PE Discipline: Civil or Environmental

In my jurisdiction, PE licenses are granted for those passing exams in general, broad disciplines....civil, mechanical, electrical, etc.
My seal just has my name, license number, and state. It makes no reference to a discipline, nor any niche area within a discipline.
The engineer is left to use his/her discretion to practice only in areas they are competent in. Now, you are free to take whatever afternoon exam you are most comfortable with in order to pass the exam. For example, I took the structural afternoon exam because that was my area of expertise. But by passing that exam, I am simply a PE.

MotorCity nailed it, once you pass an exam, a PE is a PE, the test taken has little meaning as far as I have seen. The reason for multiple different afternoon exams is to try and tailor the exam more closely to the test takers area of study (structural, geotechnical, water resources, etc), taking and passing the geotechnical exam does not mean that your PE title is only good for geotechnical work, if you have professional skillsets in structural design, you can stamp structural plans even though you passed the geotechnical exam.

You cannot legally or ethically stamp or sign documents in a discipline in which you are not competent. Some jurisdictions restrict you to the discipline listed on your PE certificate.

bimr has given the best answer. If you're not currently working (and learning) in the environmental engineering field, you can't become an Environmental PE. Your state board would likely reject your application to take the environmental exam if all of your experience is in utilities and mines.

It's true that most states don't differentiate on the stamp. I don't know of any place that would give you an environmental stamp. Some places have structural licenses, and one or two put the discipline on the PE stamp (civil, mechanical, electrical, etc.), but I haven't seen environmental. It's usually lumped in with Civil in the few places that overtly differentiate on the stamp/license. They just give a separate exam because NCEES recognized the rather large difference between stream remediation and site design. It's up to the engineer holding the license to use it ethically, and if you use it outside of your area of expertise there can be significant ramifications (fines, loss of license, etc.).