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RE: Copper in Roof Runoff

Hi Tom,

We are currently conducting a project for the City of Los Angeles related to
Stormwater BMPs.  One area of focus is commercial buildings.  It is
interesting to learn that while their individual pollutant load is so much
less than an industrial facility, they can have a sizable impact because of
their large number. I also believe that some loads, such as copper leaching
from roofs or preservatives leaching from wood have been largely overlooked.
These may have been minor issues in the past because of what else was going
on, but now they should be revisited.

As for copper roofs, they can last for hundreds of years.  "The Weathering
and Performance of Building Materials" by Simpson and Horrobin (1970) quotes
several studies of copper roofs that reported corrosion rates of 0.0006 to
0.0025 mm/yr.  The mean value was 0.0013 mm/yr. These rates are for an urban
industrial setting.

If the corrosion products are not disturbed, the corrosion rate will
eventually cease.  They report that corrosion tends to cease after 70 years
of exposure.  If the building owners remove the patina, then the rate of
corrosion will be re-established.

Copper roofs should be installed with copper or brass fittings.  Rainwater
should not be allowed to run over a copper roof and then over aluminum or
zinc.  The copper in the water will cause corrosion of these materials. The
book also states that the runoff from cedar wood should not be allowed to
run over copper. While not fully explained, I believe the tannins and acids
extracted from the wood will cause dissolution of the copper and staining.

As for concentration, no data is available.  That will depend on how long
the water remains in contact with the roof. One book I have on corrosion
states that water held for 9 hours in a 1 year old copper pipe had 1.4 to
5.0 mg/l of copper.  In piping where the corrosion film was complete, the
resulting level was 0.1 to 0.3 mg/l.  All of these levels drop to near zero
once the water starts flowing and the piping is purged.

As for other sources of leaching data, I would contact both copper roofing
and piping manufacturers.  A water corrosion specialist should be able to
help. Corrosion tests are commonly performed with flat metal coupons.
Perhaps this data can be converted to mg/l of copper per square foot of
copper per inch of rain.  Good luck.

> ----------
> From: 	Thomas Barron[SMTP:tsbarron@ibm.net]
> Reply To: 	Thomas Barron
> Sent: 	Sunday, October 24, 1999 4:46 PM
> To: 	P2tech@great-lakes.net
> Subject: 	Copper in Roof Runoff
> P2Techs:
> I'm involved in a study of heavy metals contained in runoff from
> buildings that have copper roofs, flashing, gutters, or downspouts.  The
> effort is being done for the Palo Alto Regional Water Quality Control
> Plant at the south end of San Francisco Bay.  
> Have you seen published reports or do you know of unpublished studies
> that have data on copper in runoff from buildings?  If so, can you send
> us the internet address, author's contact info, or a literature cite?
> Thanks in advance for your help.
> Regards,
> Thomas Barron, PE
> Professional Engineer
> 3351 Beechwood Drive
> Lafayette, CA 94549
> (925) 283-8121