TRANSPORT OF MERCURY FROM A MINED AREA AND UPTAKE OF MERCURY BY FISH IN THE TIBER RIVER, ITALY

Stream sediment, stream water, and fish were collected from the Tiber River to evaluate downstream transport of mercury (Hg) from inactive mines in the Monte Amiata Hg District (MAMD), Tuscany, Italy. The Tiber River is the third longest river in Italy, with a drainage basin is about 17,400 km², flowing eventually into the Mediterranean Sea near Rome. Concentrations of Hg in stream sediment samples varied from 20-1900 ng/g, and only 5 of the 17 samples exceeded the probable effect concentration for Hg of 1060 ng/g, the concentration above which harmful effects are likely in sediment-dwelling organisms. An estimated 11 kg of Hg were discharged in 2011 from the Paglia River, a large tributary of the Tiber River proximal to the MAMD. Methyl-Hg in Tiber River sediment samples ranged from 0.12-0.52 ng/g (n = 9), and although there is no guideline established for methyl-Hg in sediment, these concentrations greatly exceeded methyl-Hg in stream sediment collected from a regional baseline site (<0.02 ng/g). Concentrations of Hg in Tiber River water varied from 1.2-320 ng/L (n = 9), all of which were below the 1000 ng/L Italian drinking water guideline and the 770 ng/L guideline for Hg recommended by the U.S. Environmental Protection Agency (USEPA) to protect against chronic effects to aquatic wildlife. Concentrations of methyl-Hg in Tiber River water ranged from <0.02-0.53 ng/L (n = 9) and were generally elevated compared to the baseline site (< 0.02 ng/L). Concentrations of Hg in freshwater fish ranged from 0.052-0.56 µg/g (muscle, wet weight), mean of 0.17 µg/g, but only 17% (9 of 54 samples) exceeded the 0.30 µg/g (wet weight) USEPA fish muscle guideline recommended to protect human health. Concentrations of Hg in freshwater fish collected from the Tiber River generally decreased with increasing distance from the MAMD. Fish with the highest Hg concentrations were collected proximal to the MAMD, whereas all fish collected most distal from the Hg mines contained Hg below the 0.30 µg/g fish muscle guideline. Conversion of inorganic Hg to methyl-Hg and uptake of Hg by fish was observed on the Paglia River, but less Hg methylation and Hg uptake by freshwater fish was found on the larger Tiber River.