Man has been polluting Earth since Middle Palaeolithic: Heavy Metal Contamination caused by Neanderthal fires dates back at least 450,000 years (Sarah GRIFFITHS, MailOnline/Daily Mail UK; September 22, 2015)
Oldest Traces of Heavy Metal Pollution Uncovered (Agence France Presse; September 21, 2015)
http://www.afp.com/en/news/oldest-traces-heavy-metal-pollution-caused-humans-uncovered
Oldest Evidence of Manmade Lead Pollution Found in Michigan
(James A. FOLEY, Nature World News; June 11, 2013)
http://www.natureworldnews.com/articles/2391/20130611/oldest-evidence-man-made-lead-pollution-found-michigan.htm
Research Articles:
Earliest Evidence of Pollution by Heavy Metals in Archaeological Sites (MONGE et al., 2015)
http://www.nature.com/articles/srep14252
Lake Sediments Record Prehistoric Lead Pollution Related to Early Copper Production in North America (POMPEANI et al., 2013)
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.442.2413&rep=rep1&type=pdf
C. Mei-Ling HAMILTON - Environmental Geochemistry Issues 