dumping ground vs slag heap

dumping ground

noun
  • A landfill. 

  • Any place where unwanted items are sent. 

  • A presumably polyphyletic supraspecific taxon where species of unclear affinities are placed. 

slag heap

noun
  • A man-made mound or heap formed with the waste material (slag) as a by-product of coal mining. Can also refer to the waste by-product from a foundry or furnace, formed into such a mound. 

How often have the words dumping ground and slag heap occurred in a corpus of books? (source: Google Ngram Viewer )