In my 42 years as a Civil Engineer, working almost exclusively for large consulting firms, my experience was somewhat different from that of Mr. Windeler (above). The Environmental Engineers that I encountered frequently had masters degrees. This is a complex field and such an advanced degree certainly enhances your career. It will definitely accelerate your salary, although I don't know if this will actually "pay" for the extra 12-18 months of graduate school. But if you want the best opportunities in this Field, I do not hesitate to recommend you get a masters degree.
It is common for engineers, a bit later in their careers (perhaps five years), to obtain an MBA and this would enhance your opportunity for project management and perhaps major partnership in an engineering firm.
Good luck, Pete Sturtevant, PE