The interviewers are interested in your personal motivation(s) to enter this profession, so be honest and authentic with your response. Whether you are interested in conservation, sustainability, public health, or something else, always remember to project your excitement and enthusiasm for the position.
"For as long as I can remember, I have always been drawn to water. I love how its taste varies, I love how peaceful a body of water can be, and of course I appreciate it's immense benefits to life on Earth. I think I started to consider water as a career when I was in high school and started learning more about sustainability and water supply. When I began pursuing this career option in college, I learned of the many impacts water quality also has on public health. It is difficult to attribute my motivation to any one of those things, but I am certainly inspired by the profession and passionate about contributing to it."