translate English to French: However, a survey conducted between 1987 and 1990 found that 10% of the agricultural land in monoculture had elevated levels of heavy metals.
Cependant, lors d’une enquête menée entre 1987 et 1990, on a constaté que 10 % des terres agricoles consacrées à la monoculture affichaient des taux élevés de métaux lourds.