What happens to rain after it falls to the ground?

Once upon a time, an old tree, a smart frog, and a friendly fish lived in a stream. The water in the stream was as blue as the sky and clear as a crystal.

One day, a school was built next to the stream.

The animals loved the kids, and the kids loved the stream, but the parking lot was causing some problems.

One day, after a huge storm, the frog and fish noticed something weird about the water. Their home wasn't blue and clear anymore. It was brown and green. The frog tried to swim, but he felt sick. The fish tried to swim as well, but he couldn't see.

What happened?

The tree's leaves began to die. The tree began to feel sickly and older. The water his roots drank didn't taste the same. What happened?

The tree, fish, and frog got together and the frog decided to go search the school's parking lot. She found that the parking lot is full of nasty things. Car oil, fertilizers, pesticides, and grease. When it rained, all those things would runoff into the stream. The stream wasn't just water anymore. It had car oil, fertilizer, pesticides, and grease in it.