Proper Way to Take a Water Sample

I have mentioned in many other articles that monitoring water quality is important. The only way to effectively monitor changes in water quality is by sampling the water and following with laboratory analysis. This can all be done by you or sent off to a professional lab. There are companies that can and will come sample the water for you and arrange for a proper analysis of the sample.

If you do decide to take your own samples, here is what you will need to do.
First we need to cover some terminology. This list is far from complete, but it will give you the basics.
Grab Sample: A sample of one location and at one point in time.  These are the simplest to take, but also leave you open to missing a potential problem by sampling in the wrong place or at the wrong time.
Composite Sample: A sample composed of many separate samples taken from either different places at the same time or from the same place at different times.
Representative Sample: A sample which accurately reflects the composition in time AND space of the thing being sampled. Ideally this is what you want with water sample.  Representative samples can be either grab or composite.

Water bodies are dynamic and are constantly changing. I could write an entire blog dedicated to it alone. As a result there are significant differences throughout every body of water. Time of day, time of year both change the compositions of water bodies this is due to human activities and natural processes both organic and physical.
Location is also important. Location on the earth and location within the water body affect what will be present in a water sample. Location on earth is self-explanatory, things change the farther apart they get and water is no exception. The location within a body of water is less obvious. The closer you are to a source of pollution the more contaminated the sample will be. The depth also needs to be considered. Some things sink and others float and others still get dissolved, this will vary significantly with depth.

The next thing to do is find a way to analyze your sample. If you have the equipment and know how, there is no reason why you can’t analyze the yourself. For those who can’t that leaves laboratories. Choose one and ask the for bottles. Different parameters sometimes need different and speciality bottles size, shape, material all play a part. Follow the lab instructions and fill the bottles accordingly, some need an bit of air in the bottle others need zero air, many need a preservative added to the bottle. Many labs will not accept bottles not prepared by them. Like I said, follow the lab instructions, including paperwork and proper labeling.

The most common thing sampled for in drinking water is bacteria. E. coli and total coliforms are easy to test for and are considered indicator organisms, if they are present it is assumed that other pathogens are present. Sometimes people monitor the heterotrophic plate count (HPC). Heterotrophic bacteria are naturally occurring in treated water, they are good indicators of secondary disinfection residuals and are not pathogenic. They will also be in the natural source water. Changes in HPC indicate a change in water quality that could lead to contamination.
To take a bacteriological sample you need to first wash/disinfect all the sampling equipment including your hands. You don’t want a false positive from having poor technique.
Next you disinfect the faucet or spigot you will be sampling from. Avoid sampling from filters, hoses, and faucets with strainers or aerators as these can add bacteria to the water and give another false positive.
Third, you run the water to flush the immediate line you are sampling from an get fresh water.
Fourth, open the bottle. The bottle should have been sterilized and sealed by the lab. If it looks like it has been opened or hasn’t sealed properly, then use a different bottle. DO NOT touch the inside of the bottle or lid with anything or to anything. DO NOT put the lid or bottle down.
Fill the bottle to the correct level and reseal the bottle. DO NOT rinse the bottle before filling it, there is usually a dechlorinating agent (sodium thiosulfate) in the bottle either liquid or powder, it needs to be there.
Finally deliver or ship the sample to the laboratory. The sample needs to be temperature controlled. It needs to be kept under ten degrees Celsius (fifty degrees Fahrenheit), but still above freezing. Also the sample needs to be delivered within 48 hours of sampling. Samples received outside this window will be rejected.

The lab will then plate the sample and incubate it for twenty four and forty-eight hours then the count the number of colony forming units (cfu). For drinking water, the standard is zero for e. coli and total coliforms. With proper maintenance of your water supply and reserves these number can stay at zero, which is just another reason to stay responsible for your own water supply.


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