Metal Pollution From Medieval Mining Persists

Scientists have tracked down the source of high levels of heavy metal pollution at a site in southern France: medieval metallurgical workshops. Sandrine Baron, Jean Carignan, and Alain Ploquin of the Center for Petrographic and Geochemical Research in Nancy, France, report that the lead, antimony, arsenic, copper, and zinc residues left behind there more than 800 years ago still pose a potential health threat (Environ. Sci. Technol., DOI: 10.1021/es0606430). Mont-Lozère Massif, the site of medieval workshop remnants that they studied, is in the C??vennes National Park, where people fish, hunt, farm, and camp.

Heavy metals are distributed in the environment because of natural and anthropogenic mechanisms. The trick for Baron's team was to figure out to what extent each mechanism contributed. While it's not surprising that large quantities of heavy metals have staying power in soil and rocks, this study is one of the first to systematically attribute present-day pollution levels to an ancient source of man-made pollution. The study "certifies the responsibilities of the polluters and the origin of the toxicity," Baron points out.

The team examined leftover slag???the by-product of smelting ore to purify mined metals???and the background content of the metals in surrounding soil and granite. By comparing the lead-isotope ratios found in the slag to those found in rock and soil, Baron's team illustrated that binary mixing between slag and granite was the primary method of dispersion and verified that the medieval pollution contributed at least 40% of the lead found in the granite.

"Studies of old metallurgical pollution allow the understanding of modern pollution because we can observe the long-term effect of heavy metals in the environment," Baron says. He hopes that this research will continue to elucidate historical anthropogenic pollution by examining what he calls environmental archives, such as peat bog cores and lake sediments. He stresses that further studies will require an interdisciplinary approach, with collaboration between geochemists, archaeologists, historians of science, and toxicologists.