Alright, I got this idea from another game I play that has percentages. The percent complete for each achievement appears each time you do something to get you a step closer to the achievement. So, maybe when you reach 1000 trash picked up, you would get a little notification that says "Keepin' It Clean- 50%" or something similar, so you can keep track of where you stand. Now obviously, if they appeared too frequently it would become annoying, so maybe they would only appear as you reached 25, 50 and 75% or similar increments.
Here are a couple (terribly photoshopped) concept photos-