Arsenic distribution in the Quaternary sediments of the median valley of the Pecora stream (Grosseto,Italy)

Southern Tuscany hosts a number of ore deposits which have been actively exploited since the Etruscan period until very recently. Natural processes of mineral and rock weathering and almost 3000 years of ore mining and metallurgical activity led to the dispersion in the environment (mainly soils and waters) of toxic elements, among which arsenic is one of the most hazardous. In this paper we present the first results of a geological and geochemical survey carried out in the middle portion of the Pecora valley (MPV), aimed at establishing different contributions (“geogenic” vs. “anthropogenic”) to the observed arsenic enhancement in soils (2-335 ppm As, mean value = 70 ppm) and waters. The study area is part of a larger pyrite and polymetallic sulphide mining district; in addition, it hosts several industrial plants that have been treating pyrite for sulphuric acid production during the last fifty years. Geologically, MPV is constituted by Neo-autochtonous (Quaternary) alluvio-colluvial sediments, deposited by the palaeo- and present-day Pecora watercourse, and lacustrine, phytoclastic travertines. The Quaternary sequence overlay a suite of sedimentary rocks spanning from Trias to Oligocene, hosting magmatic rocks emplaced in Mio-Pliocene and associated with intense hydrothermal circulation which played a major role in ore formation. Quaternary alluvial-colluvial sediments and travertines are characterized by As contents constantly above the regional background value (7 ppm) and up to 947 ppm and 267 ppm, respectively. The lack of correlation between depth of samples (from the surface) and As contents seems to role out anthropogenic sources like fallout (metallurgical) ashes or insecticides and/or pesticides used for agricultural purposes. On the basis of our results, two possible “geogenic” sources may be proposed, i.e., transport of As by hydrothermal fluids and/or weathering of As-bearing ore bodies occurring in the Pecora stream catchments.