A striking lede:

New research from the National Oceanic and Atmospheric Administration (NOAA) finds that all the impervious surfaces-buildings, roads, parking lots, and roofs-in the continental United States cover an area nearly the size of Ohio.

I have a love-hate relationship with this sort of fact: it’s shocking, but I don’t quite know what to make of it. Is Ohio a lot of impervious surface, or just a little given the size of the country?

But what really interests me about the article is the pointed questions it raises about “smart growth”—particularly, whether increasing the density of new development is really better for water quality.

  • Some context is in order: impervious surface is generally bad for stream, rivers, and other water bodies. Concrete and asphalt increase runoff volumes and decrease natural filtration—meaning that the water that reaches streams is dirtier and comes in bigger bursts than it ordinarily would. And it doesn’t take a lot of pavement to hurt a stream: within a watershed, if one acre in ten is paved, stream quality declines, and once one in four is paved, streams often can’t even maintain basic channel stability or fish habitat.

    In general, urban areas have substantially less pavement per resident than do sparsely populated suburbs: the pavement is concentrated, but overall there’s less of it. “New urbanist” developments try to replicate those features, by concentrating amenities, workplaces, and homes in pedestrian-friendly developments. In theory, then, concentrated growth can have lower overall impacts on the health of water bodies than sprawling suburbs.

    But that’s the theory. In practice the specifics really matter: a new urbanist development that’s in the wrong place can have more of an impact than a sparser development that’s done just right. This should be obvious enough, and I’m sure that smart growth advocates and practitioners would agree.

    But given that this issue is being raised, what we can probably expect to see at some point is an example of a “smart growth” development that really hurts water quality. And then some enterprising reporter or think tank will blow it up into a story of how smart growth has failed.

    The question about the link between smart growth and water quality is definitely worth asking—but it’s also worth answering right.