Morro do Ferro groundwater

the couple for acetate ion can be written as a dismutation reaction, 

Models of natural waters calculated assuming redox disequilibrium generally require more input data than equilibrium models, in which a single variable constrains the system s oxidation state. The modeler can decouple as many redox pairs as can be independently constrained. A completely decoupled model, therefore, would require analytical data for each element in each of its redox states. Unfortunately, analytical data of this completeness are seldom collected. 

As an example of modeling a fluid in redox disequilibrium, we use an analysis, slightly simplified from Nordstrom et al. (1992), of a groundwater sampled near the Morro do Ferro ore district in Brazil (Table 7.2). There are three measures of oxidation state in the analysis the Eh value determined by platinum electrode, the dissolved oxygen content, and the distribution of iron between ferrous and ferric species. 

we set oxidation state in the model using the dissolved oxygen content and calculate the distribution of species assuming redox equilibrium. 

To account for the possibility of redox disequilibrium among iron species, we use the analysis for ferrous as well as total iron  

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Table 7.3. Concentrations (molal) of predominant iron species in Morro do Ferro groundwater, calculated assuming equilibrium and redox disequilibrium...

We can compare the Eh measured for the Morro do Ferro groundwater (Table 7.2) with the Nernst Eh values (Eqn. 7.1) given by the reactions for dissolved oxygen and iron oxidation, as reported in the program output ... 

Table 7.2. Chemical analysis of a groundwater from near the Morro do Ferro deposits, Brazil (Nordstrom et al., 1992)...

Nordstrom, D. K., J. A. T. Smellie, and M. Wolf. 1990. Chemical and isotopic composition of groundwaters and their seasonal variability at the Osamu Utsumi and Morro do Ferro Analogue Study Sites, Pocos de Caldas, Brazil. Pocos de Caldas Report No. 8. NAGRA NTB 90-24, SKBTR 90-15. 

Morro do Ferro, P090S de Caldas. Brazil Natural Analogue 0.15 0-85 Presence of a weathered zone. Th rich deposit. Th presence in groundwater is probably associated to colloids. Ore zone is very close to the surface. 

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