Hi,

First what is the value of Na/Cl ratio?

increasing Na/Cl ratio in the groundwater indicates that disolution of HALITE is the principal sources of salinisation.

Look at Ca and SO4 concentration, that should be have a same behaviour of Na/Cl.

We need more data to say something with sense related with your problem. Perhaps indicates ion exchange typical of GW flowing through clays.

Dears Elgattafi and Jorge Jodar, thanks for the comments. Actually in my samples, electrical conductivity varied from 647 to 9439 µS/cm, with an average 4142 µS/cm. the average molar ratio of Na/Cl = 1.81 which indicates excess of Na over Cl, which can reflecting Na+ released from silicate weathering reaction. moreover, Groundwater in the plain has a higher average ratio (0.68) of Na+K versus total cations. based on the background it seems that evaporation and silicate weathering happened in the study area.However I can't understand Na/Cl vs EC result. 

What is the geological nature of the plain?

thanks Ashwani Kumar for the paper and comment.

Dear Elgettafi. Based on the geology map, the study area consist volcanic formation (mainly of tuff, lava fan deposit, andesitic lava flows and their interaction with marl sediments) and several evaporate-sedimentary layers including red marl and gray-red sandstone, gypsum, and conglomerate.  

You can dowlaoding my work Messinian salinity crisis impact on the groundwater quality in Kert aquifer NE Morocco: Hydrochemical and statistical approaches. International Journal of Water Resources and Environmental Engineering, 4(11): 339-351.

Try plotting the data in a piper diagram and read Apello and Postma, 2010: Geochemistry, groundwater and pollution.

Have you considered cation exchange?

based on the geological nature you have a dissolution of evaporites. These rocks are not as carbonates,diden't Tp or pH  just a simple conact with water they release Ca, Na SO4 Cl.

Try plotting data in Ca and bicarbonates and look if you have correlation, if not that means evaporates dissolution.

Excuse me, try plotting data in Ca and bicarbonates digramme.

Cation exchane are always present in this type of area.

Thanks dears Elgettafi and Licha. I think both cation exchange and evaporation occur in my study area. The problem is that if evaporation occur, in the Na/cl vs EC diagram the samples should be plotted in a horizontal line parallel to EC axis. but in my samples Na/Cl ratio increase by increasing EC and they aren't parallel to EC axis. 

It's normal Na/Cl ratio increases with EC, why? Becouse salinty (EC) is related to Na and Cl in water more theses elements are present in water more EC is higher.

I think in your case disssolution of evaporates rocks occured accompanied with cation exchange.

Dear Elgetaffi,I really thankful of your answers

Do tou have readed my paper ?i think you find the same probleme in your stydy area

Yes I found it very useful. thanks for the recommendation 

I agree with the preceeded answers that probably exists a cation exchange process, that involve Ca or Mg with Na.  It's also important to note that the origin of these can be diverse and  in normal hidrogeological contexts involve the existence of  clays: vermiculites, montmorillonites, Illites, with Cation Exchanges Capacities beetween 10 to 150 meq/100g. Another important point to determinate the origin is the spatial relationship of the rNa/rCl ratio. There are some hidrogeological processes of mixing that produce a regional and termporal variation of these index, like Marine/ continental salt intrusion proceses. As  Mr. Elgettafi  comment, the litological effect could be also important in some cases.

Regards.

Mr. Ribera, Thanks for your valuable comment .

Electrical conductivity varied from 647 to 9439 µS/cm, here we are in gradual increasing of salinity. In my opinion you must combined geological kind, pieozometric map and hydrochemical data (plloting all of data in binair diagram and geographical repartition). then look what is relationship  between them, and try to explain this chimically. In this study you are a broblem of salinity? Are you used an isotopic data? In geological formation do you have a Trias formation?

Regards   

Mainly ion (cattion) exchange process and if the values comes in the coastal aquifer it has indicates saline intrusion, otherwise it is inland area, main passibilities for ion exchange process , halite dissolution and anthropogenic activities. High level of EC indicates that mainly saline intrusion.

Among confined aquifers, Na-HCO3 type fluids are typical hydrochemical facies.  Among semi-confined aquifers, Na-SO4 type fluids are also typical.  In these examples, Na is derived from cation exchange.  If GW samples are derived from water wells, it is common to observe mixed fluids in the well bore derived from multiple aquifers.  Although the screened interval may be limited in range, a gravel pack or open hole completion will allow vertical connectivity between aquifers.  You will need to run a full suite of major ion analyses to address mixed water types. 

I'm not totally agree with Sr Selvakumar and Sr Anthony. Based on the geology Kind, the study area consist volcanic formation and several evaporate-sedimentary layers including red marl and gray-red sandstone, gypsum, and conglomerate. Here we are in a geological context where mainly evaporite chemistry takes place, then cation exchange will be occured as result of this dissolution (principaly halite and gypsum). Narany has not communicat the facies of water. But according to geological natur the facies will be Na-Cl CaMg-SO4 (What do you say Narany?)

I mean what do you say about hydrochemical facies?

If you wish to address the effects of halite dissolution, then you must analyze for Br.  Dissolution of gypsum will not affect the Na/Cl ratio.

I agree with Mr. Elgettafi about groundwater facies, since it changes from Na-Mg-Ca-Cl to Na-Cl. We found the parts related to evaporite-sedimentary rocks mostly covered by Na-Cl  water facies.  

Dissolution of Halite with underground flow

To my mind Na can be derived from either ion-exchange processes or silicate weathering. I agree that plotting on a Piper plot and looking at Br/Cl ratio provide excellent clues to the potential processes. I've also found it useful to remove the "marine" component (simply remove cations in proportion to that in seawater or rainfall) and compare just the "weathering" component. Sometimes you see things much more clearly.

Dear Narany,

I hope you have found the explanation for your problem. I know the complexity of the  groundwaters salinity. Mostly, they are several processes that mainly involved.

Best for you

The increase of EC may be due to ionic content of the groundwater. The relation of Na/Cl increase with EC increase may be due to the presence of halite mineral or highly anthropogenic activities. Natural contribution of Cl to the groundwater is mainly due to halite or saline water intrusion. Human activities such as sewage ponds, solid waste disposal sites,open drainage system contribute high chloride values to the groundwater. In those cases high Na/Cl may be observed along with high EC values.  Other than those conditions there is no positive relation between Na/Cl and EC.  

On the basis of, Na/Cl ratio is directly proportion to EC, not much can be predicted. However hydrological & hydrochemical ( Complete chemical analysis ) may suggest some results.

Your question is difficult to answer because it lacks detail. Over what period was the change in water quality observed?  How many samples did you collect to observe the change in ratio.  How deep is the well and what is the length of the screened interval?

The most common hydrochemical facies for confined aquifers are composed of Na-Cl and Na-HCO3 water types. Semiconfined aquifers will tend to have Na-SO4 and Ca-Mg-SO4 water  types.   Layered aquifers bounded by effective confining layers will have differing composition because naturally occurring vertical mixing is restricted.  In such situations, a  well with an long screened interval or open hole interval will allow aquifer fluids to become mixed in the well bore environment. The mixing ratio among samples collected can change for any number of reasons that typically are related to environmental conditions such as poor grout bonding, pump use, or biofouling.  If an increase in EC is positively correlated to an increase in Na/Cl ratio, it usually means that fluids in the well are derived from more than one aquifer source and that the mixing ratio in the water will be susceptible to change.  A piper diagram will help you identify the end-member compositions of the mixtures, when sample data points fall on a linear trend. 

According to

KUMAR, P. S. & JAMES, E. 2013. Physicochemical parameters and their sources in groundwater in the Thirupathur region, Tamil Nadu, South India. Applied Water Science, 3, 219-228.

Na/Cl was decreased with an increase EC, his indicated that evaporation is not a dominant process affecting the groundwater chemistry. So i would simply say that for Na/Cl ratio which is directly proportion to EC, it could indicate that water chemistry is affected by other processes evaporation. But please confirm this by drawing the Gibb's diagram of your samples