Science Buddies: "Ask an Expert"

I am doing a science project on Tap water content, impurities and the differences between tap water sourced from different regions. I am planning on having several samples collected from each region on the island and delivered to my school. I would like an expert's help on the Controlled variables needed for my project, as I am unsure about what is necessary to be controlled.

I have already identified these variables but not sure what I should set for them:

• The amount of water collected from each tap (this should depend on how much is needed to test the sample)
  The number of samples from each region

I am not sure if these things need to be kept the same for each sample (ie if it will change the result or not)

• The container or bottle used to collect the sample
  The conditions while the container or bottle is being transported (temperature etc.)
  The period of time between collecting the sample and testing it
  The period of time between turning on the tap and collecting a sample
  The type of tap and whether there is an aerator
  The time of day the sample is collected
  The temperature of the water out of the tap

If any of the above things do not matter and I will not need to worry about them, then please tell me.

Once I am sure about the controlled variables, I will be working on the independent variables (the locations of the taps I am collecting my samples from) and the Hypothesis, and then researching the method of testing my water samples. If someone could give me some advice on this as well that would be appreciated.

PS. There are more details of my project on the right

IMO: You are thinking about this backwards. Whether any of these or other factors are important to your experiment are probably influenced by what impurities you are testing for.

Testing for impurities in the parts per billion requires a lot of expensive equipment and is as much an art as it is a science in terms of validating that you aren't contaminating your samples, containers, and/or equipment.

You should first consider what impurities are there in water sources that are high enough concentrations and simple enough to test for that will allow you to obtain some valid results. If any of your sources are municipal water sources, there should be some public data on impurity levels, especially for things that are not harmful. If any of your sources are potable well water, surrounding municipalities that use wells will also have impurity levels for their untreated water sources and you can expect private wells to have the same impurities at similar levels.

As the previous respondent says, the first thing to consider is what are you testing for? Metals, anions, or organic compounds, like pesticides, may be the "analytes." Different sampling techniques are required for each. In general, the temperature of the water when the sample is taken and degree of aertion are not important. Different sample containers are required depending on what is tested, and a means of sample stabilization may be needed. For example, for measuring the metals (Pb, Cd, etc) in water, it is common to mildly acidify the sample. Other analytes may require prompt measurement. East Bay Municipal Utility District (EBMUD) sends out a nice summary of what they test for in the waters from the various reservoirs in the East Bay. Other water companies probably do the same. You might contact one of those companies to get an idea of what is measured.