I did this study for my AP Enivronmental Science class senior year. I studied the impact of a ski resort on surrounding water systems. I collected data about water quality from three different sites. One was the snowmaking pond, one was a river which runs right below the resort, and the third site was a major wetland across the interstate from the resort. The data was collected before and after the ski season. I wanted to see if the run off from the ski runs effected the surrounding water and if it did I would be able to see if it was effecting something as far away as the wetland. This was an easy thing to reseach because it was something which interested me.
My recommendation is just do something that interests you. Think of something you like doing and then look at how it impacts or could impact the environment. Hope my example helps!