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LJM

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  1. I'm a novice using this program, and realize this may be a simple question. I'm trying to create a very generalized redox-pH diagram that will help me understand the effect of changes in dissolved oxygen concentrations on the mobilization of chromium in groundwater. Using estimated concentrations of Cr6+ in groundwater at our site, I created the redox-pH diagram below. The current [O2]aq concentration in groundwater at the site is 4 mg/L, and is estimated to change to 8 mg/L due to the use of air sparging technology to remediate a different groundwater contaminant. Unfortunately, I don't have more information regarding the concentrations of other dissolved constituents. So I've calculated the activity of O2(aq) for 4 mg/L assuming a very general activity coefficient of ~0.5. This gives me an O2(aq) activity of 6.3*10^-5, or roughly -4.2 on the y-axis of the attached diagram [log activity-O2(aq)]. My question is: are my assumptions correct? I'm confused because my understanding is that dissolved oxygen in groundwater generally ranges from 0 to 20 mg/L. How is it possible to obtain a value for log activity-02 aq greater than -30? Please let me know if I've made an obvious blunder, or if there is a better way to approach this problem. Thanks very much.
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