effect of the influent mixing zone on the spatial contaminant distributions for low-flow systems. The simulation results, which were generated for column conditions described by Khandelwal et al. (1998), indicate that the mixing zone has a significant influence on the shape of the spatial contaminant distribution and, therefore should be considered explicitly in estimating sorption parameters from spatial column data. 

Figure 2. Spatial contaminant distributions and simultaneous model calibrations for column experiments using SB amended with 5 percent humus (column conditions summarized in Table 2). Reprinted with permission from Khandelwal and Rabideau (2000). Copyright 2000, Elsevier Science B. V.

As discussed above for low-permeability systems, the analysis of spatial contaminant distributions is an appealing strategy for shortening the duration of column experiments. Again, the use of multiple columns operated over... 

Involve a statistician into the planning process for sites with highly toxic chemicals or with stratified contaminant distribution for the determination of background concentrations or for hot spot detection projects. 

Stratified random sampling, which is a variation of simple random sampling, is used for media that are stratified with respect to their chemical and physical properties. Each stratum is identified and randomly sampled. The number of grab samples and the sampling point selection depend on the nature of contaminant distribution within each stratum. Stratified random sampling is used for the characterization of multiphase liquid wastes or process waste batches that undergo stratification over time and/or space. 

Unhomogenized duplicates are soil sample aliquots sequentially collected from the same sampling point into separate sample containers. Unhomogenized duplicate sample data provide information on soil variability with respect to contaminant distribution at the sampling point. Vastly different contaminant concentrations in unhomogenized field duplicate results indicate high contaminant variability. 

---