How much CO2 can leak from an underground storage site before it poses a health and safety concern?

Although there is a substantial amount of research into the propensity of storage sites to leak CO2 over time, there is currently an insufficient number of studies addressing the health, safety and environmental (HSE) impacts of these unmitigated leaks (Stenhouse et al. 2009).  The study done by Stenhouse et al. attempts to quantify the impacts of CO2 leakage on human health and safety by examining a scenario in which CO2 leaks directly into an enclosed dwelling, causing increased indoor CO2 concentration, and a scenario whereby CO2 leaks into a source of potable water, thereby causing lead mobilization.  The model predicts that to meet Health Canada’s recommendation for indoor air CO2 concentrations, CO2 leakages into an enclosed house should not exceed 5.4 kg d–1.  Further modelling indicates that aquifers containing unpolluted groundwater can tolerate a leakage rate of 1.7e-4 kg CO2 d–1 without mobilizing enough lead to exceed regulatory limits on lead concentrations in drinking water. A comparison of the two scenarios reveals that the health risks associated with leakage into drinking water occur with a lower level of CO2 leakage than do health risks associated with elevated CO2 levels in the home; thus regulators need to set limits on CO2 leakage using the numbers derived from the water scenario.– Shanna Hoversten.
Stenhouse, M., Arthur, R., Zhou, W., 2009. Assessing environmental impacts from geological CO2 storage. Energy Procedia 1, 1895–1902.

M. Stenhouse and colleagues at Monitor Scientific LLC generate their models determining maximum allowable CO2 leakage based on data from the Weyburn Midale CO2 Storage Project.  To assess acceptable leakage rates into a dwelling, the model is based off of a small one-story house with a ventilation rate of 3.1 exchanges per day, and conservatively assumes that the entire quantity of leaked CO2 enters the dwelling as opposed to some of it getting stuck in the soil due to mass transport resistance.  To identify the effects of CO2 leakage on groundwater, a model was used to simulate the acidification that would gradually occur with the addition of CO2 and the resultant mobilization of lead.  Several simulations were performed to assess the varying impacts of the CO2 given the presence of a combination of minerals in the sediments, including calcite, goethite, and cerrusite. 
The results derived for levels of CO2 leakage rates into houses allowable before health and safety becomes a concern were conservative, but relatively straightforward. The model for the water scenario was able to generate a number for how much additional CO2 could be leaked into the water supply, however, this figure was largely dependent on the variety and concentrations of minerals assumed to be in the aquifer sediments. The model demonstrates that with the addition of 0.01 mol calcite to the water, the change in concentration of lead over time is dramatically effected. Thus safe levels of CO2 leakage into potable water will be rather disparate across a range of sediment geology. Stenhouse et al. conclude that their analysis should be used to provide regulatory bounding limits on CO2 leakage rates, but that there should be a strong site-specific component to guard against over reliance on the model.  Further, the paper goes on to suggest a number of areas that require additional research in order to gain a fuller understanding of the health, safety and environmental impacts of CO2 leakage from storage sites.  Some of these topics include: the propensity of high CO2 concentrations to cause tree kills, the effect of intermediate CO2 concentrations over long periods of exposure, and the effect of heightened CO2 concentrations on underground microbial populations and the subsequent ecological consequences. 

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