I'm not sure how to verify my answer to this one. Assuming that the water doesn't get into the storm water sewer system, I would think that the water makes its way into the soil and then...
2) contribute to the soil's moisture
3) be used up by a plant;
4) collect in an aquifer somewhere below; or
5) make its way to the Detroit River
Its another one of those questions that is easy to answer with a principle ("the ground water cycle") but very hard to answer with something specific ("water in my neighbourhood collects underground and travels out via the stream").