Explain the potential sources of error in sampling water and describe how to deal with errors?

Question

Explain the potential sources of error in sampling water and describe how to deal with errors?

Answer 1

This is an interesting question - why do you need it answered? There are entire books written on water sampling discussing all of the problems with it. I'll go with the three problems I think are the most important, although there are many more:

The first problem has to be in the analytical tools you are using. If they are not calibrated correctly, they will give you poor data. A machine that is not functioning, for instance, may indicate that a sample is clean when in fact it is not. The current solution to avoid this problem is to calibrate your analytical instruments before and after each sampling event. You calibrate them before the event to make sure they are functioning correctly, and then you calibrate them after to make sure that they are still functioning correctly. If the final calibration is off, then your data is suspect, and you note that in the results.

Your next problem will be contamination of the sample. This typically occurs during collection, but can also occur during analysis. Let's say that you are sampling a lake with a dog dish. When you analyze it, all of a sudden you find all kinds of bacteria in it. But do you know that the water in the lake had that bacteria, or did it come from the dog dish? To solve this problem we use sampling containers that 1) are only made out of a certain substance that we know we can clean, and 2) are "properly" decontaminated before use. The meaning of properly decontaminated varies by analyte. Samples can also be contaminated by rainwater, for this reason it is usually prohibited to sample during the rain.

A third problem will be the change of the sample over time. Hydrocarbons, for instance, tend to break up over time. If you collected a sample of something that was contaminated with a hydrocarbon, then you analyzed it six months after you sampled it, your result would be less then what it should be. To solve for this problem, every sample has a specific maximum hold time, and most samples are required to be placed on ice before analysis in order to slow chemical and biological degradation of the analytes.

That gives you three sources of error in sampling water, but there are many more. The EPA has a big fat book on the subject called "SW-846". You can find it here:

http://www.epa.gov/sw-846/main.htm

Most states also have sampling requirements, Florida's are here:

http://www.dep.state.fl.us/labs/qa/sops....

The extent of these rules is why we have environmental professionals. If you graduate with a degree in Environmental anything, expect to have to read at least the EPA rules.