© by PSP Volume 19 – No 1. 2010 Fresenius Environmental Bulletin 69 ASSESSMENT OF MERCURY IN SEDIMENTS AND IN THE BENTHIC INVERTEBRATE Micronecta scholtzi DOWNSTREAM AN ABANDONED PYRITE MINE Ana Raquel Agra 1 *, Sizenando N. Abreu 1 , Carlos Barata 2 and Amadeu M. V. M. Soares 1 1 CESAM (Centre for Environmental and Marine Studies) & Biology Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal 2 Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18, 08034 Barcelona, Spain ABSTRACT The drainage of abandoned mines leads to several eco- logical problems, particularly the acidification of surface freshwater systems and heavy metal contamination. Mer- cury (Hg) is of particular interest because of its high toxic- ity and its ability for bioaccumulation. This work evalu- ates the magnitude and pattern of Hg dissipation from an abandoned pyrite mines to the local water ecosystem by testing of the stream bottom sediments and water inverte- brate Micronecta scholtzi along the pyrite-cupric mine drainage system. Overall results showed a high significant correlation (Pearson coefficient = 0.82 at p<0.05) between mercury levels in biota and those in sediments for all the sampling sites. In the sediments Hg concentrations ranged from 14.4-32.4 µg g −1 dw decreasing downstream to 3.67- 5.3 µg g −1 due to natural dilution. Hg concentration in M. scholtzi showed a similar trend ranging from 3.6-12.6 µg g −1 dw at the mine pit and decreasing 5 Km downstream to 0.2-1.6 µg g −1 thus reflecting the significant Hg dissipa- tion in surface sediments. Reported Hg levels in sediments were very high and significantly above threshold limits de- fined by the legislation for the protection of aquatic biota. The Hg in stream sediments derived from such mining areas is potentially hazardous to the environment adjacent to the abandoned mine and this work proves that Hg is mobilised from the mines and accumulated by local water biota, posing a risk to surrounding freshwater systems. KEYWORDS: mercury, São Domingos mine, mine drainage, sedi- ments, bioaccumulation, freshwater, biota, Micronecta INTRODUCTION Mercury (Hg) has been one of the most studied global pollutants over the last 40 years due to its high toxicity, persistence in the environment and bioaccumulation po- tential along the trophic chains (wildlife and ultimately to humans) [1]. Although Hg is released by natural sources like volcanoes, additional releases from anthropogenic sources, like coal burning and mining activities, have led to significant increases in environmental exposure and deposi- tion. Past releases have also created a global pool of Hg in the environment and part of that Hg is continuously mobi- lized, deposited and remobilized between air, water, sedi- ments, soil and biota [2]. Once it enters the aquatic envi- ronment, the inorganic form of Hg can be methylated into its most toxic form, monomethylmercury, enhancing the risk of Hg bioaccumulation and biomagnification along the food chain and consequently posing serious threats to ecosystem and public health [2, 3]. Abandoned mercury and pyrite mines usually generate acid-mine drainages (AMD) that are the result of oxidation of significant quantities of sulphides and that constitute an environmental threat world- wide [4, 5]. In the sediment and soil environments, Hg and other trace metals have different behaviors but they are all potentially toxic depending on the element concentration and on the metals species; AMD running along contami- nated sediments/soils and tailings increases the mobility of these trace elements and, as a consequence, it increases the hazard of contamination in sediments and soils down- stream the open pit. The rainy season has been identified as a factor promoting the downstream movement of met- als and consequently, mine wastes are a potential danger to downstream environments and require urgent remediation [4]. The abandoned cupric-pyrite mine of S. Domingos (SE Portugal) was explored mainly for copper and sulphur but in fact every sulphidic ore/manifestation contains a wide spectre of elements having affinity to sulphur, including mer- cury. Moreover, gaseous Hg halo over sulphide ores is a well-documented fact [6]. Forty years after the cease of the mining activity, São Domingos water stream is still an important and continuous source of trace elements pollu- tion and acidity to the soil and to the water and river bed sediments located downstream, as the Chança River reser- voir, within the Guadiana river basin [7-9]. In fact no physi- cal barrier separates the mine hydric system from Chança River since the dam built by the mine owners (English