Risks Of Volatilization


Like all organochlorines, HCB has very low water solubility. Because the BAPE report and the environmental follow-up did not mention a covering for the tailings pond, volatilization of HCB to the atmosphere must be considered as another source of environmental pollution from the facility.
The issue was addressed through the research of models that could approximate the rates of volatilization. Several models were found, from which the General Fugacity Model as described by MacKay and Paterson (1981) was chosen due to its simplicity and its general acceptance and frequent use in other studies. Studies concerning the volatilization or pathways of organochlorines were also researched to provide insight into the validity of the results obtained.
A major limitation to the accuracy of the predictions was the inability to determine the exact dimensions of the tailings pond. While parameters such as volume and general composition were obtained, others, such as width and length had to be approximated through aerial photographs and diagrams.


Project Issue in a Broader Context / Literature Review

Organochlorines tend to have very low water solubilities, leading to high Henry's Law constants. Thus it is plausible to raise concerns over the potential for high quantities of HCB to volatilize out of the tailings pond and into the atmosphere. Because emissions from the tailings pond were not considered in the environmental impact assessments by Hatch (Magnola, Pers. Comm.) and Environnement et Faune Québec, or by the BAPE report (1998), the potential for a large contribution of HCB to the environment from this source could be cause for reassessment. Furthermore, as a potentially major source of HCB to the environment, the probability of this occurrence must be researched for reasons detailed in the section of potential ecological impacts.

Research Question / Hypothesis

What will be the rate of volatilization of HCB from the tailings pond at the predicted input rate of 53.8 kg per year?
Due to a lack of information, our study was not able to determine the details of this issue. Rather, we sought to evaluate whether there was a need for a more rigorous study into the matter and whether the concerns regarding the emission of HCB from the tailings pond were plausible.



The General Fugacity Model as described by MacKay and Paterson was used to approximate the potential volatility rate of HCB from the tailings pond. This model was used due to its simplicity and its general acceptance and frequent use by other studies. Several unknown parameters were estimated from information obtained through correspondence with Mr. Alain Bergeron, and, as it was predicted that a high rate of volatilization would result, both normal and best case scenarios were constructed.
Studies regarding the modelling of HCBs pathways and volaticity in the Great Lakes were also researched to determine the validity of the results obtained.


Analysis and Discussion

The solubility of HCB in water is only 0.011 mg/L at 24 oC. This leads to numerous problems with the reprocessed tailings plan. The basin volume is known to be 850 000 m3, and the water in the pond will therefore be able to dissolve 9.35 kg of HCB. Considering that Magnola's estimated output of HCB into the tailings pond is 53.8 kg per year, it would be plausible to forecast a high rate of volatilization.
The general fugacity model (MacKay and Paterson, 1981), estimated a high rate of volatilization, predicting that almost all the HCB would volatize out of the tailings pond, with a negligibly small amount left dissolved in the water medium. Even when the depth of the pond used in the calculations was increased to a generous 10 meters to create a best-case scenario (the current plan intends for a water covering of only one meter), the same results were obtained. While both surprising and alarming, in light of the supersaturation of the tailings pond, the high surface area to volume ratio, and the low solubility of HCB, this result becomes more credible. Due to the inability of the water to dissolve HCB and the fixed, known, volume of the basin, for the pond to dissolve all 53.8 kg of HCB the depth of the water would have to be over 2500 meters.
Since the above results indicated that virtually all the HCB placed in the tailings pond would volatilize, the rate of volatilization was calculated by dividing the total inputs over one year by one year. This resulted in a rate of volatilization of 0.0061373 kg/h.
The validity of these alarming findings was determined by comparing with other studies of the volatility of HCBs. A study that assessed the pathways of several chlorinated benzene species, including HCB, found that over 80% of the HCB input to Lake Ontario left through volatilization (Oliver, 1984). This is supportive of the prediction that the vast majority of the HCBs placed into the tailings pond will rapidly volatilize into the atmosphere.
Mr. Alain Bergeron (Pers. Comm.) has revealed that Magnola's plan is based upon research of tailings ponds for mines in northern Quebec, where basins with water coverings 1 meter deep effectively prevented the volatilization of heavy metals. However, the free form of most heavy metals such as zinc is strongly held by soil and sediment particles and is available for biological uptake (Daniel, 1995). Furthermore, heavy metals such as cadmium, mercury, lead and zinc form soluble chloride complexes and complexes with oxide and humic particles (Daniel and Koerner, 1995). Thus it is no surprise that heavy metals are much more soluble in water than HCB or other organochlorine compounds. This lends further support to the notion that Magnola's plan to place large quantities of HCB in a water medium contains serious oversights.



In light of these alarming findings, it becomes evident that this issue requires closer scrutiny. Indeed, even Noranda's vice-president acknowledged this oversight, but there has yet to be mention of an alternative plan. One suggestion may be to ship the HCB contaminated wastes to a company, which will then dispose of them safely. The Norsk Hydro magnesium producing facility in Bécancour, Quebec, currently uses this disposal method. As our study lacked information that could have resulted in very different findings, we recommend that the approximated volatility rate and analysis be viewed as a preliminary study. We also highly recommend that the risk of high rates of atmospheric HCB emissions from the tailings pond be assessed further, with more complete data, and with other fugacity models.



Magnola will sample air HCB levels at three stations north of the facility to determine atmospheric emissions. This measure is not sufficient, however, to determine the volatilization rate from the tailings pond because those measurements do not give any information on the individual contributions from the tailings pond and the electrolysis plant. We therefore highly recommend that air HCB levels be monitored directly above and near the surface of the tailings pond continuously.

Groundwater Contamination
Risks Of Volatilization
Health Effects
Ecological Effects
Glossary and Appendix
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introduction | Groundwater contamination

Risks Of Volatilization | Health Effects| Ecological Effects

Glossary | References | Our Group