Comprehensive waste characterization and organic pollution co-occurrence in a Hg and As mining and metallurgy brownfield

•Complex legacy of contamination afflicts As–Hg brownfields.•As- and Hg-rich waste analyzed in a paradigmatic study site.•Co-ocurrence of a complex speciation of As and Hg, and organic pollution (PAHs).•Arsenolite was determined to be the main source of risk at the site.•Unexpected Hg organo-compounds found.