[SimoneMeyer]

How is it possible that the amount of lead initially found is different from the amount of lead found later?

In the month of August, a school's fountain was found to have unacceptable levels of lead. But, when re-checked later on in the year, the levels of lead changed, they were now inferior to the maximum level of lead tolerated.

Take into consideration that the school was not used between the first and second check and that no canalization was replaced.

* Does this have to do with dilution of the lead or even the concentration? I actually don't have any plausible ideas or even guesses for this one. How could the lead levels have changed?

Thanks

[marquis]

If I understand the data correctly, the lead level decreased with time.  Again, it appears to have decreased to a level where lead remediation was no longer required.

Please pardon the suspicion, but this is very "convenient".  Lead remediation can be expensive.  If the original sample is available, retesting the original sample by another lab would probably be wise.  Look out if the original sample was "lost" (again, this would be very convenient).

As for how the level can change, a different sample from a slightly different location could easily have lower levels.  If one sample was nearer to a lead based solder joint, for example.  How was the second sample pulled and what were the differences?

There are many other variations (external contamination, dirty glassware in testing, etc.).  Starting with the sample location is probably best.

[SimoneMeyer]

I don't think that the answer would have to do with samples being lost, because it would then turn into a trick question (this is for my Chemistry course and is not an actual occurrence, it's merely an example).

Is there any other way (more straight forward not having to do with testing material) that could have led to the decrease?

[Lonn]

Lead can be released when water sits in the pipes mainly old ones but your problem clearly states "Take into consideration that the school was not used between the first and second check and that no canalization was replaced."

Maybe the first time they checked it hadn't been used in many many years. So the second time it had less.

[SimoneMeyer]

They probably would have stated that it had been used for the first time in years... But thank you both for trying!

I hate these questions that make me guess what's on the teacher's mind...

[Arctic-Nation]

A possibility might be that lead precipitates out in some way as other metals are being oxidized into the water... or not...