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.
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.