J Soils Sediments (2015) 15:1235–1245 DOI 10.1007/s11368-015-1096-4
SEDIMENTS, SEC 1 • SEDIMENT QUALITY AND IMPACT ASSESSMENT • RESEARCH ARTICLE
Heavy metal risk assessment after oxidation of dredged sediments through speciation and availability studies in the Reno river basin, Northern Italy Chiara Ferronato & Gilmo Vianello & Livia Vittori Antisari
Received: 21 July 2014 / Accepted: 15 February 2015 / Published online: 1 March 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Purpose The distribution of heavy metals was investigated in sediments of both natural and artificial watercourses of the Reno river basin (Northern Italy) with the aim of assessing their pollution risk before and after dredging operations. The different solubility and availability of metals in wet and dry sediments were investigated in order to identify the main critical variables controlling metal adsorption into sediments, their speciation and, therefore, their potential environmental hazard. Materials and methods Twenty-four sampling stations were selected in the Reno basin network, and sediment sampling was seasonally carried out in 2012–2013. Pseudo-total metal content was determined through acid digestion with aqua regia, and the geoaccumulation index (Igeo) of metals was estimated using regional mean background values. Leaching tests were carried out through partial dissolution techniques (deionized water and diethylenetriaminepentaacetic acid (DTPA) extraction) on wet and dry samples, while the speciation of metals was investigated by a five-step sequential extraction. All analyses were performed by inductively coupled plasma optical emission spectrometry (ICP-OES). Results and discussion Artificial watercourses presented higher contamination levels than natural ones, and a different pollution level was found when Igeo was applied. The sequential extraction showed that metals in river sediments are mostly immobilized in the residual phase (e.g. Cr), while in canals, weak bonds were found (e.g. Cd). The dredging of sediments, and their consequent oxidation, enhances the availability of
Responsible editor: Jos Brils C. Ferronato (*) : G. Vianello : L. Vittori Antisari Dipartimento di Scienze Agrarie, Alma Mater Studiorum-Università di Bologna, Via Fanin, 40, 40127 Bologna, Italy e-mail:
[email protected]
metals according to their affinity with organic matter (e.g. Cu and Pb) or carbonates (e.g. Zn). The different remobilization rate obtained by changing the oxidation status of sediments highlighted the importance of metal availability studies for assessing and predicting their environmental hazard. Conclusions The effect of oxidation processes on the availability of heavy metals depends on the geogenic or anthropogenic nature of the element, on the redox status of the sediment and on the affinity of the metal with the different mineralogical phases of the sediment. In redox changing environments, the prediction of the environmental risk from metals before and after sediment land disposal gives more useful information than the knowledge of total metal concentration. The use of leaching techniques, combined with the calculation of background values, is strongly recommended for the assessment of metal hazard in sediments. Keywords Dredged sediments . Metal availability . Metal speciation . Redox changes . Risk assessment
1 Introduction The increase of heavy metal contamination in fresh water systems causes serious environmental problems in most industrialized countries (Schwarzenbach et al. 2006). The Padanian Plain was formed after centuries of reclamation processes, and in the present time, its hydraulic equilibrium is strongly linked to the maintenance of its watercourses, which include some natural rivers (e.g. Reno river and its tributaries) and an important network of artificial canals that was built in order to control flooding (Vittori Antisari et al. 2010). The plain area is impacted by many urban and industrial settlements that worsen the quality of water courses and increase their pollution hazard. Heavy metals are mainly discharged
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from anthropogenic sources and can be adsorbed into the sediment surface at different strengths. A large amount of soil is eroded from the hilly part of the basin, transported downstream and accumulated in sediments of lower reaches (Pavanelli and Selli 2013); therefore, dredging the excess of sediments is needed to assure the hydraulic safety of the territory. In view of resource recycling, land application of dredged sediments is considered a sustainable practice if sediments are not polluted. Italian Law stipulates that when pseudo-total metal concentration exceeds the legislative thresholds (Legislative Decree 152/2006), sediments cannot be recycled into soil but must be sent to landfills, increasing the cost of dredged sediment management. The study of total metal concentration and behaviour in sediments after land disposal requires a comprehensive prediction of its potential adverse effects, which involves the study of metal speciation, mobility and toxicity. It is well known that the changing of redox status, e.g. sediment oxidation after dredging, can deeply affect metal sorption/desorption cycles and precipitation/ dissolution processes at the interface between water and sediment (Du Laing et al. 2009; Ho et al. 2012), enhancing or decreasing their mobility and, therefore, their risk for the environment. For this reason, additional investigations on metal speciation and mobility changes are needed in order to predict the toxicity of dredged sediments. The risks associated with the presence of heavy metals in dredged sediments are determined by their mobile/ available concentrations rather their total content (Salomons and Förstner 1980; Luoma and Davis 1983; Ho et al. 2012). For assessing metal hazard, their sequential extraction can be carried out according to their affinity with different mineralogical phases (Dawson and Macklin 1998; Gleyzes et al. 2002; Farkas et al. 2007). In fact, despite some criticism and lack of uniformity in the extraction procedures, this method is widely used for establishing anthropogenic contributions to sediment metal concentration (Larner et al. 2008; Prica et al. 2010). Another common tool in environmental risk analysis is the use of partial dissolution techniques with different extraction strengths (e.g. water, salts, chelating agents), which can detect the soluble and available fractions of some elements (Carlon et al. 2004; Stephens et al. 2001). These methods have shown strong correlations with the ability of biological communities to tolerate or to be affected by metal concentration in bottom sediments (Larner et al. 2008) and can be applied to both wet and dry sediments, in order to pick up the different metal hazards during oxidation processes. In this scenario, the aim of this study was to assess heavy metal hazard in sediments of both natural rivers and artificial canals of the Reno river basin (1) according to their pseudo-total
J Soils Sediments (2015) 15:1235–1245
content compared with Italian legislative thresholds and (2) according to their speciation and mobility in wet and dried sediments though partial (water and diethylenetriaminepentaacetic acid (DTPA) extraction) and sequential dissolution techniques.
2 Material and methods 2.1 Study area The Reno river basin is located in the southern part of the Padanian Plain (Northern Italy) and covers an area of 4930 km2 between the Apennines and the beginning of the plain. In the upper part of the basin, watercourses usually have a natural bed and cross low-pollution sources (Vittori Antisari et al. 2011), while in their lower reaches, rivers are characterized by high artificial embankments and cross urban and industrial/craft settlements with spread and point sources of pollution. The Reno river basin, like all the Padanian plain, is also characterized by a network of artificial canals excavated over the centuries for land reclamation needs. These watercourses are all artificially embanked and are used as collectors for different purposes, such as for draining water and wastewater from urban and industrial discharges or for transporting water for irrigation purposes. In Fig. 1, the study area is presented and the sampling sites are shown. River samples were collected in upstream positions in the Apennine hilly region (from RUp1 to RUp5) and downstream from some cities and industrial/craft settlements (from RDw6 to RDw10). Artificial canal sampling sites were located in the plain area upstream or downstream from wastewater plants and urban/industrial/craft settlements (CUp1–CUp5 and CDw6–CDw14, upstream and downstream, respectively). 2.2 Sampling and analysis of sediment Surficial sediment samples (0–10 cm) of both rivers and canals were collected seasonally for 2 years (2012–2013), using a Van Veen grab. Two samples were collected from each site in order to be tested in wet or dry conditions. The first subsample was covered with fresh water, stored at 4 °C and processed within 1 week, while the second one was air-dried before analysis. All investigations were performed on 2-mm sieved fraction. Electrical conductivity (EC, Orion, Germany) and pH (pH meter, Crison, Germany) were detected on a 1:2.5 ratio w/v with distilled water. Carbonate content (CaCO3) was calculated by volumetric method, while total organic carbon (TOC) and total nitrogen (TN) were detected by CHN elemental analyzer (EA 1110, Thermo Fisher, USA). Total content of K, P, Fe, Mn, Cd, Cr, Co, Cu, Ni, Pb and Zn was detected
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Fig. 1 Map of the study area of Reno river basin. Monitoring points are classified as rivers (R1–10) and canals (C1–14)
by inductively coupled plasma optical emission spectroscopy (ICP-OES, Spectro, Arcos, USA) after microwave digestion (Milestone 1200, USA) with aqua regia (suprapure HCl and HNO3 3:1 w/w) according to Vittori Antisari et al. (2011). All analyses were performed in duplicate and reference materials (BCR-320R and BCR142), and reagent blanks were used to check the accuracy of data and the agreement was typically63 μm) Silt (63–2 μm) Clay (63 μm) Silt (63–2 μm) Clay (
