A friend of mine applied to Columbia's M.S. program and was accepted. She double majored at her undergrad institution in environmental studies and creative writing with a GPA of 3.8. She also had numerous honors and did an internship with the New York Times. However, she ended up declining Columbia's offer and instead is attending Medill at Northwestern. I would say you have a fairly good chance of getting in, but you might also want to look at other programs, including NYU's M.A. program with the option for an Advanced Certificate in Science, Health and Environmental Reporting. NYU is also a very prestigious journalism school.
Also, this is directly from the National Geographic website:
Vacancies on our editorial staff are usually filled by men and women who have had quite a few years of experience in the newspaper or magazine publishing fields. In preparation for such work, many of our writers have made a thorough study of the sciences, journalism, and English, and in some cases know several languages. A solid academic background and a proven record of success in one's chosen field are most important. However, because we cannot know a person's particular talents nor assure employment after a completed course, we do not encourage gearing a career or educational program specifically toward employment by the National Geographic Society.