The mechanism behind Chebotarev Sequence might answer your question

In an open environment (normal P&T), the carbonate system will tend to equilibrate according to the Le Chatelier's principle. Dissolving of CO2 in the system may generate some weak carbonic acid. But, the buffering capability of HCO3 and CO3 may take up the excess H+ in the water and, hence, reduce the acidity (improve the pH). Unless, there is complete absence of HCO3 and CO3 in the water, otherwise, you won't get a H2CO3 dominate system.

Back to your question, carbonate minerals are ubiquitous in our environment. Often, you will have some amount of bicarbonate dissolved in the water (even it is a silicate dominant environment). Rainwater itself is a source for bicarbonate ions.

CO2 dissolved in water may form carbonic acid. But, the carbonic acid will also dissociate to form bicarbonate and hydrogen ions, depends on the buffering capacity of the groundwater. To sum up, bicarbonate can be generated from reaction of CO2 with water and also other sources.

if there is a lot of silicate weathering you can also have HCO3 produced. Likewise if you have microbial mediated processes, e.g. CH4 production or consumption. It's always best to look at HCO3/Cl ratios (and/or HCO3/SO4 if there's significant sulfate sources) as this tells you a lot more about HCO3 than just the concentration.

Good question. CO2 is present in rain from equilibrium with the atmosphere. As soon as the rain contacts the ground, reactions between the acidic H2CO3 and minerals will consume the acid portion (H+) leaving the HCO3 in solution (i.e. K-feldspar + H+ = clay and ions). As the groundwater moves through the aquifer more reaction will lower the H2CO3 content and increase the HCO3 and CO3 content. Note that the total C content does not change in this reaction if the aquifer is closed (not in contact with the atmosphere). This weathering reaction also increases the pH by lowering the H+ concentration. I recommend Drever's book for a better discussion. In addition, dissolution of carbonate minerals and conversion of organic matter to CO2 will increase the inorganic C content. In the case of organic matter you will increase the CO2 content and contribute to further weathering. Dissolution of carbonate minerals can be pH neutral, that is CaCO3 = Ca and CO3, but this reaction has a small equilibrium constant and contributes little overall inorganic C. Note is the system is open the equilibrium will produce a pH of about 8.4 (seawater). Higher pH values in groundwater indicate closed systems which may reach pH values of about 9. Finally, there are saline lakes where pH can reach as high as 10 due to evaporation.

I agree with the answer given by Dr Geoffrey Thyne; Oxidation of organic matter to CO2 will increase the CO3 or HCO3.

Fantastic. Thanks for very clear and useful answers.

Hi Das,

 Could you please elucidate relation between ratio HCO3/Cl or HCO3/SO4 and HCO3.

Thanks.

Hi Phuc, it can be useful to compare HCO3 to other anions. Cl is conservative, so a low Cl groundwater that has hgh HCO3 suggests a dominant source of HCO3 that may not be related to residence time or salinity controls. If your Cl concentration is relately stable, but your HCO3 concentration is changing, this might help you try and understand HCO3 sources. Remember, your HCO3 is a function of pH and salinity.....so always good to look at ratios of any ion, rather than just concentrations of individual ions. What kind of aquifers are you working with? Sedimentary? If you have saturation of some clays such as kaolinite then silicate weather in to kaolinite can produce high HCO3. I.e. The HCO3 product from the weathering reactions is not used in subsequent incongruent reactions. There is a good paper

Here's a link to the paper

https://www.jstage.jst.go.jp/article/geochemj1966/37/3/37_3_351/_pdf

Hi Des,

Thank you very much for text and paper. I have a quick look in the paper which introduces the origins of NaCarbonate water. It is useful to me in studying seepage through the hydrodam foundation which includesdifferent section of granite + weathered granite + basalt + fine stream sediment. I agree with you that we need to refer to other ions, ratios of ions, pH... In our case we also use Radon-222 which is rich in GW.

Sometimes we work with metamorphic sea water type Calcium Chloride from petroleum production.

Nice to contact you. Regards.

These articles explain this -

• 10.1016/j.microc.2019.104304
• 10.1007/s10653-018-0096-x

Bicarbonate may also be derived from two possible sources. The main one is related with the dissolution of the carbonates minerals (calcite and dolomite) by carbonic acid, whilst secondarily their concentration might be enriched by the decomposition of organic matter and root respiration in the soil zone which creates under the precipitation effect initially weak carbonic acid (H2CO3) and finally bicarbonates (HCO3-) through the sequence of following reactions:

📷CH2O+ O2 CO2+H2O

(Organic matter)

📷CO2 +H2O H2CO3-

📷H2CO3 H+ + HCO3-

(Carbonic acid)

I agree with prof Bouteldjaoui response