Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 134, No. 2 (2015), pp. 304-322, 9 figs., 4 tabs. (doi: 10.3301/IJG.2015.03) © Società Geologica Italiana, Roma 2015
Geochemical characterization of the ground waters from the former Hg-mining area of Abbadia San Salvatore (Mt. Amiata, central Italy): criticalities and perspectives for the reclamation process ORLANDO VASELLI (1), (2), BARBARA NISI (3), DANIELE RAPPUOLI (4), FRANCESCO BIANCHI (5), JACOPO CABASSI (1), STEFANIA VENTURI (1), FRANCO TASSI (1), (2) & BRUNELLA RACO (3)
ABSTRACT
INTRODUCTION
This study was aimed to geochemically characterize the groundwater system of the Abbadia San Salvatore (Mt. Amiata, Siena, central Italy) former Hg-mining area, whose activity closed at the end of the seventies, in order to accomplish the reclamation process after that the former ownership of the mining concession (E.N.I. National Agency for Hydrocarbons, AGIP Division) passed the property to the Municipality of Abbadia San Salvatore in 2008. The study area covers a surface of about 65 ha and since February 2013 old and new piezometers were used to assess the main chemical features along with the concentrations of As, Hg and Sb. Four sampling campaigns were carried out up to January 2014 and a relatively large spatial and temporal geochemical variability was observed. Apparently, the working activities related to the construction of an artificial channel (commenced in March 2013 and terminated at the end of 2013), which crosscuts the whole mining area to drain the surface waters in order to minimize the interaction between the meteoric waters and the by-products deriving by the production of metallic mercury, did not affect the groundwater system. Slag, roasted material and other by-products deriving from the local and surrounding Hg mining activities were indeed used to fill the terrain where most of the mining structures lie. The dominating geochemical facies was Ca(Mg)-SO4 and, subordinately, Ca(Mg)-HCO3, while Na-HCO3 compositions were rarely found. Dissolution of gypsum/anhydrite and carbonates and hydrolysis of sulfide minerals are likely the main geochemical process that produced the observed geochemical compositions. The contents of As and Sb only sporadically exceeded the maximum allowable concentrations intended for human consumption (98/83 EC Directive 1998), i.e. 10 and 5 mg L-1. Conversely, those of Hg were constantly above the EC directive, e.g. 1 mg L-1, with the exception of those waters located up- and downstream the groundwater flow. This indicates that the exotic filling terrains in the mining area, mainly consisting of roasting products, likely play a pivotal role in regulating the concentrations of Hg, which reached values up 853 mg L-1. The construction of permeable reactive barriers, located downstream the water flow, appears to be the most promising solution for the removal of Hg, although, according to the literature, several materials, tested with laboratory experiments, can be used and, as a consequence, it is necessary to individuate specific piezometers with different Hg concentrations where pilot investigations are to be carried out before undertaking any remediation actions.
Mining activity is not sustainable (e.g. KALIN et alii, 2006) whereas treatment of mining wastes, during and/or after the exploitation, and remediation actions are supposedly to be carried out in the respect of the environment. The most important ore metals and metalloids, mobilized and transported in this environment by circulating waters, are Pb, Zn, Cd, Tl, Hg and As and Sb, respectively (e.g. RESONGLES et alii, 2014 and references therein). A large literature has been produced about the transport of noxious elements-bearing acid mining drainage (AMD) (e.g. NORDSTROM, 2011 and references therein). Similarly, AMD remediation options have found great interest in scientific journals (e.g. JOHNSON & HALLBERG, 2005 and references therein). AMD is the result of oxidation processes of metal sulfides (e.g. pyrite) when they are exposed to air and water during and after mining operations. According to several authors (e.g. RÍOS et alii, 2008; KALIN et alii, 2006; MAYES et alii, 2009), four steps can be invoked for the formation of ADM waters: i) iron sulfide oxidation; ii) ferrous iron oxidation; iii) ferric iron hydrolysis and iv) enhanced of ferric sulfide ions, whose most evident result is the production of extremely low pH values. Conventionally, the addition of alkaline and other chemicals to AMDs is used to neutralize these solutions, thus favoring and enhancing the precipitation of metal hydroxides. Usually, acid waters from mines are convoyed to the surface; consequently, the remediation practices are relatively simple and feasible. Different are the conditions when the mining activity has affected the groundwater system due to the interaction with calcines, gangue material and other by-products, and the principal contaminant is mercury (Hg). Abbadia San Salvatore, located in the eastern part of the Mt. Amiata volcano (the most recent and the largest volcanic apparatus of the Tuscan Magmatic Province, e.g. FERRARI et alii, 1996; CONTICELLI et alii, 2004), has been the 4th largest district in the world for the production of mercury (e.g. FERRARA et alii, 1998; RIMONDI et alii, 2012) by exploiting HgS (cinnabar), the ore deposit containing 0.6-2% Hg (BACCI et alii, 1994). After crushing, sorting and roasting the ore material at temperature higher than 350°C, mercury vapors were condensed in cooling plants (e.g. VASELLI et alii, 2013 and references therein). According to NRIAGU (1979) and FERRARA et alii (1998), more than 100,000 tonnes of liquid mercury were produced from
KEY WORDS: Mt. Amiata, Hg pollution, groundwater geochemistry, abandoned mining areas, mine remediation.
(1) Department of Earth Sciences, Via G. La Pira, 4 - 50121 Florence (Italy). Corresponding Author:
[email protected] (2) CNR-IGG Institute of Geosciences & Earth Resources, Via G. La Pira, 4 - 50121 Florence (Italy). (3) CNR-IGG Institute of Geosciences & Earth Resources, Via Moruzzi, 1 - 56124 Pisa (Italy). (4) Union of the Amiata-Orcia Valley Municipalities, Via del Colombaio, 98 - 53023 Gallina, Castiglion d’Orcia, Siena (Italy). (5) SBC Geologi Associati, Via XX Settembre, 78 - 50129 Florence (Italy).
GEOCHEMICAL CHARACTERIZATION OF THE GROUND WATERS
1850 up to the seventies, when the mining activity closed down, and more than 10% of the total production was released as Hg fumes during cinnabar roasting processes (BOMBACE et alii, 1973). Several studies have highlighted the presence of anomalous concentrations of Hg in the different geological, anthropogenic and biological compartments from the Abbadia San Salvatore mining and surrounding areas (e.g. BENVENUTI et alii, this volume and references therein). For example, Hg0 concentrations >50,000 ng m-3 were measured out- and indoor air the shaft Spirek and Cermak-Spirek furnaces (VASELLI et alii, 2013). Calcines were estimated to have up to 1,500 mg g-1 of Hg, while stream and lake sediments achieved Hg contents of 1,900 mg g-1 (e.g. RIMONDI et alii, 2012; GRAY et alii, 2013). Mercury in soils was measured up to 500 mg g-1 (e.g. BARGAGLI et alii, 1987). Surface waters showed concentrations of 1,400 mg L-1 (e.g. RIMONDI et alii, 2012), although when the mining operations were active the concentrations of Hg in the waste waters reached values up to 180,000 mg L-1 (FERRARA et alii, 1991; BACCI et alii, 1978). Methyl-Hg (Me-Hg) abundances were recently measured in water stream and fish muscles (RIMONDI et alii, 2012) with values that were ranging from 0.10 to 3.0 ng L-1 and from 0.16 to 1.2 mg g-1 (wet weight), respectively. Despite the fact that after the closure of the Hg production decreasing concentrations of Hg were observed in waters, plants and mussels (e.g. FERRARA et alii, 1991), its remobilization was detected in occasion of flooding events since they affected the local rivers, which act as collectors of the surface waters draining the Abbadia San Salvatore mining area, where enhanced contents of Hg in the stream sediments were recorded (PATTELLI et alii, 2014). Finally, Hg concentrations in different plants close to Abbadia San Salvatore also displayed anomalous Hg enrichments related to the pollution of air and soils and derived from the local mining activity. Lichens, such as P. Sulcata, exhibited Hg concentrations up to 7.8 mg g-1 (dry weight), which tended to sharply decrease with the distance from the Hg production structures (BARGAGLI et alii, 1987). Similar values were measured in pine needles by FERRARA et alii (1991), whereas lower contents were determined in rosemary leafs (0.84 mg g-1, dry weight) (BARGHIGIANI & RISTORI, 1995). The effect of Hg contamination was also recognized in agricultural products, e.g. lettuce, sage and beet (BARGHIGIANI & RISTORI, 1995). In this paper we present and discuss the geochemical results of ground waters sampled from old and recently drilled piezometers located inside and nearby the former Hg-mining area of Abbadia San Salvatore. The main goals of this study were that to i) characterize the geochemical features of these waters; ii) verify at which extent the contamination of Hg was affecting the subterraneous waters and iii) suggest actions to minimize the concentrations of Hg in the framework of a remediation plan aimed to rehabilitate this area for public activities. For this purpose, major and minor dissolved species along with Hg and As and Sb (the two latter elements commonly associated with mercury) were analyzed in four surveys from February 2013 to January 2014. These results are important for the remediation actions to be shortly undertaken since they pinpointed some criticalities associated with the relatively high Hg concentrations found in this site, opening new perspectives for the reclamation process.
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STUDY AREA
With the end of the Hg production at Abbadia San Salvatore in the seventies, two were the main concerns that affected this area, whose economy predominantly relied on the mining activity and the induced market: i) the social problem of the workers who remained unemployed and ii) the contaminated mining structures and the waste materials deriving by the Hg production. At the time of the Hg exploitation, the owner of the mining concession was E.N.I. (National Agency for Hydrocarbons, AGIP Division) that in the successive years produced a series of documentation aimed to remediate the Hg extraction and processing areas. This activity was only partly performed and it was limited to the geochemical characterization of the contaminated soils, surface waters, calcines and air. Apparently, no investigations were carried out in the groundwater system underlying the production area where at least 270,000 m3 of waste material were stored adjacently the building hosting the Spirek and Cermak-Spirek furnaces. This material was consisting of post-roasting products and unprocessed materials also deriving from the other Hg exploitation areas of Mt. Amiata, e.g. Siele, Bagnore and Morone, whose gangue composition was different with respect to that of Abbadia San Salvatore (e.g. BENVENUTI et alii, this volume and references therein). A vertical Nesa furnace was installed a few years before the closure of the mining activity and was only occasionally put in production due to stability problems of the Nesa tower (M. Niccolini, pers. comm.). In 2008, E.N.I. signed an agreement with the Municipality of Abbadia San Salvatore. With this act the ownership of the reclamation passed to the public agency. According to this agreement, the remediation is expected to be occurring with an environmental rehabilitation of the whole mining area for museum purposes and public green. According to VASELLI et alii (2013), the cessation of mining activities by E.N.I., occurred without a scheduled basis, leaving the decontamination issue still open. Reclamation activities addressed to the mine tailings, metallic mercury in the mining structures, contaminated soils around the Hg production have recently started. The location of the Abbadia San Salvatore mining area is reported in fig. 1. The mining complex (about 65 ha) is depicted by a dashed red square in fig. 1A,B and includes both the mining structures and the managers and workers buildings. In May 2011, before the beginning of the reclamation process, an ultra-light flight was carried out above the mining complex (fig. 1C), which allowed to observe the state of abandon the mining area was left after more than 30 years from its closure. Fig. 1D, taken during a flight survey in July 2013, shows the effects of the cleaning operations performed in a couple of years along with the construction of an artificial channel, whose path is highlighted by yellow arrows and traced with a blue and a yellow dashed curve in fig. 1B and fig. 1C, respectively. This channel was initiated in March 2013 and it was aimed to act as main drainage system of the surface waters, thus limiting the interactions with the contaminated terrains of the mining area. Finally, in July 2014 a new flight was operated to highlight the artificial channel (fig. 1E) after its completion (December 2013). The numbers in fig. 1E indicate the main mining structures: 1) dryers; 2) Cermak condensers; 3) the building
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Fig. 1 - The former Abbadia San Salvatore Hg-mining area and location of the piezometers (A) from where the water samples were collected (red circle) and the artificial channel (blue line in B), whose construction commenced in March 2013 and terminated at the end of 2013; C) and D) aerial photos taken from an ultra-light vehicle in May 2011 and July 2013, before the beginning of the reclamation process and during the construction of the artificial channel, respectively; E) aerial photo (July 2014) from an ultra-light vehicle after the completion of the artificial channel. The numbers in fig. 1E refer to the main mining structures: 1) dryers; 2) Cermak condensers; 3) the building hosting shaft Spirek and Cermak-Spirek and 4) Nesa furnaces while sports ground where the very first reclamation action was undertaken in 2007 (5) when this site was the sole area of public property. The path of the artificial channel is reported with yellow arrows.
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Fig. 2 - Schematic NW-SE oriented geological cross-section of the mining area as reconstructed by the piezometer logs reported in the fig. where the main mining structures are also described. LEGEND: 1) Coarse roasting products, whose matrix is mostly sandy. The shallower layers contain remnants of concrete and bricks. The color is whitereddish. Occasionally, the furnace products are well rounded; 2) Lenses of organic-rich clays and silts. They are normally black, although dark-grey clay layers were also observed; 3) Fluviolacustrine deposits consisting of fine sand and silt and organic-rich clay silt layers. Coarse-grained fragments are rare; 4) Trachyte characterized by highly variable lithology. Presence of compact and fractured volcanic material, loose tuffs and fine-grained terrains, which are topped by a weathered layer with up to metric isolated trachytic boulders; 5) Groundwater surface, which occasionally seeps within some of the mining buildings.
hosting shaft Spirek and Cermak-Spirek furnaces, later replaced by tower furnaces connected to the condensers and 4) Nesa furnace. It is worthwhile to mention that the very first reclamation action was undertaken in 2007 by constructing an up to 15 m deep physical barrier (jet grouting technique) equipped with an impermeable cover in a small portion (ca. 4,500 m2) located in the centraleastern part of the mining area (FACCENDI et alii, 2009). In 2007, this site was the sole area of public property and presently it is used as sports ground (number 5 in fig. 1E). A schematic NW-SE oriented geological cross-section and the groundwater surface were reconstructed on the basis of some piezometer logs (fig. 2). The mining buildings located in the NW sector lie above the trachytic altered products of the volcanic complex of Mt. Amiata, while the furnace building (fig. 1E) is mostly positioned on fluviolacustrine deposits, which are interbedded with organicrich clays and silts. The south-eastern part of the mining area is characterized by the presence of coarse roasting products within which fragments of concrete and bricks were also recognized. This anthropogenic layer is up to 15 m deep and lies above the fluvio-lacustrine deposits, which, at their turn, overlie the volcanic products.
SAMPLING AND ANALYTICAL METHODS
Four surveys were carried out in February, May, September 2013 and January 2014 during which 32, 33, 25 and 27 water samples from previously existing and new (drilled in 2012) piezometers were collected. The location of the sampling site is reported in fig. 1A. The great majority of the old piezometers (B, F, XY, S3, S8, S10N, S30, S40, S60, S65) was located close to the mining structures, while the recent ones (S101, S102, S103, S104, S105, S106, S107, S107, S108, S109, S110, S111, S113, S114, S115, S116, S117) were distributed in both the mining area and upstream (e.g. S102, S103 and S155) and downstream (e.g. S30, S40, S60 and S111) the groundwa-
ter flow. The water flow direction is supposedly WE-oriented up to the Spirek and Cermak-Spirek furnaces (fig. 1E) and it turns to SSE afterwards, although interferences with a NE-SW oriented water flow cannot be excluded, likely affecting some of the collected waters (e.g. S107) (BIANCHI et alii, 2012). Finally, six piezometers (S1N, S2est, S2int, S4N, S6N and S7N) were drilled in the area where the reclamation process was completed in 2007 (fig. 1E). The water table depth was measured prior the water sampling with a portable Universal phreatimeter reel mounted on the inside, which housed a circuit board to condition the tone when the probe contacted the water. The water sampling was conducted out in a dynamical mode by means of a portable sampling set up consisting of a tubing reel connected to a double valve pump power supplied by a car battery. The piezometers were up to 20 m deep and the water head was up to 4 m from the surface. Once the pump was inserted into the well, the water volume discharged before sampling was at least three times that of the tubing system, although for some piezometers this operation failed due to the lack of water and a few hours were needed before recovering enough water. This has also affected the sampling of some waters during the monitoring period. In other cases, some of the pre-existing piezometers were destroyed during the operations related to the construction of the artificial channel as testified by the decreasing number of waters collected from February 2013 to January 2014. Three filtered (0.45 mm) aliquots were collected at each site and transferred to double sealing polyethylene bottles with insert caps. The first one (125 mL) was for the analysis of the major and minor anions (Cl-, SO42-, NO3- and F-) and NH4+, while the second one (50 mL) was acidified with 0.5 mL of Suprapur HCl for the main cations (Na+, K+, Ca2+ and Mg2+). Finally, the third aliquot (50 mL) was acidified with 0.5 mL of Suprapur HNO3 for the analysis of As, Hg and Sb. The polyethylene bottles for the analysis of As, Hg and Sb were cleaned in
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laboratory by adding 5 mL of HCl and then, abundantly rinsed with MilliQ water. Temperature and pH and electrical conductivity (E.C.) were determined in the field with a Hg normal thermometer and a portable Eutech PC650 waterproof instrumentation, respectively. Bicarbonates were also determined in situ by acidimetric titration with HCl 0.01 M and methyl-orange as indicator. The content of CO32- for those samples with pH>8.3 was calculated stoichiometrically. The main anions and cations were analyzed by ion chromatography (761 Compact IC-Metrohm and 861 Advanced Compact IC-Metrohm respectively), with the exception of NH4, which was determined with the Nessler method by molecular spectrophotometry. The analytical error was
