Mercury in the unconfined aquifer of the Isonzo/Soča River alluvial plain downstream from the Idrija mining area
Graphical abstract
Introduction
Mercury (Hg) is characterised by a complex biogeochemical cycle in which slight changes in chemical, physical, biological, and hydrological conditions may determine substantial variations in its chemical forms (Krabbenhoft et al., 2005). Of particular interest to the environmental sciences are organic species of metal such as methylmercury (CH3Hg+) since it is characterised by a strong (neuro)toxicity even at low concentrations, which are promptly accumulated in aquatic organisms and biomagnified along the trophic chain. Through the consumption of fish and molluscs, CH3Hg+ can reach humans where its harmful effects are particularly apparent in the nervous system (Fitzgerald and Lamborg, 2007, Beckers and Rinklebe, 2017). It is therefore vital to understand the main pathways in which mercury enters various environmental compartments as well as its transfer mechanisms and transformation.
Groundwater is a major source of environmental concern, especially in regard to its protection from various sources of urban, agricultural and industrial contamination. Fewer studies on Hg in groundwater have been performed and published in comparison to other environmental compartments (soil, atmosphere, fluvial and marine waters). In the absence of known sources of contamination, several studies have found that Hg concentrations are low, in the range of nanograms per litre (e.g. Krabbenhoft and Babiarz, 1992, Barringer and Szabo, 2006, Kowalski et al., 2007). On the contrary, other studies have shown that the regional levels of Hg in groundwater can be high, in the range of micrograms per litre (e.g. Protano et al., 2000, Barringer and Szabo, 2006) when Hg is supplied by anthropogenic sources. Historical mining presents a significant environmental impact on the Hg-deposit area, not only on surface soils but also in surface water and groundwater (Grassi and Netti, 2000, Ordóñez et al., 2011, Porcel et al., 2015, Vaselli et al., 2015, Dadová et al., 2016). In some cases, contamination may also proceed from industrial facilities that either use Hg in their fabrication processes (e.g., chlor-alkali plants as in Richard et al., 2016b), or produce Hg compounds such as Hg-fulminate plants (Arbestain et al., 2009) or mercury(II)chloride (HgCl2) as an antiseptic used in wood preservation (Bollen et al., 2008). Soil characteristics, such as pH, carbon content, mineralogy, permeability and texture, play a major role in Hg retention or mobility and the potential transfer of Hg inputs to the land surface reaching the water table (Barringer et al., 2005). The atmospheric deposition from industrial activities such as a coal fired-power plant or cement factory, can also act as a source of mercury, which is first accumulated in soils and subsequently may be mobilised and moved through the soil column to the aquifers (e.g. González-Fernández et al., 2014). Hazardous solid waste landfill sites have been enriched in total Hg and CH3Hg+, not only in the leachate but also in effluent water and groundwater from the landfill site, thus suggesting that bacteria may cause active methylation under the reducing conditions in the anaerobic landfill (e.g. Han et al., 2017). Oxidation-reduction, precipitation-dissolution, aqueous complexation, and adsorption-desorption reactions will strongly influence and complicate the fate and transport of Hg in groundwater and the environment (Barringer et al., 2013).
In this context, the Isonzo River (called the Soča in Slovenian) drainage basin is well known as one of the freshwater systems most impacted by past Hg mining activities of the Idrija mine (Gosar and Teršič, 2012). As a consequence of over 500 years of Hg ore mining, mainly cinnabar (HgS) excavation and smelting, Hg contamination has affected areas located far from the primary source. Large quantities of tailings and waste still contain high concentrations of trace metals due to inefficient extraction and smelting activities (Biester et al., 1999) and may act as long-term sources of Hg, since single flood events can readily remobilise and convey contaminated material downstream (Covelli et al., 2007, Baptista-Salazar et al., 2017). Several previous studies focused on different compartments of Hg contamination, not only in the terrestrial environments of the river drainage basin such as air (Kotnik et al., 2005), soils (Gosar et al., 2006), the Idrijca/Isonzo riverine sediments (Gosar and Žibret, 2011) and freshwaters (Horvat et al., 2002, Kocman et al., 2011), but also in the coastal marine environments of the northern Adriatic sea, from the Gulf of Trieste (Horvat et al., 1999, Covelli et al., 2001, Covelli et al., 2006, Covelli et al., 2007) to the Marano and Grado Lagoon (Covelli et al., 2008, Covelli et al., 2012, Acquavita et al., 2012).
In this work, we focused on the study of Hg in the groundwater of the Isonzo river alluvial plain. Prior to this research, there have been no published reports or attempts to directly quantify Hg in the alluvial plain groundwater system and to depict areal distribution and sources. Since the highest sector of this alluvial plain is characterised by good permeability, water losses from the riverine course would be the main source of the phreatic aquifers (Calligaris et al., 2016), and it is hypothesized that such groundwater has higher Hg concentrations than the natural background. Phreatic aquifers in the alluvial plain are often exploited for irrigation and to provide a supply of drinking water to the local inhabitants. The aim of the study was first to determine the spatial distribution of Hg concentrations from several piezometers and wells in the study area. The second aim was to explore the relationships between Hg concentrations and the major constituents of the water along with the water isotopic composition to highlight the flow paths and local contribution from other sources. The last aim was to reveal the potential mobility and reactivity of the metal in the solid phase due to the hypothetical variability of the water table level by performing a preliminary investigation on Hg speciation in water and alluvial sediments.
Section snippets
The study area: geological and hydrogeological setting
The Isonzo River plain represents the easternmost part of the Friulian-Venetian alluvial plain. It is bordered by the Torre River to the West and by the carbonate plateau of the Classical Karst Region to the East (Jurkovšek et al., 2016). It is limited southward by the Adriatic Sea and northward by the Italian and Slovenian hills, both constituted by flysch lithologies, a sedimentary sequence of rhythmically interbedded shales and sandstones. The Isonzo alluvial plain is a thick quaternary
Field work and sample preparation
Surface freshwater from the Isonzo River and groundwater samples from existing wells and piezometers were collected during four campaigns between 2014 and 2015 (Fig. 1). Sampling operations of groundwater were performed both in static and dynamic mode. In the first case, a bailer sampler was employed to avoid any perturbation in terms of physico-chemical boundary conditions for the collection of 37 unfiltered water samples to analyse total mercury concentrations (THg). Thirteen samples were
Mercury in surface water and groundwater
All THg concentrations (Tables S1 and S2) detected in groundwater samples were found to be below the Italian regulatory threshold limit of 1.00 μg L−1 for drinking water (Italian Legislative Decree 31/2001), reaching the maximum concentrations of 855 ng L−1 and 78.77 ng L−1 at sites GS9 and GI9 respectively, collected in static and dynamic mode. The statistical variability of THg and FHg concentrations in Italian and Slovenian groundwater samples collected in static and dynamic mode are
Conclusions
Mercury concentrations, either as total or as filtered dissolved form, in groundwater of the Isonzo River alluvial plain are far lower than the recommended limit of 1.00 μg L−1 for drinking water according to the Italian Regulations (National Legislative Decree 31/2001) and the National Primary Drinking Water Regulations proposed by US EPA. Total and filtered Hg concentrations in groundwater do not significantly differ as is usually observed in the surface waters where the majority of Hg is
Acknowledgements
Major ion content and isotopic composition were kindly provided by the Regional Environmental Agencies of the two countries (ARPA FVG and ARSO for Italy and Slovenia, respectively) within the framework of the European CBC Projects Italia-Slovenia 2007–2013: GEP (http://www.gepgis.eu), ASTIS (http://astis.ung.si) and HYDROKARST (http://www.hydrokarst-project.eu), funded by the European Union.
The authors are grateful to Barbara Stenni and Marzia Michelini from the Department of Mathematics and
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