Contamination of drinking water in areas close to several fracking sites in the Marcellus and Barnett Shales, USA, is caused by structural problems leading to wells leaking natural gas into aquifers, a new study suggests. The researchers measured trace amounts of chemicals, called noble gases, which formed signatures of the sources of gas contamination in over 130 samples. These findings suggest that, rather than coming from natural sources, such contamination in the area of the investigation is an engineering problem to do with the wells themselves, the researchers say. They stress the importance of ensuring the structural integrity of the wells, which can be done in an affordable manner.