In some words the natural site-specific parameters which influence the concentration of iron dissolution are first of all related to the concentration in soil/sub soil [also in water yet dissolved obviousley], so you have to take in count Eh, Ph, concentration at the equilibrium, redox and others, I'd like to highlights the european BRIDGe project related to natural background level in groundwater
In most surface waters there will be a correlation between iron and colour and DOC. In these waters the iron is "complexed" with NOM (natural organic matter). In clasic reports this iron is suggested to be in the ferrous state!
You can correlate iron concentrations with the pH value. The correlation will be negative, because the lower pH tou have, the hiher concentrations in soluble iron.