Katie - this is a really cool idea! It could also be interesting to compare these two types of plastic bottles to an aluminum water bottle and a glass bottle. That could give an idea of which type of reusable bottle might affect the water the least. One thing you could test for would be BPAs, see this link from Mayo Clinic.

It's also possible that trace amounts of aluminum could get into the water, so you could check for metal ions. Aluminum I don't think is a health risk as much as some other metals are, but could be interesting to check anyway. I'm not super familiar with what tools you have available to you to test the water, but at UCSB I might suggest using ICP - inductively coupled plasma The tool we have at UCSB in the MRL works on liquid samples. Maybe you could ask if anyone could help you run some of your samples! I think this would detect Al, but I'm not 100% sure it would detect BPAs, worth looking in to.

I'm not sure. Typically, in science, you think of a question you could answer with the available materials, and then design the experiment.

I might consider trying to measure contaminants, e.g. measure the density of the water after being in the bottles for several days, and see if different types of plastic leak different amounts of contaminants.

None of the water bottles should make the water contaminated. We wouldn't use the bottles for water if they contaminated the water.