Industrial Mercury Pollution in a Mountain Valley: A Combined Geophysical and Geochemical Study

In the present study we report on mercury contamination in the narrow alpine valley known as canton of Valais valley. Waste materials and contaminated sediments were deposited in the valley for many years. An electromagnetic conductivity survey allowed a definitive mapping of the extent of such practices because an EM conductivity contrast was clearly detectable between the added silty-clay material and the natural silty-sand soil. High EM conductivity correlates quite well with high Hg surface soil content. Cores drilled at hotspots and along the canal were analyzed for total mercury, methyl mercury, and core sedimentary features recorded. While up to 70 cm thick, dredged, Hg-contaminated material was found to lay above the original sandy soil in housing areas, the mercury contamination extends down to 1.5 m depth, i.e. down to the water table, probably caused by the high sediment material organic content (OC), and OC-Hg enhanced transport. Methylation reductive conditions were observed, together with high MeHg relative abundance ([MeHg]/ [HgT] > 0.1%), in two environments, namely the zone of fluctuating water table and the canal water/sediment interface. Groundwater quality was not impacted, because of large groundwater fluxes observed in these glacial alpine valleys.