The cation/anion balance looks fine, supporting your assertion about the quality of the analysis.  The obvious answer is that there is a S source in the catchment.  The pH may give you a clue as to whether the source is sulfate or sulfide. Does the catchment contain marine or mineral deposits?

Na may come from i) from sea water ii) presence of sodium containing mineral in the catchment area iii) some anthropogenic source which discharges Na-containing material (may be along with S) into the river. But as the Cl contain in your sample is not so high, so the case of marine contamination may not be appropriate. So the high Na contain may be caused due to the last two reasons. The form of S will be governed by the prevailing redox condition of the river (pH will be an indicator). 

Dear Dr. Milham and Mr. Sarkar,

Thank you for your helpful comments. pH value fluctuates between 6.6 and 7.9. There is no see around and the lithology consists of marl, mudstone and conglomerate. there is no igneous rocks, therefore no mineralization.

What about the industries present on the river bank?

That's a vast area. I am not sure about industrial activity but I couldn't find any city around in that region, so it is not probable.

I talked to someone who had tested the same river water 3 years ago he had observed this anomalous results that time too. 

There may be some industry beside the river which has been discharging this. You can take several sampling sites along the 20Km course the river. By this you can find the concentration gradient and it will suggest the regeion where the concentration of Na and S is maximum. Carrying out a survey within that regeion you may find the reason for the enriched Na and S concentration you are having in the river water. 

River water may be enriched in Na and Sulphate probably of Anthropogenic sources. However water- Minerals interaction can not be ruled out. A detailed study and chemical analysis may solve the problem.

One thing that I am sure about is that sodium and sulfate comes from the mineral dissolution because we have absolutely high amount of the same ions in nearby rivers which passes through the same petrologic unites. This fact refutes anthropogenic hypothesis.

This could be following reasons

Natural: Water rock interaction, - analyse rock sample for presence of sulfide/sulfate minerals (XRD/XRF analysis). Also- check pH and redox potential and of the water sample. Presence of marine minerals.

Activity of sulfate reducing/ oxidizing bacteria  

Anthropocentric sources:   gather information about industries near your sampling  points  and effluents from these industries. Are nallas  connected with river? these may carry effluent of industries ( such dye and textile)

Excess Usage of sulfate fertilizers by farmers in near by the river banks.

it seems water is saline, indicates the possibility of seawater intrusion( if your area of investigation is close to see). Otherwise it may be anthropogenic may be due hospital waste and laundry (please check)

I hope that this may help you to find out the cause of  high concentration of sulfate. and salinity.

Some river sections can be either gaining (groundwater discharge) or losing (groundwater recharge, infiltration). The composition of groundwater discharge will vary depending on fluid residence times in the aquifer from recharge to discharge. Recharging groundwater is likely to change composition along the flow line from Ca-Mg HCO3  and Ca-SO4 type fluids (in more arid environments) to NaSO4 type fluids (typical in alluvial aquifers) and then to Na-HCO3 and Na-Cl type fluids. These hydrochemical changes are primarily governed by bacteria, cation exchange, junction potential type reactions that occur as aquifer water changes confinement from unconfined, to semi-confined, and finally confined conditions.  Sulfate at recharge most commonly originates from weathered shales and coals containing pyrite. Sulfate reducing bacteria living in aquifer matrix will gradually consume sulfate along the flow line of in semi-confined aquifers until it is exhausted.  In most basins, sulfate will not be detectable beyond a distance from recharge of a few kilometers or less than 1/2 to 1 km at depth.  So, you may be seeing discharge of water into the river from semi-confined aquifers.  By doing mass balance calculations (using milliequivalents in solution) you should be able to differentiate whether the sulfate discharge is derived from unconfined (Ca-SO4 type fluids) or semi-confined (Na-SO4) type fluids.  Your studies can be supplemented with baseflow analytical methods to help identify zones of discharge.  Here are some useful review articles that will help your understanding.

Sophocleous, M., 2002, Interactions between groundwater and surface water: the state of the science: Hydrogeology Journal, v. 10, p. 52–67, doi: 10.1007/s10040-001-0170-8.

Winter, T.C., 1999, Relation of streams, lakes, and wetlands to groundwater flow systems: Hydrogeology Journal, v. 7, p. 28–45.

Potter, K.W., 2001, A simple method for estimating baseflow at ungaged locations: Journal of the American Water Resources Association, v. 37, p. 177–184, doi: 10.1111/j.1752-1688.2001.tb05484.x.

Smakhtin, V.U., 2001, Low flow hydrology: a review: Journal of Hydrology, v. 240, p. 147–186.

Dear Dr. Gorody,

Thank you for the detailed explanation and papers.

I think you explained my study case were accurately: "(in more arid environments) to NaSO4 type fluids (typical in alluvial aquifers)". my area is arid with unconsolidated sediments, sandstone and brown marl. The brown marl in the area may have organic material (hydrocarbon) content which is a strong source of S. Additionally I don't have any information about the groundwater level in area, this lack of information makes it hard to talk about surface and underground water contamination.

I added a location coordinate for whom wants to check the lithology and structural settings of area.

Thank you.  I recommend that you convert All the major ion concentrations (Na, K, Ca, Mg, Cl, CO3, HCO3, and SO4) to milliequivalents and run a charge balance. Use your pH data to estimate CO3 and HCO3 concentrations from your total alkalinity data making sure to correct for total alkalinity as CaCO3. If CAB results are below +/- 10% then run a mass balance calculation to estimate water type volume contributions. Assume first that Ca & Mg ions assigned to bicarbonate. If there is extra Ca , then assign it to Ca-SO4 type fluid.  Extra sulfate would then be derived from Na-SO4 type fluids, and remaining sodium and bicarbonate meqs should balance  if derived from Na-HCO3 type fluids.

BTW I made some editing mistakes during cut and paste while writing.  The "typical of alluvial aquifers" refers to dilute Ca-Mg-HCO3 and Ca-SO4 (more arid climate)  type fluids, not to higher concentration Na-SO4 type fluids discharging from semi-confined aquifers.

Преобладание этих ионов типично для зоны континентального засоления. Уже на широте южной Украины это общее правило. Если вода имеет минерализацию более 1,5 г/дм3, то это обычная картина. кроме того, источником натрия может быть ионный обмен с кальцием при метаморфизации атмосферных вод в  отложениях морского происхождения.

Dear Dr. Valeriy

Sorry, I don't know any Russian and google translate is not effective enough. Anyway thank you for the comment.

Dear Dr. Gorody,

I applied the charge balance for the samples,  CO3 is zero and HCO3 is around 0.01 meq/l other ions are normal. After balance only a great amount of Na and SO4 remains, I am looking for it's source. Water type is Na- SO4 according to the Piper diagram.

Dear Dr. Missoni,

I don't know much about biological processes involved in ion concentration. Please introduce me the resources which can help me to have a vision on the subject.

Kind regards

QED Mr. Safari, your water sample is principally derived from groundwater discharge. Look at the red soils and red colored weathering in the area...lots of iron.  I doubt all the sulfur is derived from organics. If you wish to address the source of sulfate, analyze the stable isotope ratios of sulfur.