In my study, the fluoride (F) shows a significant negative relation with total hardness (TH) (r= -0.635). F also exhibits significant negative correlations with Ca and Mg.
TH is positively correlated with Ca, Mg and HCO3.
1) In this case what salts are responsible for hardness of groundwater? Are these bicarbonate salts, Ca (HCO3)2 and Mg (HCO3)2 ?
2) Can we say that the negative relation of F with TH is due to the precipitation of Ca and Mg ions in the TH as carbonates and/or ion exchange of Ca with Na?
But Ca/Na exchange will not change bicarbonate, and also is not true for precipitation.
This is very interesting study. There is no direct correlation between F and TH. However some individual data may support. It is better to collect published F and TH data and evaluate the correlation.
I agree with Mr. Virendra Kumar Saxena. You can also search the Ca and Mg content. Then you can identify the relation.
Im not aware of any direct or indirect correlation of some sought between F and Total hardness ,i think extensive literature review on this subject can give you a better view on the correlation .
In my dataset based on the groundwater samples from drilled wells in Central Finland there is only a weak positive (though highly significant) correlation between TH and F (r=0,198 p
Dear Dr. Jorma Mäkelä,
My study area is in continental set-up; far away from sea - no possibility of saline water intrusion. The groundwater with excess fluoride is found at several sites. The correlation coefficient value (TH & F) is high (r= -0.635). Please refer to my question - I think this high negative correlation is due to the Ca and Mg. Because, hardness reflects the the Ca and Mg salts.
Thanks
Hi,
I agree with Mr. Virendra Kumar Saxena and Mr. Azharul Haque Chowdhury! There is a relationship between the hardness and the cation components Ca, Mg. Fluoride anions are in very low concentrations relative to other anions associated with Ca and Mg. If you want, you can download our papers on fluoride in my RG page.
The authors of the following publication point out that fluoride contamination of groundwater can be removed by adsorption onto various calcium salts, including calcite:
Turner BD, Binning P, Stipp SLS (2005) Environmental Science and Technology 39: 9561-9568.
They present evidence to the effect that fluoride removal proceeds via a combination of adsorption and precipitation mechanisms.
Perhaps these mechanisms are responsible for the negative correlation you find between Ca and Mg on the one hand, and F on the other hand. For example, if bedrocks and mineral soils are rich in calcium carbonate then the groundwater will be high in Ca but depleted in F due to precipitation and/or adsorption reactions of F onto CaCO3. Given enough information about the major-ion chemistry concentrations, you might even be able to model the amount of F removed from solution.
Dear Asit Kumar, I'm sure you are familiar with the articles below where the negative relations between high F concentrations and Ca, Mg and total hardness are described. However, other question followers may find them interesting so let´s put them here
http://www.iisc.ernet.in/~currsci/jan252007/240.pdf
ftp://ftp.ecn.purdue.edu/vmerwade/class/GDT/uploaded_documents/India/Kundu%20-%20Geochemical%20appraisal%20of%20fluoride%20contamination.pdf
Firstly, hardness is nt a measurement, but a water quality index that is calculated (commonly) from the concentrations of calcium and magnesium ions, so the common relationship with Ca / Mg and TH is not surprising. Another observations that may be of interest (although it doesnt explain why there is a negative relationship in concentrations) - in surface water quality assessments it is known that the toxicity of F to aquatic organisms is reduced in waters with high hardness (or more correctly high Ca concentrations) due to complexation of Ca and F. that is F may still be present in solution, but it is not as bioavailable at high TH..
Dr. François L L Muller and Dr. Jorma Mäkelä - Thanks for your attachments - these are useful. Regards
Ca binds to F as CaF2. That's why you find a negative correlation between these two ions. If you plot logaritmic concentrations of these two you could find a linear relation. Check with a good textbook on water chemistry; my favorite is Stumm- Morgan Aquatic Chemistry (a bit old now, but still good).
Agrees with Andrew Butler.
Anders Wilander
