Blue billy is a chemical or mineral deposit often encountered in contaminated land.
The name is a reference to its distinctive bright blue colour, which can make it immediately obvious in other mud or soil. Chemically it is a form of Prussian blue. Although several chemical industry processes could produce it, it is particularly associated with coal gasification and older town gasworks. It has been recognised as a problem effluent and land contaminant since the early industrial period and the first gasworks at the start of the 19th century.
As a visible proof of contamination, blue billy is an important signifier in contaminated land remediation projects and its discovery, especially if undisclosed, may be a cause of contract disputes.