Dear Asit Kumar Batabyal ,

In order to give a sound explanation, you should be aware of the full background, such as hydrogeological regime, lithology, potential contamination sources, etc. However, some assumptions can be addressed, even though description lacks some critical details (e.g. which is the depth of water table, the lithology of the aquifer, climatic conditions, etc)

Taking into account that no external contamination occurs, I can think of at least three reasons that elevated concentrations of Fe may occur in unconfined aquifers.

1.       If the aquifer is rich in organic material, then there is a high possibility for depletion of oxygen due to organic decomposition. This, subsequently, will create (locally or not) reducing conditions, which in turn will enrich groundwater solution with Fe2+.

2.       Seasonal variations (e.g. recharge) that affect hydrological/hydrogeological conditions are a critical factor which may affect DO content, and in turn Fe concentrations.

3.       You should also consider the potential hydraulic connections (with surface or groundwater) which may alter the initial redox conditions. These effects may have seasonal trends too.

Best Regards,

Evangelos

Interesting question...

The extent to which Fe and Mn dissolve in groundwater depends on the amount of oxygen in the water and, to a lesser extent, upon its degree of acidity, i.e., its pH. Iron, for example, can occur in two forms: as Fe2+ and as Fe3+. When levels of dissolved oxygen in groundwater are greater than 1-2 mg/L, iron occurs as Fe3+, while at lower dissolved oxygen levels, the iron occurs as Fe2+. Although Fe2+ is very soluble, Fe3+ will not dissolve appreciably.

If the groundwater is oxygen poor, iron (and manganese) will dissolve more readily, particularly if the pH of the water is on the low side (slightly more acidic). Dissolved oxygen content is typically low in deep aquifers, particularly if the aquifer contains organic matter. Decomposition of the organic matter depletes the oxygen in the water and the iron dissolves as Fe2+. Under these conditions, the dissolved iron is often accompanied by dissolved manganese or hydrogen sulfide (rotten egg smell). When this water is pumped to the surface, the dissolved iron reacts with the oxygen in the atmosphere, changes to Fe3+ (i.e., is oxidized) and forms rust-colorediron minerals. Dissolved manganese may form blackish particulates in the water and cause similar colored stains on fixtures.The relationships between oxygen levels and precipitation rates of iron and manganese are the main key in solving this problem. Instead of some well-known chemical treatments, an innovated technique would help in resolving such problem.

One PDF enclosed for further reading...

You can read out the following book....

https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/site_page/C-12U.pdf 

Dear Dr. Anoop Kumar Srivastava,

Thanks for your kind reply. Your explanation, in general, is good. But my specific question is on the occurrence of excess iron in unconfined groundwater. High iron content in case of unconfined aquifer is possible? If it is so, then what's the explanation?

Dear Azharul,

Thanks for your attachment. I will go through the same.

Dear Asit Kumar Batabyal ,

In order to give a sound explanation, you should be aware of the full background, such as hydrogeological regime, lithology, potential contamination sources, etc. However, some assumptions can be addressed, even though description lacks some critical details (e.g. which is the depth of water table, the lithology of the aquifer, climatic conditions, etc)

Taking into account that no external contamination occurs, I can think of at least three reasons that elevated concentrations of Fe may occur in unconfined aquifers.

1.       If the aquifer is rich in organic material, then there is a high possibility for depletion of oxygen due to organic decomposition. This, subsequently, will create (locally or not) reducing conditions, which in turn will enrich groundwater solution with Fe2+.

2.       Seasonal variations (e.g. recharge) that affect hydrological/hydrogeological conditions are a critical factor which may affect DO content, and in turn Fe concentrations.

3.       You should also consider the potential hydraulic connections (with surface or groundwater) which may alter the initial redox conditions. These effects may have seasonal trends too.

Best Regards,

Evangelos

Dear Dr. Batabyal,

Have a look at the article referred below. You may find it interesting and useful.

Postma D, Boesen C, Kristiansen H and Flemming L (1991) Nitrate Reduction in an Unconfined Sandy Aquifer: Water Chemistry, Reduction Processes, and Geochemical Modeling. Water Resources Research 27 (8): 2027–2045.

Dr Evangelos Tziritis - Good explanation. Thanks

Dr. Dilip Kumar Kundu  -  I will go through the article. Thanks

Turbidity is very important factor affecting Fe (and some other metals and phosphorus too) concentrations in oxic waters. Even slightly elevated turbidity in water samples (say 10 FNU or NTU) can give Fe-concentrations as high as 1 mg/l while in filtered samples there is practically no iron at all. That is because in the analysis also the insoluble iron held by solids, humus complexes etc. will dissolve with the result of high apparent iron concentrations.

I recommend the following link for further reading:  Hatva, T. 1989: Iron and manganese in groundwater in Finland : Occurrence in glacifluvial aquifers and removal by biofiltration.

https://helda.helsinki.fi/handle/10138/24894?locale-attribute=en

Dear Dr. Jorma Mäkelä - It's a good point that you have mentioned regarding the high concentration of iron in unconfined groundwater. Thanks

I would like to add on what has been said, and would like to generalize Evangelos Tziritis 's point on seasonality - heterogeneity I think is key - groundwater is not the same everywhere through time and space. Even if the aquifer is oxic per se, there might be "pockets" or patches, even if minute, that are reduced in oxygen, e.g. because of some (microbiological) process happening just in these places. The microbes' transformations at these places might lead to products which may be unexpected when just assuming groundwater as a well-stirred pot of water which you sample at one end and know what's happening at the other end. If, in the case of Fe++, enough organic substance is around to complex the ion, it might be stable for some time.

Dr. Susanne I Schmidt - "Even if the aquifer is oxic per se, there might be "pockets" or patches, even if minute, that are reduced in oxygen"

Madam - this is a good explanation on the occurrence of high iron in unconfined groundwater. If there is any published work on this topic, please send me.

Thanks 

A very interesting, extreme example that does not fit the iron topic but gives you a hint on what's possible in the wold of microbes, there are microbially active micro aquatic phases within an oil phase: 

https://www.ncbi.nlm.nih.gov/pubmed/25104386

High Fe concentration in the groundwater of the tropical Africa with exemption from external influence is usually associated with thick lateritic overburden in the case of unconfined aquifer and the formation type of the aquifer.

Dr. Susanne I Schmidt - Thanks for attachment.

Dr. Lizzy Okoyeh - In hot & tropical climate, the formation of  laterite at or near the surface is found in many areas. The laterite is  formed by the weathering of the underlying parent rock. The laterites or lateritic soil may be the source of iron in the unconfined groundwater  of these areas. Can you provide  any published work (tropical Africa) on groundwater of lateritic terrain? Thanks

Если в водоносном горизонте есть органика, то она окисляется, забирает весь наличный кислород и среда становиться бескислородной. В таких условиях мобилизуется железо двухвалентное. Поэтому, даже в колодцах, которые открывают грунтовые воды, иногда можно встретить растворенное железо.

Another important parameter that has to be considered in the hydrogeochemistry of iron is pH. Low pH (

During the transition from ‘confined’ to ‘unconfined’ groundwater, the change in composition and redox conditions certainly favours the oxidation of Fe(II) to Fe(III). Although, thermodynamically, Fe(III) is expected to precipitate (first with phosphates/arsenates, and then as ferrihydrite), it can still remain within the 0.4-µm filterable fraction provided that precipitation results in the formation of nanoparticles or colloids, perhaps stabilised by organic matter (OM). For example, fulvic and humic substances can effectively maintain Fe(III) nanoparticles in solution (or suspension) through a combination of the steric and electrostatic effects of their negatively charged macromolecules. Depending on factors such as the Fe/org C ratio, a fraction of the total Fe may also bind the humic macromolecules and form truly soluble complexes. Equilibrium constants for the formation of Fe(III) complexes with humic groups are much larger that the corresponding constants for Fe(II). Therefore, once these complexes are formed (either by direct interaction between Fe(III) and OM or by oxidation of a pre-existing Fe(II)-OM complex) then Fe(III) can resist precipitation and contribute to the elevated concentration of Fe that you and other researchers have reported in oxygenated ground and surface waters [1, 2, 3].

[1] Jirsa F et al. (2013) Limnologica 43, 239-244.

[2] Muller FLL et al. (2015) Biogeochemistry 126, 343-362.

[3] Krachler R et al. (2016) Science of the Total Environment 556, 53-62.

Unconfined means oxygen supply is restricted. It is almost anoxic. In anoxic environment  specially when organic matter present, acidic conditions in water is inevitable. It is the best the ferrous ion can have to sustain.

A number of factors can cause reducing conditions in unconfined aquifers.  Especially in contaminated aquifers or with excessive nutrients (e.g. phosphate, nitrate) that can stimulate microbial activity.  General water quality parameters (e.g. pH, SpC, DO, Eh) along with major cations / anions can point to conditions favorable for elevated concentrations of trace metals.  Since iron is usually abundant, that may be the most obvious.  If you are seeing elevated iron concentrations, you should also look for elevated levels of Mn, As, and other redox sensitive trace metals.

Dear Dr. Mark D. Williams - You  have rightly mentioned the  factors that may cause reducing conditions in unconfined  aquifers - additional  information on the topic.

Yes it is right - due to microbial  activities,  the concentration of redox sensitive trace metals like Mn, As, Fe etc. in ground may increase. 

Thank you very much for providing important information on the topic.

Dr. François L L Muller - You have provided lot of information on the topic; would be useful for many researchers. I am highly thankful to you for the papers you have attached. I will read the papers. With regards - Dr. Asit

Thanks to all other researchers who  have addressed the question and provided information on the topic. Regards - Dr. Asit

beside of many reasons that turn back to the quality of groundwater, one of most important factors is high content Fe geological formation that can have a great effect on the quality of groundwater. As a key guide if your Fe anomaly is detected in a vast area, geological conditions should be checked in details.  

О последней рекомендации. Практически в большинстве горных пород содержание железа в тысячи раз превышает содержание его в воде. Поэтому в вопросе железа в воде главным являются параметры среды, а не содержание железа в водосодержащих породах. 

I have seen very high Fe in tropical gw which most likely attributed by the geology of the aquifer. But I wonder whether 100mg/l of Fe is logical?

But I wonder whether 100mg/l of Fe is logical? (Dr. Saim Suratman)

100 mg/L - I have not read.

Thanks for showing interest to the question.