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Context: (Not necessary to answer the question)

For an assignment I am currently testing two samples of water. One is a pre-contamination sample and the other is a post-contaimnation sample. Between tests they are stored in small white slightly transparent water tanks which are exposed to moderate to small amounts of sunlight.

Using a nitrate ion selective electrode (probe), it was found that the pre-contamination sample had nitrate levels of around 3mg/L whereas the post-contamination sample had above 2000mg/L of nitrate.

Initial tests were done 4 days after collection. Water quality tests advise these tests be done as soon as possible as they will become inaccurate as time goes on.

Due to concerns that our probe wasn't working properly based on peer results on similar pre-samples (their pre sample readings for nitrate was around 15mg/L), another test was done around 24 days after with another probe.

In the second test the pre-contamination sample amount was 6mg/L of nitrate and the post-contamination sample was above 2400mg/L of nitrate.

Stoichiometry involving TDS tests, assuming the contaminant is sodium nitrate would indicate nitrate levels of around 14000mg/L, which like our peer's is higher than our readings. I'm worried our initial results were lower than they should be.

My reasoning is that if nitrate in water decreases over time, then the nitrate levels closer to the collection of the date would have to be higher than the second post sample's 2400mg/L which means the initial reading was incorrect.

Otherwise, if the nitrate level in water increases, then this indicates the the initial reading of 2000mg/L for the post wasn't necessarily incorrect as the amount of nitrate is expected to increase in the second test as it did.

Question

My main question is, how do nitrate levels in a relatively controlled environment such as in the described water tank, change over time? Additionally, what causes these changes to occur?

Water samples are stored in small white slightly transparent water tanks which are exposed to moderate to small amounts of sunlight.

If it's helpful information it was observed that the post-contamination sample began to grow algae as time went on.

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  • $\begingroup$ Even only your last sentence explains many things, though 2000 mg/l or similar values are really high concentrations, with enough oxygen and necessary organic or ammonium/nitrite nitrogen, nitrate levels of course increases (nitrification), providing you also have nitrosomonas and similar bacteria. There is an ongoing research on nitrate fate in general, yet I refer you to Nitrogen Biogeochemistry related textbooks for detailed information. $\endgroup$ – Güray Hatipoğlu May 24 '18 at 6:52
  • $\begingroup$ What is the upper limit on the ISE? Is it calibrated at those high concentrations? What is the source water? Is it artificial (just sodium nitrate and DI)? $\endgroup$ – prof.kvothe May 24 '18 at 11:58
  • $\begingroup$ @prof.kvothe The upper limit of the ISE is 14,000mg/L. The source of water is from the hinze dam and afterwards an unknown contaminant (one compound) was added to the water. $\endgroup$ – cjeccjec May 25 '18 at 2:24
  • $\begingroup$ @cjeccjec sounds like something is wrong with you ISE. You need to make a calibration curve and include QA/QC controls in your samples. N will be consumed by microorganisms, I doubt you will be seeing increased levels over time $\endgroup$ – prof.kvothe May 25 '18 at 21:01
  • $\begingroup$ the Nitrate ISE was calibrated with a 1mg/L standard and a 100mg/L standard, the second time round the calibration was performed with special care as a dodgy calibration is what I believe caused errors in the first tests. $\endgroup$ – cjeccjec May 26 '18 at 5:11

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