Contaminant Release

In many cases, a trace element retained on the subsurface soM phase may undergo chemical reactions that induce a hysteresis phenomenon during the release process. A relevant example of hysteresis due to precipitation of some of the initial contaminants is given by the behavior of Cr(VI), an industrial contaminant, which in the subsurface environment may be subject to reduction reactions. When an available source of electrons is present, such as organic matter, Cr(VI) is reduced to Cr(III) the rate of this reaction increases with decreases in pH (Ross et al. 1981). 

Stollenwerk and Grove (1985) report the adsorption and desorption of Cr(VI) in an alluvial aquifer. From Fig. 8.44a, we see that, over the first 10 pore volumes, all the Cr(VI) in water contaminant was adsorbed by the alluvium. A rapid increase in the effluent concentration of Cr(VI) then occurred, until the capacity of alluvium for contaminant retention was exhausted ( 25 pore volumes). Leaching the alluvium column with 10 pore volumes of Cr-free water caused the release of about 

50% of the adsorbed Cr(VI), and further leaching with 80 pore volumes of ground-water, over 232 days, removed only an additional 34% of the adsorbed contaminant. Stollenwerk and Grove (1985) attributed the difficulty in removing part of the adsorbed Cr(VI) to the presence of specific adsorption sites and possible reduction to Cr(III) followed by precipitation. 

A difference in the rate of adsorption and desorption of Cr(VI) by alluvium was also observed in a batch experiment (Fig. 8.44b). On the basis of these two experiments, Stollenwerk and Grove (1985) concluded that the quantity of Cr(VI) adsorbed by alluvium is a function of its concentration as well as of the type and concentration of other anions in solution. The Cr(VI) adsorbed through nonspecific processes is desorbed readily by a Cr-free solution. Stronger bonds that are formed between Cr(VI) and alluvium during specific adsorption result in very slow release of this fraction. The Cr(Vl) desorption from the alluvium material illustrates the hysteresis process that results from chemical transformation of a portion of contaminant retained in the subsurface. 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural 

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Vents and flares are intended to take contaminants released from safety valves away from work areas. However, if an elevated vent is at the level of an occupiable platform on the same or an adjacent unit, a worker may, under certain wind conditions, be subject to the nearly undiluted effluent of a vent. Whereas such elevated platforms may rarely be occupied, a heavy exposure from a vent could incapacitate a worker or cause a fall. Tanks that vent only when being filled are common causes of this concern. The usual solution is to raise the vent above any occupiable platform or, at greater cost, to scmb the vent effluent. 

Air contaminant releases from hazardous waste sites can occur from wastes placed aboveground or belowground. The following are categories of air contaminant releases ... 

Figure 16-1 and 16-2 present the decision networks that guide contaminant release screening analysis. Figure 16-1 deals with contaminants in or under the soil and Fig. 16-2 addresses aboveground wastes. Any release mechanisms evident at the site will require a further screening evaluation to determine the likely environmental fate of the contaminants involved. 

The atmospheric fate of contaminants must be assessed whenever it is determined that significant gaseous or airborne particulate contaminants are released from the site. The atmospheric fate of contaminants released originally to other media, but eventually partitioned to the atmosphere beyond site boundaries, must also be assessed whenever this intermedia transfer is likely to be significant. 

The aquatic fate of contaminants released from the CERCLA site as well as those transferred to surface water from other media beyond site boundaries must be considered. 

The fate of contaminants in the soil medium is assessed whenever the contaminant release atmospheric, or fate screening, assessment results show that significant contamination of soils is likely. 

The latter information is important in evaluating the size of the rxrcupied zone that can be effectively ventilated by inclined jets. It was proposed that the occupied zone of rooms is well ventilated by inclined jets (particularly in industrial rooms with contaminant release) if air velocity in the occupied zone exceeds 0.1 m/s. 

TABLE 8.4 Heat and Contaminant Release to the Air in an Industrial Hall... 

Assume the use of mechanical ventilation with full mixing ventilation in the room, and calculate a rough estimate of ventilation airflow found by assuming dilution of contaminants released to the room air to one-third of the TLV level given for the substance in question. 

The addition of a uniform flow with the sink flow creates a dividing streamline (Fig. 10.19), so that the contaminants released inside the dividing streamline would be captured while the contaminants released outside would escape. The... 

Since the low-momentum supply system should enhance the efficiency of an exterior hood by supplying low-momentum airflow to a source, the system can be applied to practically any sources where an exterior hood can be used. In particular, it is effective to apply the system when an exterior hood cannot be placed close to a source or the exhaust direction is different from the initial contaminant release direction. 

Occupants Characterization Characteristics of smoking or other contaminant-releasing activities... 

Hot water injection via injection wells heats the soil and groundwater and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery. 

Fate and exposure analyses. 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 is used for determining the distribution of a chemical in the environmental compartments. An overview of the partitioning among the liquid, solid and/or gas phases of individual compartments is presented in Fig. 7. The exposure model encompasses... 

Buhl, K.J. and S.J. Hamilton. 1990. Comparative toxicity of inorganic contaminants released by placer mining to early life stages of salmonids. Ecotoxicol. Environ. Safety 20 325-342. 

Scenario Development and Simulation The typical procedure for assessing the consequences of an airborne contaminant release is as follows ... 

Routes by which one may be exposed to contaminants released from these weapons... 

The apparatus and containers must be scrupulously cleaned to provide low blank levels and to lower losses due to absorption or the ingress of contamination released from the vessel walls. 

Genuine (true) and apparent hysteresis may be considered to explain contaminant release from the subsurface solid phase. Genuine hysteresis assumes that observed data are real and the equilibrium results can be explained on the basis of well-identified phenomena. Apparent hysteresis results from an experimental artifact due, for example, to a failure to reach retention or release equilibrium. 

Genuine hysteresis is considered when contaminant release results only from desorption. Experimental data can be interpreted in terms of genuine desorption only when the system is at equilibrium and released molecules are those adsorbed onto the solid phase surface. Molecules brought back into the solution as result of dissolution, diffusion out of the solid matrix, or biotic/abiotic transformation cannot be considered desorbed molecules. In the subsurface, it is almost impossible to distinguish between desorbed molecules and molecules that were not subjected to adsorption and desorption. 

Several other explanations have been put advanced to explain retention hysteresis, including (1) surface precipitation of metallic cations whose hydroxides, phosphates, or carbonates are sparingly soluble (2) chemical reactions with solid surfaces, including organic surfaces, which form complexes with metallic cations and (3) incorporation into the subsurface organic matter through chemical reactions and biochemical transformation. For the case described by Fig. 5.9 or explanations (1) and (2), the contaminant release always exhibits a hysteresis... 

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