"(...) The one participant who answered yes said the evidence
s/he had come across was ‘high levels of atm[ospheric]
barium in a remote area with standard ‘low’ soil
barium’. (...)"
http://iopscience.iop.org/article/10.1088/1748-9326/11/8/084011/meta;jsessionid=81C700692BB4F708F0E3F79B15942753.c4.iopscience.cld.iop.org page 3

Source: http://www.sauberer-himmel.de/2016/...-existenz-der-chemtrails-bewiesen-haben/