Word your environment essay for your audience. if you're writing the essay as part of a school unit on the environment, you can assume your reader already knows what you're talking about. Don't waste space explaining simple concepts that you have already gone over in class. For example, if your class has spent a unit studying aquifers, you don't have to explain what the term "aquifer" means or the basic characteristics of aquifers -- your class and your teacher already know.
If you are writing for an audience that may not know as much about the environment as you do, however, you should give more background information. For example, if you write a letter into the local newspaper asking for your community to protect its aquifers, explain what aquifers are, what the history of your local ones is and why they need protection.