In order to put the results obtained from well leakage modeling in some perspective, scoping calculations were carried out to address two potential environmental impacts:
• Indoor air CO2 concentration: CO2 leakage from an abandoned well
(point source) directly into the basement or lower floor of a dwelling through cracks in the foundation. The maximum acceptable CO2 flux from the wellbore was determined by the limiting CO2 concentration in air (Health Canada, 1989).
• Mobilization of trace metals into drinking water: CO2 leakage from an abandoned well directly into a potable groundwater, changing the water chemistry, which in turn promotes the release of a toxic trace metal (lead) into the water supply. The maximum acceptable CO2 flux in this case was determined via geochemical modeling, taking into account an interim drinking water standard for dissolved lead (Langmuir, 1997).
Evaluation of the above cases indicated that the more restrictive example is CO2 leakage into a potable aquifer, yielding a limiting CO2 leakage rate of similar magnitude to the mean cumulative leakage described in the previous section.
Given that only scoping calculations were carried out, more detailed treatments of these environmental impacts are required, particularly in the case of trace metal release into drinking water, before making a definitive statement on an acceptable CO2 leakage rate to the biosphere.
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