If you are uncertain, I would say environmental engineering. You will have far better job opportunities with an engineering degree than a science degree. To actually work as a scientist, you typically need at least a masters, which you can still get after doing engineering anyway -- your MS does not have to be the same field that you got your BS in. But at least if you study engineering, you will have excellent job prospects even if you don't end up getting an advanced degree. A lot can happen in the next four years -- marriage, etc. -- that may make you not want to continue on to grad school.

I graduated with a degree in environmental engineering, and while I liked it, I now work as a software engineer. I know others who went into civil, or mechanical. Engineering is engineering to many employers -- but of course you will be most qualified to do what an environmental engineer does: Think air pollution control systems, wastewater treatment, drinking water treatment, groundwater and soil remediation, and solid/hazardous waste management. It's basically applying engineering principals to reduce humanity's impact on the environment, and in so doing, helping everyone be healthier.

I know know, why you tell me? You should know the difference by now