Contamination can come from a variety of sources on- and off-site. This is a basic knowledge-based question, though you will want to ensure that your response suits the area in which you're applying. Your interviewers will appreciate your demonstration of local knowledge beyond answering generically.

For xample: "There are many sources of contamination to consider in water quality testing. For the purposes of this question, I'll share three potential sources of contamination in this region. Given the number of farms in the area, I would consider local land-use practices as a potential source for contamination; this can include pesticides, fertilizers, or even livestock. We also have two nearby factories, so I would also consider manufacturing processes to potentially contribute to water contamination in this area. Of course, a third source of contamination could simply come from naturally occurring chemicals and minerals, such as arsenic or radon."