Passage Summary: Rainwater picks up oil and dirt as it washes over city streets, making it a bigger polluter than factories; therefore, water is essentially polluting itself.

Conclusion: Water itself is one of the primary sources of water pollution.

Reasoning: Rainwater runoff picks up pollutants from urban surfaces, and in many areas, this contributes more contamination to bodies of water than industrial waste does.

Analysis: The statement in question is a premise that provides a factual comparison to support the final conclusion. It establishes the magnitude of the problem by showing that runoff is actually worse than industrial discharge. I categorized it as a premise because it provides the 'data' that the author uses to justify the somewhat paradoxical claim that water is a polluter. It isn't the main point, but rather the evidence that makes the main point plausible.