Arsenic contamination in domestic wells in the southern Versilia (Tuscany): investigation and hypothesis on origin and causes

High arsenic concentrations reaching about 1 g/l have been measured in domestic wells in a localized area of southern Versilia (Tuscany, Italy), nearby a stream receiving acid mine drainages from a past mining site. The arsenic is likely of natural origin, possibly related to As rich pyrite initially present as mineralized components in the alluvial sediments. Pyrite weathering is hence supposed to have been the primary source for As in groundwater, even if the amount of pyrite in sediments must be better investigated. However, the highest As concentration is at present measured in groundwater reflecting reducing environments and it is often coupled with high amount of iron and ammonium ions. This suggests the occurrence of secondary As sources such as hydrous ferric oxides and organic matter likely in peat layers at depth, and a complex fate for As through the role of adsorption/desorption processes.