When de-icing takes place there is a problem of contamination with the glycol run off, it is part of the conservancy measures. There are various articles available on a google search of "Heathrow balancing reservoir".

Google:
http://www.heathrowairport.com/about...ironment/water

http://books.google.co.uk/books?id=b...athrow&f=false

http://books.google.co.uk/books?id=v...=0CDoQ6AEwAzgK

There is a nice Civil Engineering article here:
http://www.nce.co.uk/breath-of-fresh-air/704902.article