Sidewalk Pits

Last time it rained I noticed water was draining off of a sidewalk awning, onto the sidewalk - and apparently forming pits in the sidewalk. WTF?



Acidic precipitation, while not unknown in California, is rather unusual, mostly because there is usually enough alkaline calcium carbonate-bearing dust around to neutralize the acid present. (That's the missing ingredient on the East Coast, so that's why it's a problem there.)

Nevertheless, these pits bother me, because they suggest acid is present anyway. So, maybe the acid isn't in the rainwater, but in the runoff instead.

Even when it's not raining, acidic materials can still dry deposit onto surfaces during the long intervals between storms. That's the source of much of the gunk we see on cars that haven't been washed in a while. That gunk falls on rooftops and sidewalk awnings too, just waiting to get washed into the gutter by the next storm.

Wow, intense enough to form pits.....