I'm assuming by "environmental science" and "environmental consultant" you're referring to a career closer to an engineer than a hard scientist (as studentdoc2 assumed); site remediation, energy consultant for building owners, water resources etc. If I'm wrong about that, please disregard everything I have to say.
If you're near a school that has a decent program and has night classes, getting a masters part-time and having your employer paying for it is a slam dunk. I'm a civil engineer (so, not a totally different planet than you), and went that route. I did it one-class-a-semester and started up a year after I graduated/started working. It's definitely true that going back to school gets harder, but it's not a cliff; after a year or two it's no problem. Where it starts to get tough is if you get married, or have a kid. It's really about whether you have people waiting at home for you.
In my field, I have found getting a masters to be helpful and relatively painless. Grad classes, especially night classes, are a lot different than undergrad. The students are grown-ups with jobs who are there because they want to learn something, not because they need the credits. The coursework was a lot more practical (at least in my program). The professors generally worked (either actively or recently) in the industry. Having the degree really did help when I applied for a new job, and is a tangible point in your favor for salary negotiation.
All that said, it makes absolutely no sense if there's a chance you'll decide to leave this field, so I'd think hard about your last sentence, lest you waste a whole lot of time pursuing a degree you won't end up using.