Hi Sangunathan,

Did you see bubbles coming out of your sample? What temperature was your sample? As the temperature increases, DO would bubble out of solution if it were 15 mg/L, and generally water only holds 15 mg/L when the water is fully saturated with DO at near-freezing temperatures or if abundant photosynthesis is going on in the water, not likely the case though in groundwater. What type of instrument were you using or were you doing Winkler titration? One quick way of just testing your instrument if you are using a probe would be to check your water temperature, shake up your sample to create bubbles, put a lid mostly over the sample, then measure the DO and see what result you get. The result should be close to 100% saturation on the attached figure, and if it dramatically exceeds 100%, your instrument may be out of calibration. http://www.uky.edu/WaterResources/FF/Nutrient%20Management/images/q08.gif  

Dear Sangunathan,

15 mg / l you indicated is very surprising. Groundwater does not contain dissolved oxygen or very little.

With my best regards

Prof. Bachir ACHOUR

Mostly Dissolved oxygen will be very less. I think u just check the result after calibrating your instrument. U may also try for 5 day BOD which will help u to find the nearer value of accurate result....

This is almost attributed to the intensive existence of fissure systems and karstic conduits which open ventilates with air.

Two things have a dominant control on dissolved oxygen, aquifer (screened interval) depth and well dynamics.  If the aquifer (screened interval) is shallow, e.g. an alluvial aquifer, you will be sampling in a relatively oxic environment.  Did you measure oxygen from a monitor well or from an domestic water well?  A domestic water well tapping groundwater can have elevated DO if the screened interval is near the air-water interface. Shallow well venturi type pumps could entrain more oxygen than submersible pumps in deeper wells. A well with a high rate pump will allow native groundwater to mix with water from the air-water interface.  Same is true for a well with a high working volume relative to the volume of static water in the well.   You should go beyond DO and look at both Ar and N2 to improve your interpretation.  Here is one reference with a helpful bibliography:

Ramesh Kumar, A., and P. Riyazuddin, 2012, Seasonal variation of redox species and redox potentials in shallow groundwater: A comparison of measured and calculated redox potentials: Journal of Hydrology, v. 444–445, p. 187–198, doi: 10.1016/j.jhydrol.2012.04.018.

Dear Sangunathan Ulaganathan

What was the temperature? I think surely the measuring is wrong. Maximum DO in near to zero in surface water could be about 14.4 mg/l, therefore the result is not logic

Dear,

- Calibration of the instrument ?

- Treatment of the water with oxydant (Na hypochoirit or other ?) directly in wells ?

- In karstic media with more than X100 m between spring and feedind zone, with very fast vertical flow, temperature present negative anomaly and OD positive anomaly at the spring.

Всегда - атмосферный кислород. Если его содержание велико, значит вы имеете дело с быстрым поступлением атмосферных вод, малой мощностью и высокой проницаемостью грунтов зоны аэрации, низким содержанием органических веществ.

The concentration of oxygen in groundwater in equilibrium with the air depends on the temperature. As stated by Seyed Hadi the highest concentration is reached at the lowest temperature, but even at 0 deg.C will not reach 15 mg/l. Only when the pressure is higher than atmospheric, a higher oxygen concentration can be reached.

Your results seem unlikely. Possible explanations that I can think of:

- the measured concentration is not reliable (e.g. air bubbles disturbing the measurements)

- a change in the oxygen concentration between the aquifer and the sample (air bubbles entering the sample)

- elevated oxygen concentration due to some kind of chemical treatment

- a significant rise of the groundwater table that resulted in trapped air bubbles and dissolution of these bubbles at a higher pressure

- air bubbles that have been dragged to depth during infiltration of the water

- air lifting during drilling or sampling (a compressor is used to inject air below the water table)

При бурении скважин используют воду из других источников. Обычно это речные или водопроводные воды, которые имеют повышенное содержание кислорода. Если бурение шло долго, то могла образоваться интрузия. Другая возможная причина - попадание в грунтовые воды свежей дождевой воды по каналу снаружи скважинных труб при плохой их изоляции.

Dear all,

    Thanks for your valuable suggestion , i will confirm that the actual value during next sampling.