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High TDS values coupled with Low Total Hardness : Causes ?

In some water samples (rarely) it has been noticed that TDS is very high 1500-1600PPM but the Total Hardness is low 100-150 PPM. What could be  the reason behind this?



1. High TDS waters with low total hardness

A water with the characteristics you have highlighted has been encountered by me only a few times in the last 4 decades.Like you have said in your query,it is rare.However,it is hard to explain. I believe it is due to the composition of the minerals present in the earth in which the acquifer containing the water is located.A complete test report of this water as mentioned by you would be of interest to me.If you can, please mail it and I will comment on it.


2. Dear Amit,   Water dissolves

Dear Amit,


Water dissolves the minerals present in the strata of soil it filters through in the case of ground water and, in the case of surface water, the minerals present in the soil over which it flows (rivers/streams) or over which it stands (lakes, ponds, reservoirs).The dissolved minerals in water are commonly referred to as Total Dissolved Solids (TDS). The TDS content of any water is expressed in milligrams /litre (mg/l) or in parts per million (ppm).

The minerals are basically compounds (salts) of Calcium(Ca), Magnesium(Mg) and Sodium(Na) What is commonly called as ‘hardness in water’ is due to the compounds/salts of   Ca and Mg such as  Calcium or Magnesium Chloride, Calcium or Magnesium Sulphate ( CaSo4, MgCl, etc).In some areas of India, there are ground waters which contain fluoride salts of Ca and Mg. Fluoride in water above 1.5 mg/l is dangerous and causes a disease called ‘Fluorosis’ which affects the teeth and the bones of humans who consume water with high levels of fluoride. Iron is another contaminant/impurity which is not safe for human consumption if it is present in water in excess of 0.3 mg/l. In several parts of eastern India, Arsenic is an impurity which has been found in ground water and needs to be removed as it is a slow poison. In water samples (rarely) it has been noticed that TDS is very high 1500-1600PPM but the Total Hardness is low 100-150 PPM because total hardness depends on Ca and Mg whereas TDS refers to the matter either filterable or non filterable that remains as residue upon evaporation.


With best regards,



Abhishek Mendiratta


Water Resource

New Delhi


3. Dear Amit,   Some water may

Dear Amit,


Some water may have low Ca & Mg content and hence low hardness. This could be for e.g. a sugar solution with high salt content but low hardness.


With regards,



Associate Fellow

Water Resources Policy and Management

Water Resources Division

The Energy and Resources Institute (TERI)

New Delhi

4. Sodium chloride type of water

Dear   Ranaji,

The high TDS is due to the presense of high amount of Sodium and chloride ions. Low hardness with high TDS is often found in the samples of coastal India and in saline water samples.

Sibasis Panda


sibasis Panda

5. High TDS and Low Total Hardness

Total Hardness is due to dissolved calcium and magnesium salts in the form of bicarbonate(temporary hardness), sulphate and chloride (permanent hardness).
However, TDS is beacuse of different types dissolved inorganic salts.
So, it is clear that high TDS need not be associated with high Total Hardness.
With regards
Diponkar Bordoloi

6. Dear Amit Kumar Singh,   TDS

Dear Amit Kumar Singh,


TDS is an expression for the combined content of all substances (organic as well as inorganic) contained in a liquid, which are present in a molecular, ionized or colloidal form. In drinking water sources, organic components are generally negligible.   Depending on the source, main inorganic components are

  • Sodium, calcium, magnesium and potassium as cations.

  • Carbonate, bicarbonate, chloride, sulphate and nitrate as anions.

   Only a portion of the minerals that are dissolved in water are Calcium and Magnesium (the ions that make up water hardness). 

Thus, TDS in some water samples can be high in the absence of high hardness.A detailed water sample analysis has to be carried out to know specific components.




Leela Iyengar






7. Dear Amit Kr. Singh,   TDS

Dear Amit Kr. Singh,


TDS (total dissolved solids) may be due to any soluble matter. Hardness is due to the presence of Calcium and Magnesium.

If the soluble matter does not have Calcium and Magnesium, hardness will be low while the dissolved solids can be high.


With regards,  


Paritosh Tyagi

Chairman (Retired)

Central Pollution Control Board

New Delhi


8. Dear Amit Kr. Singh,   The

Dear Amit Kr. Singh,


The Hardness in water is contributed by salts of divalent cations whereas the TDS is contributed by salts of diavalent as well as monovalent cations. Therefore a water can have low hardness but high TDS if it contains low concentration of divalent cations (Ca, Mg) and high concentration of monovalent cations (Na, K).


With regards,



Environmental Engineer

Pollution Control Implementation Division - III

Central Pollution Control Board

New Delhi


9. Dear Amit Kumar Singh,   TDS

Dear Amit Kumar Singh,


TDS is total dissolved solids - i.e all that is dissolved in water measured interms of the conductivity thus quantifying ions that conduct electricity.

Hardness is caused by Calcium and Magnesium ions mostly all higher valence ions. Hence, there can be other ions like Sodium in excess which is monovalent which can cause high TDS but not high hardness.


Hope I have answered the question.


Dr. Indumathi M. Nambi

Assistant Professor

Department of Civil Engineering

Division of Environment and Water Resources Engineering

Indian Institute of Technology

Madras, Chennai


Thanks for all the comments. The answers were according to my ideas which I was thinking behind this phenomenon.

regards to all the contributors. 

Amit Kr. Singh,
Govt. of Haryana,
Public Health Engg. Department,
Water Testing Lab,Karnal-132001(haryana)

11. Natural waters with high TDS and low hardness


Dear Amit,

You can find a question and answer similar to the question you posed at

The TDS (total dissolved solids) of natural waters (seawater, river/lake water, and groundwater) is mainly the sum of the major alkali and alkaline earth cations (calcium, Ca, magnesium, Mg, sodium, Na and potassium, K) and the major anions (chloride, Cl-, sulphate, SO4-- and bicarbonate, HCO3- & carbonate, CO3-- as CO3--); while hardness is computed mainly from Ca and Mg. The present discussion is limited mostly to these four major cations.

The ionic potential, IP (z/r), where z is the ionic charge and r the ionic radius expressed in angstrom units, determines the mobility of ions in the surficial rocks, soils and natural waters with solubility increasing with decrease in IP. Those with IP less than 3 occur as soluble cations or anions, between 3 and 10 occur as insoluble oxides and hydroxides, and over 10 as soluble complex anions such as SO4--, HCO3- and CO3--. The IP of major cations is 0.7 for K, 0.9 for Na, 1.9 for Ca and 2.5 for Mg, indicating greater solubility of K and Na over Ca and Mg.

The concentration of these ions in natural waters is governed by:

1.      Abundance in the Earth’s crust,

2.      Dissolution of minerals in which they occur,

3.      Hydrogen-ion concentration, pH,

4.      Dissolution or precipitation of some of their compounds,

5.      Ionic adsorption and

6.      Biomineralisation.

The weight percentage of these ions in the Earth’s crust is 3.63% for Ca, 2.83% for Na, 2.59% for K and 2.09% for Mg, while other alkali and alkaline earths are in trace amounts. Although some elements, such as silicon, Si and aluminium, Al, occur in much greater abundance in the Earth’s crust, their rarity in natural waters is due to their occurrence in highly stable primary and secondary minerals, besides higher IP making them to occur mainly as insoluble oxides and hydroxides. All this explains why these four cations alone occur mostly in natural waters. Of these ions, sodium is not only most easily leached from rocks and soils but remains very long in water without getting removed through precipitation, adsorption or biomineralisation.

It is interesting to note that seawater with a salinity (TDS) of around 35,000 milligrams per litre (mg/L) has unusually high Na of 30.8% and unusually low K of 1.1% and low hardness with Mg of 3.7% and Ca of 1.2%. The low concentration of Ca in seawater is partly because of its tendency to get precipitated as inorganic CaCO3, besides biomineralisation making marine organisms particularly to use CaCO3 to build their skeletons. As rightly pointed out by Sibasis Panda, groundwater subjected to seawater intrusion along coastal areas is characterised by high TDS and low hardness.

The low concentration of Mg and K in seawater on the other hand is attributed to their adsorption by clay minerals and low-temperature metasomatism that makes Mg to react slowly with CaCO3 to become dolomite, CaMg(CO3)2.

Groundwater with high TDS and low hardness is particularly found in arid and semiarid areas with conditions favouring formation of alkaline soils. Such areas are characterised by high fluctuations in groundwater levels with water logging in rainy season and steep decline of groundwater levels in summer leading to deposition of salts in the soils. This results in high pH, high alkalisation, precipitation of inorganic CaCO3 in the substrate, and cumulative accumulation of salts such as sodium carbonate both in the topsoil and groundwater. High sodium and low calcium/magnesium groundwater results in loss of tilth and permeability of soil and make them infertile. The TDS as known from its specific conductance (or electrical conductivity, EC in micromhos/cm at 25oC) is a useful index of the salinity hazard, while sodium-adsorption-ratio (SAR) is a useful index of the sodium hazard of irrigation water. The SAR of irrigation water is calculated by dividing Na by the square root of one-half of the sum of Ca and Mg, where the concentration of Na, Ca and Mg are expressed in milliequivalents per litre (me/L).

The U.S. Salinity Laboratory has rated the irrigation water on the basis of salinity hazard (EC) and sodium hazard. Salinity hazard is low when EC is under 250, moderate in between 250 and 750, medium in between 750 and 2250, high in between 2250 and 4000, and very high over 4000. Sodium hazard is low when SAR is under 10, medium in between 10 and 18, high in between 18 and 26, and very high over 26. It is not uncommon to find groundwater in lands subjected to high alkalinity to have very high salinity hazard and very high sodium hazard. Such waters will have very high TDS and very low hardness.

Best wishes



Dr. R. Jagadiswara Rao, Professor of Geology Retired, Sri Venkateswara University, Tirupati, AP 517502, India

12. Dear Amit,   Total Dissolved

Dear Amit,


Total Dissolved Solids means it is equal to the total content of dissolved substances in water samples. In any water sample Calcium and Magnesium when combined with bicarbonate, sulphate and other species, contribute to the hardness of natural waters. All waters in the environment contain dissolved salts. TDS cannot be related to hardness of water. In the source the dissolved constituent may be any type of salts and need not be calcium or magnesium bicarbonate or sulphate. The other salts of the form of sodium or potassium chloride or sulphate. Hence, TDS may be high irrespective of hardness and there is no link between hardness and TDS.


With regards,


M.V. ShashiRekha

Chief Chemist

Department of Mines and Geology


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