Is an analysis of 61mg/L Fe (total) plausible from a deep confined siliciclastic aquifer ~420 m below ground level? Normally precipitation of FeS and FeS2 under redusing conditions will limit iron concentrations from getting anywhere near that high (i.e. 5 - 10mg/L Fe is often considered very high) . Can bacteria affect iron concentartions?
Iron in rural groundwater supplies is a common problem: its concentration level ranges from 0 to 50 mg/l, while WHO recommended level is < 0.3 mg/l. The iron occurs naturally in the aquifer but levels in groundwater can be increased by dissolution of ferrous borehole and handpump components. Iron-bearing groundwater is often noticeably orange in colour, causing discoloration of laundry, and has an unpleasant taste, which is apparent in drinking and food preparation.
Iron dissolved in groundwater is in the reduced iron II form. This form is soluble and normally does not cause any problems by itself. Iron II is oxidised to iron III on contact with oxygen in the air or by the action of iron related bacteria. Iron III forms insoluble hydroxides in water. These are rusty red and cause staining and blockage of screens, pumps, pipes, reticulation systems etc. If the iron hydroxide deposits are produced by iron bacteria then they are also sticky and the problems of stain and blockage are many times worse. The presence of iron bacteria may be indicated by rusty slime inside headwork’s, reduced water flow form the bore and unpleasant odor from water pumped from the bore, slimy deposits blocking main and lateral lines, severe staining on pavements, walls foliages.
See François L L Muller's reply
https://www.researchgate.net/post/Elevated-concentration-of-iron-Fe-in-groundwater
Hi,
in order to be able to answer seriously, it would be good to know more information. For example, what is the pH of the water, was the sample filtered before analysis, how was the sample taken, what is the geology of the area, etc. So deep in the well, massive bacterial oxidation by neutrophilic bacteria is probably not to be expected. The same probably applies to sulfide-oxidizing bacteria. So, for now, the only possibility left is the oxidation of the well casing. When the well will be properly pumped out, much can be learned from the change in chemistry.
Thanks Dalibor and Mostafa,
The pH is circum neutral 6.7 and the sample was unfiltered - it was noticeably reddish in colour - but still wasn't expecting such a high reading of ~61 mg/L (ie 5 mg/L is considered quite high). As far as we know there is nothing very usual about the aquifer - mature, quartz dominanted, Jurrassic-aged, siliciclastic sediments, trace pyrite is common - we need to check whether oxidation of the steel well screen maybe by bacteria mediated processes is causing the problem. The sample also contains 24 mg/L SO4 which may be unusal from such a deep sample and may be indicating some sort of redox process is taking place?
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