Geochemistry of trace elements in surface waters of the Arno River Basin, northern Tuscany, Italy

Applied Geochemistry 24 (2009) 1005–1022

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Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

Geochemistry of trace elements in surface waters of the Arno River Basin, northern Tuscany, Italy Gianni Cortecci a,*, Tiziano Boschetti b, Enrico Dinelli c,d, Rosa Cidu e, Francesca Podda e, Marco Doveri a a

Instituto di Geoscienze e Georisorse, Area della Ricerca CNR, Via Moruzzi 1, I-56124 Pisa, Italy Dipartimento di Scienze della Terra, Università di Parma, Via G.P. Usberti 157a, I-43100 Parma, Italy c Centro Interdipartimentale per le Scienze Ambientali (CIRSA), ‘‘Alma Mater Studiorum” Università di Bologna, Centro di Ravenna, Via Sant’Alberto 163, I-48100 Ravenna, Italy d Dipartimento di Scienze della Terra e Geologico-Ambientali, ‘‘Alma Mater Studiorum” Università di Bologna, Piazza Porta San Donato 1, I-40126 Bologna, Italy e Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino 51, I-09127 Cagliari, Italy b

a r t i c l e

i n f o

Article history: Received 27 July 2008 Accepted 4 March 2009 Available online 14 March 2009 Editorial handling by W.M. Edmunds

a b s t r a c t Trace element geochemistry of the Arno River and its main tributaries was investigated on the basis of two sampling campaigns carried out in November 1996 and June 1997. By analyzing filtered and unfiltered water samples, Fe and Al are found in solution mainly as colloidal particles of size lower than 0.45 lm. In June (lower flow rate), Fe and Al are enriched in the filtered waters from the main river, and this feature was interpreted in terms of higher water temperature promoting the formation of smaller particles, thus reducing their aggregation properties. Iron and Al show perfectly synchronous downstream profiles along the Arno River, correlate quite well each to other, and display abrupt concentration increases near to Florence, where the lithology of the catchment changes from siliciclastic dominated to clay-sand (lacustrine-marine)-dominated. The same behaviour is shown by most of the other trace elements in the river, thus supporting a general lithological control. Trace elements in the final part of the Arno River are influenced by flocculation processes in addition to mixing. Adsorption phenomena on oxy-hydroxides are denoted by good elemental correlations with Fe (and Al). Sporadic anomalous concentration values, possibly related to anthropogenic contributions, may prevent such correlations. Referring to the quality of waters for potable use and fish life, toxic elements are below the acceptable limits of current European regulations, with few exceptions for Hg exceeding guideline values. Multivariate analysis groups trace elements according to geochemical affinities and natural or anthropogenic sources, thus distinguishing contaminated from uncontaminated samples. The results achieved in this work will help regional and national Authorities for compliance with the EU water policy, especially in assessing the water quality at the river basin scale and its vulnerability to human activities. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The chemical composition of stream water depends on natural factors, such as bedrock geology, morphology and structural setting of the catchments, and climate, which controls the weathering rates and hydrological features. Other important factors include density and type of vegetation cover (e.g., Salomons and Förstner, 1984) which may be influenced by man, and a variety of anthropogenic activities that may change the dynamics of the fluvial system and/or add a variety of contaminants. When attention is focused on inorganic contaminants, all these factors need to be considered, along with chemical speciation which affects chemical/biochemical reactivity, and adsorption–desorption reactions on and from chemically reactive substrates such as clay minerals, Al–Fe–Mn oxides and hydroxides, and organic molecules, all these substances strongly influencing metal mobility (Förstner and Wittman, 1979; * Corresponding author. Fax: +39 0503152323. E-mail address: [email protected] (G. Cortecci). 0883-2927/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2009.03.002

Miller and Orbock Miller, 2007). Moreover, considering that significant proportions of the Fe oxy-hydroxide content of riverine solid material are present as potentially bio-available micro particles, the Fe cycle is a focus of interest as the metal acts as a limiting nutrient for phytoplankton activity (Raiswell, 2006). Within a research project aimed at monitoring the geochemistry of waters and river-bed sediments of the Arno River Basin (northern Tuscany) by elemental and isotopic analyses with repeated sampling in different seasons during November 06–09, 1996 and June 23–25, 1997, the results on the trace element composition of waters from the main stem and its principal tributaries are reported here. Data on the major chemistry of waters and S isotope composition of dissolved SO4 (Cortecci et al., 2002), and results on major and trace elements just in river-bed sediments (Dinelli et al., 2005) have already been published. Previous investigations on the Arno River Basin waters (Bencini and Malesani, 1993; Cortecci et al., 2002) indicated that at the scale of the basin, major ions Na, Cl and SO4 significantly increase downstream due to anthropogenic contributions both in the main stem and tributaries,

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the exceptions for SO4 being represented by the Elsa and Era tributaries due to the presence of evaporites and thermal springs in the sub-basins; also, potentially harmful metals (Pb, Zn, Ni, Cr, Mn) in the main stem may be largely affected by anthropogenic sources. The present study was undertaken in order to investigate (1) the sources of a number of trace elements in the stream network of the catchment, thus complementing the data obtained by Bencini and Malesani (1993), (2) the variation of possible anthropogenic signals with season and hydrometry, (3) the adsorption and transport roles of colloidal particles, based on samples collected during the June 1997 campaign, and (4) the chemical quality of the investigated waters referring to humans and health of fish. Inferences concerning this point are clearly out-of-date, as in the meantime the chemical composition of waters may have changed. Nevertheless, the data here presented can be of interest for forthcoming regional comparative studies, especially considering that part of the Arno River water is intercepted just upstream of Florence for drinking water purposes, and elsewhere for assessing the water quality at the river basin scale and its vulnerability to human activities. According to Chapman (2007), the term contamination (and derived terms) has been used throughout the text instead of pollution (and derived terms), this is because it was not demonstrated that the concentrations of the studied elements in the Arno River Basin resulted in adverse biological effects.

2. Background 2.1. River Basin description and lithology The Arno River is a river with flow extremes. It reaches the Ligurian Sea after travelling 242 km (Fig. 1). The hydrographic basin covers an area of 8228 km2, 86% of which is lower than 600 m a.s.l. A few peaks up to 1650 m a.s.l. occur in the upper part of the basin along the Apennine chain. The Arno River Basin (Fig. 1a) can be subdivided into an upper part including the Mugello, Casentino, Val di Chiana and Valdarno Superiore sub-basins (a total of 4078 km2, that includes the first 108 km tract of the Arno River), a middle part (Valdarno Medio; 1383 km2; from 108 to 153 km of the river) and a lower part (Valdarno Inferiore; 2767 km2; from 153 km to the river mouth). As summarized in Dinelli et al. (2005), sedimentary rocks dominate the Arno River Basin (Fig. 1b). Sandstones occur in the upper reaches of the catchment, with marls and clays in the Chiana valley. Sandstones, shales and calcareous rocks with chaotic clays along with scattered ophiolitic blocks prevail in the central part of the basin around Florence (Valdarno Medio). Clastic lacustrine deposits occur in the plain area between Florence and Pistoia. Downstream of Florence (Valdarno Inferiore), the right-hand tributaries drain mainly sandstones and limestones; the left-hand tributaries drain fine-grained marine and lacustrine clayey and sandy deposits. Triassic limestone and gypsum-anhydrite along with Messinian gypsarenite occur in the headwaters of the Elsa and Era sub-basins, where ophiolitic rocks are also present. At the end of the catchment, north of Pisa, quartzite outcrops on the right side of the Arno River. The coastal plain around Pisa consists of a graben filled by alluvium. 2.2. Climate and hydrology The climate is of sub-littoral type inland and Mediterranean close to the coast. Mean annual air temperature ranges between 15 °C along the coast and 11 °C inland at an elevation of about 1000 m a.s.l., with maximum (July) and minimum (January) mean

annual values of 20 and 4 °C, respectively. Annual precipitation is generally between 700 and 900 mm, with values up to 2400 mm in the upper Apennine area. Rainfall maxima usually occur in October–November, and the minima in July–August inland, and close to the coast maxima occur in winter and the minima occur in summer (July–August). Snowfall is important only in the Apennine sector of the basin. The annual discharge of the Arno River at the Giovanni alla Vena hydrometric station (36 km from the mouth; 99.5% subtended basin) averages 100 m3/s, with 4–6 m3/s during low-water in summer with flooding values up to 3000 m3/s in autumn. Mean flow rates of the tributaries (Pulselli and Bagato, 1976) are between 16 m3/s (Sieve tributary) and 0.6 m3/s (Pesa tributary). Additional hydrological information on the catchment can be obtained from the Autorità di Bacino del Fiume Arno (http://www.adbarno.it; accessed 16.01.09). 2.3. River-bed sediment mineralogy and geochemistry The semi-quantitative estimates of mineral abundances in the river-bed sediments, collected during the 1997 survey (Dinelli et al., 2005), basically match the compositional features reported by Bencini and Malesani (1993), i.e. mainly quartz (9–44%), feldspars (7–32%), calcite (1–38%) and clay minerals (11–80%); dolomite occurs in the Era, Elsa and Pesa tributaries. Data on organic C (Corg) and N in sediments suggest a small content of inorganic N, the low C/N values emphasizing the role of urban and animal waste effluents (Dinelli et al., 2005). The distribution of Corg, N and P2O5 is consistent with that of potentially harmful metals, but the relationships between Corg and N, P2O5, S, and potentially harmful metals indicate a mixed origin for the organic matter (industrial, agricultural, municipal) and point to a major role of inorganic sources, both geogenic and non-geogenic, in providing metals to the stream sediments. 2.4. Human impacts Residents in the Arno River Basin number around 2.6  106, a third living in the major towns. They contribute a variety of contaminants like organic substances from domestic, agricultural and industrial sources, phosphate and coliform bacteria from municipal sewage and animal waste, detergents from municipal discharges, laundries and textiles, and potentially harmful metals from paper-mills, textiles, tanneries, electrochemical workings and urban drainage. The contaminant load is estimated to be equivalent to 8.5  106 inhabitants. With this high contamination load, that corresponds on average to 1033 potential polluters per km2, no section of the Arno River can be regarded as a totally uncontaminated section. Major anthropogenic inputs in the Arno River occur downstream of Florence (Consorzio Pisa Ricerche, 1998), the direct discharge of the Florence city domestic black and white waters corresponds to a contamination load equivalent to 106 inhabitants. This contamination source increases the COD (Chemical Oxygen Demand) from 10 to 30 mg/L, N–NH4 from trace to 8 mg/L and phosphates from 0.1 to 1 mg/L. The Bisenzio and Ombrone tributaries carry wastewaters from textile industries and nurseries around Pistoia and Prato. Contamination load due to wastewaters (about 80% treated) from textiles in the Prato area is equivalent to 1.4  106 inhabitants. In addition, the Bisenzio receives domestic untreated waste waters from the Vaiano municipality (about 9  103 inhabitants) in the upper part of its basin, as well as from the northern part of Florence through the Macinante Canal. All the domestic waters conveyed to the Ombrone are treated. Cortecci et al. (2002) found TDS values to be increasing downstream from 290 to 499 mg/L in the Bisenzio and from 332 to 1057 mg/L in the Ombrone. The Usciana tributary receives

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Fig. 1. (a) Map of the Arno River catchment and its major tributaries, with the largest towns and the geographic subdivisions of the basin. The sampling stations are shown according to the sampling year. (b) Schematic lithology of the Arno River catchment (modified from Bortolotti et al., 1987).

discharges (95% treated) from many tanneries in the leather district, corresponding to a contamination equivalent load of about 3  106 inhabitants. The tributary also receives treated waters from numerous paper-mills. At 39 km from the confluence, the TDS in the stream was 308 mg/L and at the confluence it had increased to 2179 mg/L (Cortecci et al., 2002). The notable influence of the effluents from the treatment plants (Aquarno/AA and

Cuoiodepur/CD) in the leather district for SO4 and Cl in the Arno River in the lower part of the basin is shown in Fig. 2, where also the influence of seawater in the final tract of the river is manifest. The Aquarno and Cuoidepur plants convey their effluents into the Arno respectively through the Usciana and Malucco tributaries, the latter discharging between S. Croce and Castelfranco di Sotto.

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3. Sampling and analytical methods The location of the sampling sites is reported in Fig. 1 and Table 1, subdivided according to the sampling periods. Water discharge for the Arno River during sampling surveys, measured at the hydrometric station of San Giovanni alla Vena at about 36 km from the mouth, was 36.1 ± 3.0 m3/s in November 1996 and 28.2 ± 2 m3/s in June 1997. The hydrometric data were provided by the Servizio Idrologico Regionale-Centro Funzionale di Monitoraggio Meteo-Idrologico of Pisa. 3.1. Chemical methods

Fig. 2. Sulfate to Cl relationships in the Arno River and tributaries compared to important sources of elements, like seawater and effluents from treatment plants (CD = Cuoiodepur plant;.AA = Aquarno plant; in = waste water to be treated; out = treated waste water) discharging via tributaries in the Arno River. Data on rivers are from Bencini and Malesani (1993) for February 1989, La Ruffa and Panichi (2000) for March 1997, and Cortecci et al. (2002) for November 1996 and June 1997, whereas annual data means on waste waters are from Aquarno WWTP (2005) for AA and http://www.cuoiodepur.it/impianto/descrizione.htms for CD (accessed 22.01.09).

Upstream of Florence, contamination in the Arno River mainly derives from the Chiana tributary, which receives wastewater from electrochemical plants processing Au in the Arezzo district and untreated effluents from intensive agricultural-breeding activities. The electrochemical activity caused a Hg anomaly in the stream sediments in the area (Dall’Aglio, 1971). At about 30 km from the confluence, TDS values increased from 552 to 736 mg/L, and then slightly decreased to 684 mg/L (Cortecci et al., 2002).

Water samples for trace element analyses were stored at 4 °C after filtration (0.45 lm) and acidification with supra-pure HNO3 in the field, and then analyzed by inductively coupled plasma mass spectrometry (ICP–MS; Al, B, Li, Rb, Sr, Ba, Mn, Cd, Cr, Co, Ni, Cu, Pb, V, Mo, Tl, U, Te, Se, Ag, Bi) and optical emission spectrometry (ICP– OES; Si, Fe, Zn, B, Sr, Ba, Mn, Cd, Cr, Co, Ni, Cu, Pb, V). Data on Li from ICP–MS were verified by AAS. Mercury, As and Sb were analyzed by flow-injection ICP–MS. Detectable concentrations derived by different techniques were found to be in good agreement. Detection limits (DL = 10  standard deviation of the blank mean) were calculated for each analytical sequence. Waters with salinity >2 g/L required dilution prior to analyses, thus implying higher DL values depending on the dilution factor. Concentrations of Ag, Be, Bi, Cd, Ga, Te and Tl were almost systematically below DL (usually 0.1 lg/L). Values of V in the seawater contaminated samples were disregarded because determination can be affected by an isobaric interference of 35Cl16O on 51V using ICP–MS. Analytical accuracy and precision were tested by means of certified reference materials SRM1643d (NIST; filtered and acidified freshwater) and SLRS-3 (NCR Canada; river water) and multistandard MSQ6 (for Hg), with results for most elements within ±10%. In the 1997 survey, aliquots of unfiltered and acidified water were also analyzed to investigate the partition of elements between adsorbed (‘‘total” or ‘‘unfiltered”) and dissolved (‘‘filtered”) forms. Nitrate in solution was also measured following the method of Bencini (1977) with a precision better than 3%. COD was deter-

Table 1 Sampling sites of waters and years of survey for 40 stations in the Arno River catchment. In 1997, some tributaries were sampled at the confluence and upstream. Arno River

Tributaries

Sample

Locality

Distancea

1996

1997

Name

Sample

Locality

Distancea

1996

1997

A23 A22 A21 A20 A19 A18 A17 A16 A15 A14 A13 A12 A11 A10 A09 A08 A07 A06 A05 A04 A03 A02 A01

Stia Rassina Subbiano P.te a Buriano Laterina S.Giovanni Valdarno Rignano sull’Arno Rosano Firenze East Firenze West FS S.Donnino Ponte a Signa Camaioni Empoli Fucecchio S. Croce sull’Arno Castelfranco di Sotto Pontedera S. Giovanni alla Vena Caprona Pisa East Pisa West Vallentina (boatyard)

12.5 35 47.5 60.5 69.5 88 108.5 118 131.5 139 145.5 149.5 156 168.5 181 184 188 200 206 215 229 232 239

+ + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + +

Era Usciana

I II II-1 III IV IV-3 V V-1 VI VI-1 VII VII-2 VIII IX X X-2 XI

Pontedera Cateratte 3 km N of Pescia Ponte a Egola Ponte a Elsa Ulignano Montelupo Fior. Sambuca FS Carmignano P.te S. Felice S. Mauro a Signa Vaiano Le Briglie Ponte a Greve Pantassieve (PSF) Pranatico San Leo Foiano della Chiana Tulliano-Montanina

202 192 39 182 176 36 162 28 151 31 147 22 143 117 64 30 40

+ +

+ + + + + + + + + + + + + + + + +

Egola Elsa Pesa Ombrone Bisenzio Greve Sieve Chiana Salutio

+ + + + + + + + +

a For the Arno River samples, distance from the source. For the tributaries, distance of the confluence point from the Arno River source; in italics, distance of the sampling site from the confluence point in the Arno River. Geographic coordinates of the sampling sites can be found in Dinelli et al. (2005).

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mined by treating the water with K2Cr2O7 and measuring the excess Cr (VI) (APAT-CNR IRSA, 2003). 3.2. Thermodynamic and statistical treatments of the data

Table 2 Range of chemical composition of minor and trace elements in the Arno River and its tributaries in November 1996 and June 1997. The ranges on tributaries include the confluence and upstream water samples. Element (lg/L)

The PHREEQCI code, version 2.8.0.0 (Parkhurst and Appelo, 1999), combined with LLNL thermo.com.V8.R6. thermodynamic database, was adopted to compute chemical speciation, and to evaluate the saturation indexes of waters relative to mineral phases. The Principal Coordinate Analysis (PCoord; standardized) using PAST (Hammer et al., 2009), Principal Component (PComp; standardized and unstandardized) and Hierarchical Cluster (HCA) analyses using SPSS 14 (Norusis, 2005) were applied to element concentration datasets adequate for multivariate analysis. In the statistical analysis applied to the Arno River an exotic variable represented by the distance from the source (DS) of the water samples was included, and the concentration data were standardized. When the main stem Arno and tributaries were treated together (and variable DS omitted), the concentration data were unstandardized. The statistical significance of the linear regression between two variables was tested using the critical value tables for r (Pearson’s correlation coefficient), and a degree of freedom df = N 2, where N is the number of the samples (e.g. Fisher and Yates, 1974).

4. Results and discussion The compositional ranges of filtered waters sampled in November 1996 (hereafter autumn waters) and June 1997 (hereafter summer waters) are reported in Table 2. The results on filtered (dissolved) and unfiltered (total) samples in June 1997 are reported in Table 3 for the Arno River and Table 4 for tributaries. The complete data set for 1996 water samples is available upon request from the authors. Based on the saturation indexes (SI), waters from the Arno River and tributaries are nearly saturated to supersaturated (SI = 0–1) relative to calcite, depending basically on pH; unsaturated relative to gypsum, with a downstream SI increasing trend in the Valdarno Inferiore due to the important contribution of Ca and SO4 discharged by the tributaries; and saturated to supersaturated relative to clay minerals (like kaolinite and montmorillonite) and supersaturated relative to hematite. The SI values behave similarly along the Arno River, even if with different trends in autumn and summer. The autumn trends appear more regular (less spiked) than in summer, as expected according to the higher flow rate and related turbulence of the river. 4.1. Dissolved and adsorbed trace element proportions As observed by Cidu and Biddau (2007) using unfiltered and filtered water samples, the concentration of trace elements in river water can be strongly influenced by the amount of adsorbing colloidal particles. In this study, the concentrations of elements such as Li, B, Mn, Rb, Sr, Mo, Ba and U were found to be higher in June at low flow (i.e. low rainfall), whereas the concentrations of elements such as Al, Fe, Co, Ni, Cu, Zn, Cd and Pb were generally higher in January at high-flow (i.e. heavy rains), these features testify to true solution in the first case, and for transport via adsorption in the second case, likely on Fe and Al oxy-hydroxides. The relationships between total and dissolved concentrations are graphically shown as box plots in Fig. 3. It appears (within the analytical uncertainty of about 10%) that filtered samples are unaffected or depleted with respect to unfiltered samples. Calculated average removal percentages by filtration are 1–6% for Li, B, Rb, Sr, Ba, Mo and U (group 1), 9–13% for Si, Mn, Cu, Pb, Ni, Co

pH Eh (mV) Si (mg/L) Al Fe Mn Zn Cu Pb Cr Cd Ni Co As Hg B V Se Mo Sb U Li Rb Sr Ba NO3 (mg/L) COD (mg/L)

Arno River

Tributaries

November 1996

June 1997

Min

Max

Min

7.4 322 0.8 15 <13 <3 <3 <2 <1.6 <1.5 <0.1 <1.6 <0.2 <0.1 <0.3 19 <1.2 <2 0.3 <0.2 0.5 2.4 0.7 359 60 1.8 7.0

8.2 427 4.4 540 356 143 30 9.1 6.5 5.5 <0.5 34 1.0 1.0 2.3 870 3.7 2.6 2.2 2.0 1.0 31.3 17 1450 99 11.6 32

7.3 457 1.3 16 8.0 7.0 – 1.0 <0.6 <0.4 <0.1 <1.2 0.1 <0.3 <0.4 18 <0.6 <0.6 0.3 – 0.5 2.3 0.7 326 57 0.4 1.0

November 1996

June 1997

Max

Min

Max

Min

Max

8.1 533 4.3 1100 1400 168 <3 5.7 2.4 8.7 <0.6 9.7 1.2 1.2 1.2 293 4.8 1.5 3.8 <0.2 1.1 14 4.2 580 90 11.7 11.0

7.6 368 1.7 7 <13 <3 27 <2 <1.6 <1.5 <0.1 <1.6 <0.2 <0.1 <0.3 16 <2.2 <2.4 0.2 3.8 0.3 0.9 0.4 272 37 4.3 8.0

8.2 434 7.5 1700 1280 264 – 15 3.6 27 0.2 15.6 2.3 1.0 1.4 1120 3.6 4.6 2.9 – 2.2 57 17.8 2700 120 13.0 59

7.4 456 1.0 13 <24 2.0 – <0.9 <0.6 <0.4 <0.2 <1.2 0.1 <0.1 <0.4 16 <0.6 <0.6 0.2 – 0.4 <1 0.4 274 39 0.3 0.0

8.0 627 6.3 1000 1160 490 – 8.8 3.4 32 <1.2 19 2.6 1.8 1.7 1290 7.9 2.2 22 – 3.3 116 19 2670 108 4.7 65

– = not determined. In 1997, the Arno River was not sampled downstream of Pisa. Data on V and Se in seawater affected samples were omitted due to analytical uncertainty.

and V (group 2), and 17–29% for Cr, Al and Fe (group 3). Group 1 elements are basically not retained by the filter, that is their dissolved concentrations are not influenced by adsorption processes on particles of size greater than 0.45 lm; group 2 should be appreciably affected by adsorption processes by particles greater than 0.45 lm, and are probably present in the filtrate mainly as adsorbed ions, and finally group 3 elements occur in the studied waters in the form of colloids, with a considerable fraction greater than 0.45 lm in size. Iron is retained more (29%) than Al (19%), this suggesting a larger mean size of Fe-particles in the unfiltered water, and lower sizes for Al, which therefore is enriched relative to Fe in the filtrate. Chromium may be retained as colloid as well as adsorbed anion (see later). Silica is also depleted in filtered samples with respect to total, probably reflecting polymerisation phenomena (Iacopini et al., 2005), producing colloids of size greater than 0.45 lm, coupled with adsorption on Fe(OH)3 (Hiemstra et al., 2007). 4.2. Iron, aluminium and silicon Based on Eh–pH diagrams, Fe and Al in the studied waters should be present basically as oxy-hydroxides. Excluding waters affected by seawater influx and related effects (samples A01– A04), Fe and Al in the Arno River show similar downstream profiles with higher concentrations in the Valdarno Inferiore (Fig. 4), are more abundant in summer and display significant correlations both in summer and autumn (r = 0.99 and 0.97, respectively). The fact that Fe and Al correlate well in the main stem and tributaries both in autumn and summer (Fig. 5) suggests a common main source at the scale of the basin. This source may be the weathering

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Table 3 Analytical results on unfiltered (total) and filtered (dissolved) waters from the Arno River sampled in June 1997. For symbols see Table 1. Arno River

Si

Al

Fe

Li

B

Rb

Sr

Ba

Mn

Cr

Cu

Pb

Ni

Co

Mo

V

U

Se

A23

3.3 3.1 – 2.0 1.4 1.5 – 1.8 – 2.0 2.6 2.3 – 2.0 – 2.2 1.9 1.7 1.5 1.3 – 1.7 1.7 1.2 – 2.0 1.8 1.7 2.7 2.5 – 2.1 2.5 2.5 – 2.3 4.9 4.3 4.3 3.7

155 110 – 38 30 33 – 400 – 140 210 150 – 16 – 140 200 160 170 130 – 120 390 400 – 350 371 455 610 510 – 490 800 720 – 485 1200 1110 890 930

206 125 – 44 30 31 – 387 – 113 233 180 – 18 – 105 188 60 162 114 – 140 590 472 – 335 530 451 970 688 – 650 1043 833 – 555 1670 1400 1290 1100

<3.9 2.3 <3.9 3.2 – 2.6 – 3.1 – 4.9 <3.9 4.9 – 6.0 – 5.1 4.0 4.7 4.5 4.7 – 4.3 11 10 – 7.2 5.5 6.0 7.8 8.1 – 7.8 8.4 8.4 – 8.6 13.7 14.0 13.2 13.4

20 18 – 42 40 40 – 47 – 106 91 95 – 133 – 86 109 109 100 100 – 195 158 163 – 244 196 199 197 199 – 192 233 229 – 198 296 293 284 282

0.8 0.7 – 1.0 1.0 1.0 – 1.1 – 1.7 1.5 1.4 – 1.3 – 1.1 1.5 1.4 1.4 1.2 – 2.3 2.2 2.2 – 3.2 2.6 2.6 2.5 2.6 – 2.3 2.9 3.1 – 2.4 4.3 4.2 4.0 3.8

353 326 – 391 385 367 – 367 – 380 379 363 – 374 – 380 363 352 370 356 – 372 391 375 – 410 400 390 579 580 – 570 599 570 – 550 565 550 569 550

63 57 – 85 85 80 – 81 – 78 81 77 – 75 – 76 73 73 76 71 – 77 87 83 – 86 90 88 88 84 – 85 91 84 – 83 96 90 96 90

16 7.5 – 13 10 10 – 61 – 56 34 31 – 7 – 24 34 22 39 33 – 103 107 102 – 109 112 112 132 114 – 111 132 122 – 87 180 168 161 148

1.0 0.8 – 0.7 1.3 0.4 – 2.0 – 0.9 0.9 0.8 <0.4 <0.4 – – 0.8 0.5 1.3 0.8 – 0.8 3.4 3.3 – 6.1 4.3 4.4 5.2 4.3 – 4.2 5.3 4.7 – 3.9 8.9 8.7 7.6 8.0

1.1 1.0 – 1.2 1.9 1.6 – 3.2 – 3.2 3.1 2.6 – 2.4 – 3 3.1 3 3.2 2.5 – 3.8 5.2 4.6 – 4.2 3.6 4.0 4.9 4.0 – 3.8 4.7 4.3 – 3.7 6.2 5.7 5.6 5.0

<2.5 <0.6 – 0.8 <2.5 1.2 – 1.0 – 1.0 <2.5 0.6 – <0.6 – 2.4 0.6 0.6 0.7 0.6 – 1 2.2 2 – 1.5 1.9 2.2 2.5 1.7 – 1.7 <2.5 1.9 – 1.7 3.4 2.7 2.6 2.3

<1.9 <1.2 – <1.2 <1.9 <1.2 – 2.2 – 2.6 3.1 2.2 – 2.1 – 2 2.7 2.3 2.5 1.8 – 3.2 5.5 4.8 – 6.0 5.7 5.8 9.0 6.5 – 6.4 8.6 7.8 – 6.7 10.7 9.7 9.7 8.7

0.2 0.1 – 0.1 0.1 0.1 – 0.4 – 0.3 0.3 0.3 – 0.2 – 0.3 0.3 0.3 0.3 0.3 – 0.3 0.6 0.5 – 0.6 0.6 0.6 0.9 0.7 – 0.7 1.0 0.9 – 0.8 1.4 1.2 1.2 1.1

0.3 0.3 – 0.4 0.4 0.4 – 0.4 – 0.7 0.6 0.6 – 0.9 – 0.9 0.7 0.7 0.7 0.7 – 0.7 0.8 0.7 – 0.8 0.8 0.8 0.8 0.8 – 0.8 2.3 2.3 – 2.2 3.7 3.8 3.2 3.2

0.8 <0.6 – <0.6 <0.6 <0.6 – 1.2 – 3.1 1.7 1.5 – 1.5 – 1.5 1.9 1.7 1.9 1.7 – 1.7 2.5 2.4 – 2.4 2.7 2.6 3.5 3.0 – 3.0 3.7 3.4 – 3.3 5.1 4.8 4.7 4.2

0.5 0.5 – 0.5 0.5 0.5 – 0.6 – 0.9 1.0 0.9 – 1.1 – 0.9 0.9 0.9 0.9 0.9 – 0.8 0.9 0.8 – 0.8 0.8 0.8 0.9 0.9 – 0.9 1.0 0.9 – 0.9 1.0 1.0 1.0 0.9

<1.2 <0.6 – <0.6 <1.2 <0.6 – <0.6 – 0.7 <1.2 0.8 – 0.9 – 0.7 0.7 0.7 0.7 0.8 – 0.8 0.7 1.0 – 1.3 1.2 1.3 1.2 1.3 – 1.1 1.7 1.5 – 1.4 1.5 1.4 1.4 1.0

A22 A21 A20 A19 A18 A17 A16 A15 A14 A13 A12 A11 A10 A09 A08 A07 A06 A05 A04

Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss.

– = Not determined. Concentrations are in lg/L, except Si in mg/L.

of country rocks, followed by homogenization of the contributions in the main river. This interpretation would be supported by the good positive relation of the two elements with aqueous silica in November (Fe–Si, r = 0.84; Al–Si, r = 0.74), but does not explain the lower correlation coefficients in June (Fe–Si, r = 0.72; Al–Si, r = 0.60). A disturbing factor could be the flourishing of diatoms in the warm water during summer (Montagnes and Franklin, 2001), which may account for the removal of variable silica amounts from solution. The growth of diatoms should have been particularly developed in the Valdarno Inferiore waters, which show summer silica contents dramatically lower than in autumn. Tributaries do not show any Fe and Al time-spatial-trend, with the highest concentrations of 1000–1700 lg/L being displayed by the Egola stream both in autumn and summer. The sharp increases of Fe and Al in the Arno River (and tributaries) passing from the Valdarno Medio to the Valdarno Inferiore (at Ponte a Signa station A12, about 150 km from the source) correspond to an abrupt change in the lithology from siliciclastic dominated to clay-sand (lacustrine-marine)-dominated, thus supporting a main lithological control of Fe and Al in the catchment waters, and a subordinate role of human activities. Much higher concentrations in the Arno River are observed in June for Fe and Al especially downstream of Ponte a Signa (that is about 4 km from Florence). Considering the similar flow rates, the water temperature is possibly a critical factor, which determines the size of particles and their tendency to settle. The water

temperature in June was about 10 °C higher than in November, and may have promoted the formation of more breakable (in terms of size reduction) flocs, thus favoring smaller and lighter colloid particles in solution (Fitzpatrick et al., 2004). Iron and Al decrease sharply in the final part of the Arno River affected by seawater downstream of Pisa (autumn samples), due to flocculation phenomena promoted by salinity; in fact both elements were found to be enriched in local bed-sediments (Dinelli et al., 2005). In these waters, flocculation determines a decrease of the SI values relative to clay minerals and hematite, suggesting that both Al and Fe are likely present in solution as colloids rather than minute mineral particles. 4.3. Human impacts and Natural baseline The downstream concentration profiles of possible technogenic elements B, Ni, Cu, Cr, Zn and Mn along the Arno River and their concentration at the tributary confluences are shown in Fig. 6. At the observed Eh–pH values, Cr in all the waters sampled in June 1997 should be present totally as Cr (VI). In November 1996 Cr (III) occurs in the Arno water sampled in the final tract from Santa Croce (A08) to Caprona (A04) with percentages from 26% to 52%, and in the Usciana (58%) and Ombrone (82%) tributaries. In contrast, Mn in solution in both the Arno River and tributaries is exclusively in the form of Mn (II). The profiles of other elements possibly affected by anthropogenic contributions (V, Pb, Mo, Co, Sb, As, Se

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G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

Table 4 Analytical results on unfiltered (total) and filtered (dissolved) waters from the Arno River tributaries sampled in June 1997. The tributaries include the confluence and upstream water samples. For symbols, see Table 1. Tributaries

Si

Al

Fe

Li

B

Rb

Sr

Ba

Mn

Cr

Cu

Pb

Ni

Co

Mo

V

U

Se

I

6.3 6.1 5.2 4.6 3.7 3.6 6.8 6.3 6.6 4.4 1.2 1.0 – 3.3 6.0 6.2 2.7 2.3 – 2.9 2.9 2.0 2.7 2.8 3.1 2.7 4.9 3.9 – 2.8

370 290 700 590 210 180 1090 1000 453 290 70 60 – 126 750 800 170 126 – 13 360 190 270 245 210 78 840 550 – 17

616 389 234 52 51 26 1670 1160 602 396 55 53 – 80 580 580 282 195 – 36 344 220 251 235 277 135 1040 593 – <24

26.5 26.4 91 87 <3.9 <1 16.1 15.4 27.4 26.5 <3.9 2.9 – 2.4 20.2 20.2 7.9 7.0 – 8.8 5.6 4.1 6.0 4.7 11.3 9.1 12.8 10.3 – <3.9

273 267 1300 1290 30 32 194 188 338 337 117 116 – 46 344 341 231 216 – 44 520 470 81 75 219 203 305 290 – 16

2.3 2.2 19.4 19 0.8 0.8 1.4 1.3 3.2 3.1 0.3 0.4 – 0.3 6.7 6.9 2.4 2.3 – 1.0 3.7 3.3 1.1 1.1 3.4 3.2 7.6 7.0 – 0.4

2140 2000 565 560 133 127 806 770 2450 2300 446 430 – 470 449 460 389 369 – 343 510 449 508 461 491 461 680 645 – 274

71 66 69 65 47 42 44 42 67 64 96 92 – 98 91 90 113 106 – 98 91 81 107 98 115 108 56 53 – 58

236 235 500 490 14 10 280 269 96 89 2.4 2.6 – 23 149 153 79 76 – 16 75 63 43 43 209 173 280 268 – 2.0

1.9 1.2 29 29 0.8 0.4 4.8 4.9 2.3 1.4 <0.7 <0.4 – <0.4 33 32 3.9 3.8 – 8.3 1.0 0.7 0.7 0.7 0.8 0.6 3.7 2.9 – 0.4

3.6 3.0 9.1 8.8 1.6 1.6 5.0 4.5 3.5 3.1 1.8 2.0 – 1.6 7.1 8.1 5.6 5.0 – 1.9 3.8 3.1 3.1 2.6 6.8 5.5 2.6 2.0 – <0.9

<2.5 1.1 <5 2.2 <2.5 <0.6 <2.5 1.2 <2.5 1.0 <2.5 0.6 – <0.6 <3.3 3.3 <2.5 3.1 – <0.6 <2.5 <2.5 <2.5 <2.5 0.8 <2.5 1.1 <2.5 – <2.5

8.7 7.8 19.7 19 <1.9 <1.2 9.3 8.4 6.4 6.0 <1.9 <1.2 – <1.2 7.5 7.3 3.6 3.2 – <1.9 9.4 8.2 3.4 2.9 5.6 4.6 6.5 5.5 – <1.9

0.8 0.7 2.6 2.6 0.1 0.1 1.3 1.2 0.9 0.8 0.1 0.2 – 0.2 1.0 1.0 0.5 0.4 – 0.3 0.8 0.7 0.4 0.4 0.7 0.5 1.0 0.8 – <0.1

1.3 1.3 22.1 22 0.3 0.3 1.2 1.3 1.4 1.4 0.3 0.3 – 0.2 0.7 0.8 0.7 0.7 – 0.6 1.1 1.1 1.8 1.7 1.5 1.4 0.8 0.7 – 0.2

2.3 2.2 8.4 7.9 0.9 0.9 3.0 2.7 3.6 3.2 0.7 0.7 – 0.6 2.9 <3.3 2.3 2.1 – <0.6 3.2 2.7 2 1.7 3.3 2.8 4.6 3.9 – <0.6

2.4 2.3 0.9 0.9 0.2 0.3 3.5 3.3 1.7 1.6 0.5 0.5 – 0.4 0.6 0.7 0.5 0.5 – 0.5 0.7 0.6 0.9 0.8 1.4 1.3 1.8 1.6 – 0.4

1.3 1.2 <2.5 2.0 <1.2 <0.6 <1.2 0.6 <1.2 0.7 <1.2 <0.6 – <0.6 3.0 2.2 <1.2 <1.2 – <1.2 <1.2 <1.2 <1.2 <1.2 1.4 1.9 0.7 1.4 – <1.2

II II-1 III IV V V-1 VI VII VII-2 VIII IX X X-2 XI

Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss. Total Diss.

– = Not determined. Concentrations are in lg/L, except Si in mg/L.

and U) are not given because their concentrations were usually very low. In the Arno River, Mn and B show similar profiles that mimic those observed for Fe and Al, thus suggesting a common, mainly natural origin of these elements. Boron concentration steeply increases in the final tract of the river due to the influence of seawater in this part of the river as confirmed by a variation of the B/Cl (weight ratio) from 7  10 4 just before Pisa to 3.9  10 4 close to the mouth, compared to 2.3  10 4 in seawater (Bruland and Lohan, 2004). In tributaries, relatively high concentrations occur in the Usciana, with values up to 490 lg/L for Mn and 1120 lg/L for B. These concentration spikes may denote contamination, in keeping with the high concentrations reported recently for Mn (1.2–2.1 mg/L) and B (1.5–3.2 mg/L) in treatment effluents from the Aquarno plant (http://sira.arpat.toscana.it/sira/bandat.html; accessed 16.01.09). Similarly, the relatively high concentration of Mn in the Chiana (140–170 lg/L) may be in part related to local agricultural practices (biocides, fertilizers; Reimann and de Caritat, 1998). Manganese in the tributary waters was higher in summer, which appears consistent with lower dilution effects due to lower rainfall. On the other hand, B was depleted in these waters, this matching with its important physiological role in plants (Tanaka and Fujiwara, 2007) and the expected higher uptake during summer. In the Arno River, Cr, Cu, Zn (only autumn samples) and Ni increase downstream showing similar profiles (particularly Cu and Zn in November). The good correlations between Cr and Ni observed in June (r = 0.93) and November (r = 0.78) suggest a common source likely from ultramafic debris in sandstone, which is common in the units outcropping in the basin (Dinelli et al., 1999). These features indicate that geological sources contribute significant amounts of Cr, Cu, Zn and Ni in the main river. The only dramatic spike occurs with Ni at station A12 with a value

of 34 lg/L in November, possibly due to a point-source contamination event. In the tributaries, contaminant sources may cause, specifically: Cr in the Usciana (27–29 lg/L) and Ombrone (15– 32 lg/L), Cu in Chiana (15 lg/L), Zn in Usciana (34 lg/L) and Ombrone (40 lg/L), and Ni in Usciana (16 lg/L) and Greve (17 lg/L). This interpretation is supported by the Usciana tributary, where discharges of treated wastewaters with up to 230 lg/L Cr, 290 lg/L Ni and more than 200 lg/L Zn are documented (http://sira.arpat.toscana.it/sira/bandat.html; accessed 16.01.09). Copper and Ni in the tributaries were depleted in summer probably due to adsorption and settling effects by colloids. In contrast, higher Cr concentrations occur in summer samples in the Usciana and Ombrone tributaries, as well as in the Arno River downstream of S. Croce. According to speciation modelling, this can be related to the decrease of Cr (III) as hydroxide in these waters. The influence of seawater downstream of sample A04 is evident on Cr and Cu, whose concentrations fall below DL by dilution and possibly co-precipitation with Fe and Al oxy-hydroxides. This is not the case for Ni and Zn, likely due to contamination by the intense local shipyard activity. In the Arno River, concentrations of As, Co, Sb and U are usually lower than 1 lg/L. In the Valdarno Inferiore, concentrations of Mo, Pb and V may reach 4–5 lg/L, and Sb 2 lg/L. In the tributaries, relatively high concentrations of Mo, V and Co (22, 8 and 2.6 lg/L, respectively) occurred in the Usciana, Sb (5.6 lg/L) in the Ombrone, and As (3.6 lg/L) in the Chiana, that might all be partially of anthropogenic origin. Analytical uncertainties (i.e. potential interferences on the determination of V) and deterioration of the DL values (Se, and especially Pb) in diluted samples, caused undetectable concentrations in many cases. Therefore, these three elements will be not further discussed in terms of sources and transport.

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B

Ba

Sr

total

diss.

200

400

600

800

1000 1200

500

1000

1500

2000

2500

Cr

40

60

80

100

Mo

V

total

diss.

5

10

15

20

25

2

30

4

6

8

Si

5

10

15

20

Fe

Al

total

diss.

1

2

3

4

5

6

200

400

600

800

1000

Mn

1200

500

1000

1500

Rb

Li

total

diss.

100

200

300

400

500

20

40

60

80

Co

100

5

120

10

15

Cu

U

total

diss.

0.5

1.0

1.5

2.0

2.5

2

4

6

8

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Se

Ni total

diss.

Pb 5

10

15

1.0

1.5

2.0

2.5

3.0

1.0

1.5

2.0

2.5

3.0

Fig. 3. Box plots for shown elements between total and dissolved samples, collected in 1997 in the Arno River and tributaries. Each box includes the 25th and 75th percentiles with the median displayed as a line, outliers as circles. Box plot for Zn was not reported as many concentration data are undefined. Concentrations are in lg/L, except Si in mg/L.

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G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

Fe (µg/L)

Si (mg/L)

km

Al (µg/L)

km

km Fig. 4. Downstream concentration profiles along the Arno River for Fe, Al and Si, and concentration values in the tributaries (X = Chiana; VI = Ombrone; IV = Elsa; III = Egola; II = Usciana; I = Era) close to the confluence point. The data are compared with the Drinking Water European Regulation (EUreg; Council Directive 98/83/EC), and the available URWA (Unpolluted River World Average) concentration (Gaillardet et al., 2003). Marks FI and PI refer to the sampling stations just before Florence and Pisa, at 131.5 and 229 km from the source, respectively.

The role of contamination of other trace elements of technogenic interest (Ag, Bi, Cd, Hg, Te and Tl) cannot be assessed because concentrations are found below DL values (0.1–0.5 lg/L) at the scale of the whole basin. However, it must be noted that six samples (Arno River at Ponte a Signa and Caprona, and Chiana, Sieve, Bisenzio and Ombrone tributaries) show Hg contents in the range of 1.1– 2.3 lg/L, i.e. higher than 1 lg/L representing the mandatory upper limit in surface waters (Italian regulation n. 152, 2006; see later).

4.4. Geogenic trace elements: Rb, Li, Sr and Ba These elements should be basically geogenic in the Arno River catchment waters. Their downstream concentration profiles are shown in Fig. 7. Average concentrations of Rb in the Arno River are significantly different upstream and downstream of Florence, in keeping with the chemical composition of tributaries. Among these latter, the

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G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

1400 Arno 96 1200

r = 0.98

Arno 97

1000 800

r = 0.95 600

Al (Og/L)

400 200

0 1800

Tributaries 96

1500

Tributaries 97

r = 0.90

1200 900

r = 0.90

600 300 0 0

500

1000

1500

Fe (µg/L) Fig. 5. Al vs Fe plots in the Arno River and tributaries, with regression lines and correlation coefficients.

highest concentration was shown by the Usciana in November (17.8 lg/L Rb). The relationships with K (data from Cortecci et al., 2002) are good both in the Arno River and tributaries, in keeping with a common natural source of both elements. The distinction of the Rb–K data with respect to Florence, and the downstream highest values, indicate a lithological control of Rb in these waters. Lithium behaves like Rb, including its high concentration in the Usciana tributary (57–116 lg/L). Its relatively high concentration in Elsa and Era tributaries (16–28 lg/L) cannot be related to the presence of evaporites in the sub-basin, as these rocks are practically devoid of Li. This element should derive from interaction of water with silicates. Thermal spring waters spread in the Elsa and Era sub-basins show Li contents as high as 319 and 479 lg/L, respectively (Bencini et al., 1977), so feeding the stream network. The Li/Cl ratio in the Arno River shows downstream decreasing trends (not shown), nearly parallel in November and June. The variation is mainly controlled by Cl, which in turn should be largely controlled by the downstream increasing anthropogenic load (industry and population), whereas Li should be basically geogenic. In the final part of the river, seawater ingression is the controlling factor. Spikes at Fucecchio and Santa Croce in 1996 are related to the upstream Elsa tributary. In June, these spikes are not observed, possibly because of a lower flow in the Elsa tributary. The Sr/Cl ratio behaves comparably, including the above mentioned spike values, in agreement with the natural origin of both Sr and Li in the Elsa sub-basin waters. Strontium concentrations around 370 lg/L in June and 410 lg/L in November were observed in the Arno River up to the confluence of the Elsa tributary, after which the concentration increased to 580 lg/L and 680 lg/L, respectively. The Sr concentration in the Elsa tributary at the confluence was 2300 lg/L in June (and 2670 lg/L at 36 km upstream) and 2700 lg/L in November. In this

tributary Sr derives substantially from dissolution of evaporites, as also supported by its positive correlation with aqueous SO4 and the S isotope composition of the latter (d34S = 11.6–12.9‰; Cortecci et al., 2002, 2007). Evaporite dissolution was also involved in the water feeding the Era tributary, as indicated by high Sr (2000 lg/ L) and S isotope composition (d34S = 5.9‰) in June compared with those observed in November (790 lg/L Sr; d34S = 8.5‰). The Egola tributary also showed a high Sr content of 770 lg/L in June, but the d34S of aqueous SO4 ( 6.2‰; Cortecci et al., 2002) excludes evaporite dissolution as a source, in keeping with the absence of this kind of rock in the sub-basin. Detrital-organogenic limestone outcrops in the upper reaches of the sub-basin, and this may be the source of Sr in the Egola tributary. The lower Sr concentration in November (530 lg/L) can be attributed simply to dilution effects, as also supported by the constancy of the d34S of aqueous SO4 ( 6.6‰; Cortecci et al., 2002). The Ba profile along the Arno River is strictly controlled by tributaries, particularly those draining siliciclastic terrains, like the Sieve and Bisenzio on the right side and Chiana, Greve and Pesa on the left side of the Arno River, thus suggesting a natural origin of Ba from drainage of sandstones and alteration of plagioclases. All tributaries are saturated to supersaturated with respect to barite. In the main stem, waters vary downstream from slightly undersaturated to supersaturated, with similar SI profiles in November 1996 (higher values) and June 1997 (lower values). The SI values appear to be controlled by Ba concentrations in the Valdarno Superiore-Medio from the source (SI = 0.25–0.39), and by SO4 concentrations (Cortecci et al., 2002, for data and distribution) in the Valdarno Inferiore (SI = 0.09–0.70). In the final part of the Arno River, seawater intrusion affects Rb, Li and Sr, greatly increasing their concentration, in agreement with their relatively high concentration in seawater. Accordingly, weight ratios to Cl in the Arno River water near to the mouth (7.5  10 6 Rb/Cl; 1.4  10 5 Li/Cl; 6.4  10 4 Sr/Cl) approach the values in seawater (6.1  10 6 Rb/Cl; 0.9  10 5 Li/Cl; 4.1  10 4 Sr/Cl; Bruland and Lohan, 2004). 4.5. Downstream variations of trace elements in the tributaries This section deals with the Chiana, Bisenzio, Ombrone, Pesa, Elsa, Era and Usciana tributaries sampled in summer at two or three sites. Discussion focuses on dissolved concentrations of Cu, Zn, Cr, Ni, Mn, Al, Fe and B, which are elements of environmental concern and at the same time show appreciable to substantial positive or negative downstream variations (see Table 4). Downstream increasing concentrations of bracketed elements are found in the Usciana (Cu, Zn, Mn, Ni, Fe, Cr, Al, B), with Cr (0.5–30 lg/L), Mn (10–490 lg/L), Fe (26–1040 lg/L), Al (180– 590 lg/L) and B (32–1290 lg/L) showing variations up to two order of magnitude; the Ombrone (Cu, Mn, Cr, Al, B), particularly for Mn (2.3–153 lg/L), B (25–341 lg/L) and Al (43–800 lg/L); the Bisenzio (Fe, Mn, Al and B), these elements varying by an order of magnitude from tens to hundreds lg/L; the Elsa and Era, with Mn, Fe and Al increasing from tens to hundreds of lg/L; and the Pesa (B), the element increasing from 46 to 116 lg/L. These progressive accumulations of elements can be in part justified by the increasing human load along the sub-basins. Notable downstream decreasing concentrations occur in the Pesa (Mn, Fe, Al) and Chiana (Cr, Mn, Fe, Al, B). These trends may indicate upstream contamination followed by downstream dilution of the contaminants. For other elements, major downstream enrichments occur with Rb and Mo (Usciana) and Sr (Era), whereas major downstream depletions were observed for Li (Usciana), Sr (Elsa) and Ba (Chiana). The higher V content in the Usciana near the confluence both in 1996 and 1997 was possibly influenced by the much higher Cl

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B (µg/L)

Ni (µg/L)

G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

1997-06

Zn (µg/L)

Mn (µg/L)

Cr (µg/L)

Cu (µg/L)

1996-11

km

km

Fig. 6. Downstream concentration profiles along the Arno River for B, Ni, Cu, Cr, Zn and Mn, and their concentrations in the tributaries close to the confluence point. The data are compared with the (EUreg) and (ITAreg; low by decree n. 31 of February 2, 2001) for drinking water, and URWA concentration (see Fig. 4 for EUreg and URWA acronyms and references), as well as with the Freshwater Fish Life Regulation (Italian Public Law by decree n. 152 of April 3, 2006; F-Sreg for salmonids; F-Creg for ciprinides). For symbols, marks FI and PI, and Roman numerals, see Fig. 4.

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G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

X

10000

IV III II I

VI

X

100

VI

IV III II I

Rb (µg/L)

Sr (µg/L)

10

1000

1996-11

URWA = 1.63 1996-11

1

1997-07

1997-07

0

50

0.1 100

150

200

0

250

50

100

X

150

200

250

km

km

140

PI

FI

PI

FI

URWA = 60

100

IV III II I

VI

X

100

Ba (µg/L)

VI

IV III II I

Li (µg/L)

120

100 10 1996-11

80

1996-11

1997-07

1997-07

60

URWA = 1.84

1 40

20

URWA = 23 PI

FI

PI

FI

0.1

0 0

50

100

150

200

250

km

0

50

100

150

200

250

km

Fig. 7. Downstream concentration profiles along the Arno River for Sr, Rb, Ba and Li, and their concentrations in the tributaries close to the confluence point. The data are compared with the URWA concentration limits (see Fig. 4 for the acronym). Symbols, marks FI and PI, and Roman numerals, as in Fig. 4.

content (due to effluents from a treatment plant), and therefore may have been overestimated. 4.6. Elemental relationships with iron At the observed Eh and pH values, the predominant Fe-species in the studied unfiltered or filtered waters should be Fe(OH)3 in the form of colloids. The adsorption of ions onto these particles is a function of several factors, including the ionic potential of the sorbed element. Therefore, a good correlation with Fe should be

indicative of significant adsorption for a specific element in solution. Not considering the seawater influenced A1–A4 samples (and some largely anomalous samples), the correlations between Fe and Cu, Ni, Co, Mn and Cr in the Arno River (Fig. 8) are significant with r values of 0.7 (Cr; 1996 samples) to more than 0.9 (Co; 1996 and 1997 samples). Significant correlations (not shown) are obtained also with Al. These features indicate that these ions are associated with Fe–Al colloidal particles. The latter should have been more abundant in the November water samples in keeping with the higher flow of the river, thus yielding the higher slopes

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G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

40

10

Arno 96

A12

Arno 96 A12

Arno 97

Arno 97

A11

8

r = 0.92

6

Ni (µg/L)

Cu (µg/L)

30 r = 0.79

4

20 r = 0.85

r = 0.91

10 2

0

0 0

500

1000

0

1500

500

1.4

200

r = 0.97

1500

r = 0.87

180

r = 0.95

1.2

1000

Fe (µg/L)

Fe (µg/L)

160 140

Mn (µg/L)

Co (µg/L)

1.0 0.8 0.6

r = 0.86

120 100 80 60

0.4

40

Arno 96

0.2

Arno 96 20

Arno 97

Arno 97

0

0.0 0

500

1000

0

1500

500

Fe (µg/L)

1000

1500

Fe (µg/L) 10

r = 0.92

9 8

Cr (µg/L)

7 r = 0.69

6 5 4 3 2

Arno 96

1

Arno 97

0 0

500

1000

1500

Fe (µg/L) Fig. 8. Relationships of Cu, Ni, Co, Mn and Cr relative to Fe in the Arno River, with regression lines and correlation coefficients. Samples affected by seawater (A01–A04) are excluded. Regression does not include labeled samples.

observed for the relationships in November with respect to June (Fig. 8). This means that the increase in sorbed metals is directly related to the fraction of colloidal Fe (60.45 lm). The correlation of Cr with Fe in 1997 is also significant with r = 0.92. In these samples Cr should be present basically as CrO24 , so supporting the idea that this anion can be appreciably sorbed even at near neutral pH (e.g. Rai et al., 1989). Lithium and Rb behave similarly, displaying good correlations with Fe (r = 0.56 to 0.87), and higher slopes in November. Strontium and Ba do not show any significant correlation with Fe. Also,

B, V, Mo, U, As, Se and Sb do not show any clear-cut correlation with Fe as they, at the measured pH of 7.3–8.2 and Eh of 0.32– 0.53 V, should be present in solution as negatively charged, large and low ionic potential oxy-ions, which may undergo substantial sorption only at low pH. The relationship of a specific trace element with Fe in the tributary waters might be altered by point enrichment (natural and/or anthropogenic) occurring along the stream, considering that the Fe-particles in the water can adsorb a portion of the added element, this resulting in high ‘‘anomalous” element/Fe ratios.

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Perhaps for this reason, defined relationships with Fe were not found even for the divalent metallic ions. The only exception being Mn (r = 0.91 and 0.80 in 1996 and 1997, respectively), testifying to a common major source with Fe.

tical treatment of trace and major elements represents an intriguing test. PComp and PCord analyses (Fig. 9) of the Arno River 1996–1997 datasets, compared with the 1989 one (Bencini and Malesani, 1993), yield the main features summarized below:

4.7. Italian and EU regulation limits Based on the Italian public law by decree n. 31 of February 2, 2001 (see also decree n. 152 of April 3, 2006), that received the EU Directive 98/83/CE on the quality standards for surface water intended for human consumers, the concentrations of Fe and Al (Fig. 4) and Mn (Fig. 6) in the Arno River and tributaries were often higher to much higher than guidelines, particularly in the Valdarno Medio-Inferiore. The Cu, Zn, Cr (total), Ni and B were lower than the currently acceptable values (Fig. 6). The only exception was Ni (34 lg/L) at Ponte a Signa (A12) in November 1996. No health protection level has been issued by regulatory agencies for Cr (VI) in surface waters, this Cr species prevailing in these waters. It can only be observed that the concentrations in the main stem and tributaries downstream of Florence are higher to much higher than the acceptable limits of 5 lg/L and 11 lg/L set for groundwater by the Italian law by decree n. 471 of October 1999, and EPA ‘‘Ambient Water Quality Criteria” (ATSDR, 2000a), respectively, but lower than the limits of 20 lg/L reported for drinking water by Merian et al. (2004) and 50 lg/L by the WHO and EPA ‘‘Water Quality Criteria” (ATSDR, 2000a). The concentrations of As, B, Se, V and Cd in both the Arno and tributaries are lower than the acceptable values (10 lg/L for As; 1 lg/L for Hg; 10 lg/L for Se; 5 lg/L for Cd; 50 lg/L for V) established by the Italian public law by decree n. 31 of February 2, 2001. As pointed out above, exceptions occur for Hg which is slightly above the limit at 6 sites. With reference to the use of the Arno River water just upstream of Florence for potable purposes, it has to be remarked that all the above elements complied with regulations. Very recently, a list of priority substances to be monitored in surface waters was published in the EU Directive 2008/105/EC of December 16, 2008, dealing with environmental quality standards in the field of water policy. This list, along with many organic compounds, includes some metals (and their compounds) such as Cd, Pb, Hg and Ni. New guidelines were established for Cd (60.45– 1.5 lg/L, depending on hardness) and Hg (0.7 lg/L). It may be of interest to compare the concentration of potentially toxic elements with those accepted for freshwater fish life (Italian public law by decree n. 152 of February 3, 2006). The levels were within the limits for As (<50 lg/L), Cu (<40 lg/L), Zn (<300– 400 lg/L), Ni (<75 lg/L), Pb (<10–50 lg/L) and Cd (<2.5 lg/L). For salmonids (20 lg/L; 100 lg/L for ciprinides), unacceptably high Cr values of 29 and 32 lg/L were found in the Usciana and Ombrone tributaries, whereas in several sites along the tributaries and Arno River, Hg was too high (>0.5 lg/L) for both salmonids and ciprinides. When compared with the available URWA (Unpolluted River World Average) concentrations (Gaillardet et al., 2003), determined elements show higher concentrations (e.g. Table 2; Fig. 4, 6 and 7). Some exceptions are Mn, As and Rb especially at sites upstream of Florence. 4.8. Multivariate analyses: natural vs anthropogenic contributions of elements Statistical treatments of the trace elements were carried out along with the major elements. According to Gaillardet et al. (2003), the coupling of trace elements with major ions in rivers should support a natural origin of the former, provided that the latter are natural rather than anthropogenic. This is not the case for the Arno River system, where locally some major elements in solution derive from human activities (e.g. Fig. 2). Therefore, the statis-

1. Apart from small differences due to Varimax rotation in PComp, the distributions of samples in the PComp and PCoord plots are the same. 2. Samples near Florence are easily distinguished, and constitute a sharp demarcation between the upstream less contaminated part of the river and the downstream more contaminated part, where strongly contaminated tributaries (Bisenzio, Ombrone and Usciana), dissolution of evaporites (Elsa and Era sub-basins) and seawater ingression notably influence the river chemical composition. 3. The PComp and PCoord plots discriminate quite well between Ca–HCO3, Na–HCO3 and Na–Cl waters. The enrichment in Na (and metals) of HCO3 waters just below Florence clearly indicates a localized significant influence of the city on the chemical composition of the river due to municipal discharges. 4. The distance from source variable (DS) groups with Mn, Zn and Ni, thus implying a progressive downstream contribution of these metals to the river, instead of point sources. The DS variable does not correlate with the major ions Na, Cl, SO4, Ca, K and Mg, thus not supporting a progressive accumulation of the latter. The roles of natural and anthropogenic sources in the chemical budget of the river are impossible to define on statistical bases only. An evaluation of these roles for SO4 in the studied waters was carried out by Cortecci et al. (2002), combining chemical and S isotope composition data, and assuming that Cl and Na are basically anthropogenic (except in the vicinity of the mouth). 5. The discriminating roles of Fe and Cu are due basically to their increased concentrations in the river between the Ombrone and Usciana tributary outlets. On the other hand, the different distribution of Pb in the PComp 1989 and 1996–1997 may be ascribed to the introduction of unleaded gasoline in 1992. In the PComp analyses of 1996 and 1996 + 1997, Pb clusters with SO4, this suggesting that an important part of Pb in the Arno River may derive (downstream of Santa Croce) from the plants treating waste waters from tanneries and domestic sewages in the leather district. In fact, high mean annual concentrations of Pb (110 lg/L; Cuoiodepur plant; http://www.cuoiodepur.it/ impianto/descrizione.htm.) and SO4 (1200–1600 mg/L; Aquarno plant; e.g. Aquarno WWTP brochure 2005) are released by these plants. When the tributaries are included in the sample PComp (unstandardized) treatment (Fig. 10), the following features can be seen for the Arno River and tributaries (1996+1997 samples (1) the Elsa and Era are paired in keeping with the common origin of Ca, Sr and SO4 from dissolution of evaporites; (2) outliers with respect to the centroid include the contaminated tributaries, i.e. the Usciana, (anomalous Na, Cl and SO4 from treated wastes from tanneries and paper-mills), the Ombrone (anomalous Cu, Fe and Mn from treated wastes from textiles and nurseries) and the Chiana (anomalous Zn, Cu, Ni and NO3 from breeding and agriculture). The Egola tributary should also be included in this group, the major contaminants in this stream apparently being Fe and Al. This interpretation matches with the low position of the tributary in the ecological rank elaborated by ARPAT (2002) for the Arno River Basin waters on the basis of chemical and biological parameters. Other outliers are the river samples collected at the estuary, highly affected by seawater; (3) the elements Fe and Al are closely associated on the PComp2 axis, in agreement with their significant

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3.0

1.0

2.5

0.8

totN

Zn

Pcomp2 (28.096 %)

cAlk/Ca > 1 cAlk/Ca < 1

1.5 1.0 0.5

FI

0.0 -0.5

-1.5 -1.5

-0.5

0.0

0.5

Pb

-0.4

A2

A1

1.0

1.5

2.0

2.5

3.0

-1.0 -1.0

3.5

-0.8

-0.6

-0.4

0.0

0.2

A5

FI

A4 A13 FI

-0.5

Zn

Ca

Pb

NO3 SO4

0.2

Na K

0.0

Mg

-0.2

Cl

-0.8

from source to Chiana samples

B2

B1 1.0

1.5

2.0

2.5

3.0

-1.0 -1.0

3.5

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Pcomp1 (61.433 %)

2.5

1.0 A12-96

cAlk

Cu

0.8 A5-97

DS Mn

Ni

0.6

1.5

Ca Pb

Fe A11-96

1.0

Pcomp2 (31.147 %)

Pcomp2 (31.147 %)

Mn

Fe

0.4

Pcomp1 (61.433 %)

PI

0.5 0.0 FI-96

-0.5 -1.0

DS

-0.6

-1.5

2.0

1.0

-0.4

Arno estuary

-1.0

0.5

0.8

cAlk

0.6

PI

0.0

0.6

Ni

0.5

-0.5

0.4

Cu

0.8

Pcomp2 (23.178 %)

1.5

Pcomp2 (23.178 %)

-0.2

Pcomp1 (53.049 %)

Pcoord2 (19.353 %)

2.0

-2.0 -1.0

Fe

-0.2

1.0

0.0

Mn Mg

0.0

A12

1.0

Ca SO4

Cl

0.2

Pcomp1 (53.049 %) 2.5

Na

-0.8

from source to Chiana samples

-1.0

cAlk

0.4

-0.6

Arno estuary

-1.0

K DS

Cu

0.6

2.0

Pcomp2 (28.096 %)

Ni

0.4

SO4

NO3

0.2 Mg

0.0

NaK Cl

-0.2 -0.4

FI-97

-0.6

-1.5

-0.8

C2

C1 -2.0 -1.0 -0.5 0.0 0.5 1.0 1.5

2.0 2.5 3.0 3.5

4.0 4.5 5.0

Pcomp1 (45.071 %)

-1.0 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Pcomp1 (45.071 %)

Fig. 9. Standardized PCA-component (Pcomp) and PCA-coordinate (Pcoord) analyses of the Arno River 1989 (A), 1996 (B) and 1996 + 1997 (C) samples and variables (major and trace elements). The 1996–1997 data grouping was decided because Zn and estuary samples were not analyzed in 1997. Pcoord plots are shown as insets in the Pcomp plots. Open square: Ca–HCO3 waters; open diamonds: Na–HCO3 waters; open triangles: Na–Cl waters. Variable DS is the distance of the sampling stations from the river source. The treatment deals with a few selected trace elements, this to obtain an adequate ratio between samples and variables.

correlation observed in Fig. 5. Mn, Co and As also plot close to the PComp2 axis, probably because their concentration in the water is influenced by adsorption phenomena on colloids (ATSDR, 2000b,

2001), like Fe- and Al-hydroxides. The proximity of Si and U (and in part U-cAlk) indicates a control on the U in solution by suspended clay and carbonate complexes, the latter being supported

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4

1.0

Elsa

river

Egola

tributaries

3

U

Fe

Si

0.8

Ca-bicarbonate Ca-sulfate

0.6

Ca

As

Al

Sr

cAlk

SO4 Co

1

Pcomp2 (22.843 %)

Pcomp2 (22.843 %)

Na-chloride 2

Ca-sulfate dissolution Ombrone A8 estuary

Usciana

Era

0 Chiana

A2

anthropic input / seawater entry

-1

A3

source

A1

0.4

0.0

Li Ni

Sb

Zn

Pb Mg Rb V B

Mo

K

COD

Na Cl

-0.2 Ba

-0.4 -0.6

FI

-0.8

Salutio

A2

A1 -1.0 -1.0

-2 -1

0

1

2

3

4

-0.8

-0.6

-0.4

-0.2

5

1.0

river

tributaries

Al

Ca-bicarbonate

Egola

4

0.2

0.4

Cr As

0.6

Ca-sulfate

Pcomp2 (27.227 %)

3 Na-chloride

A4-97

Chiana

A7-97 Ombrone Usciana

1

0.8

1.0

Co Mn

A5-97

0.6

Fe

0.8

Na-bicarbonate

2

0.0

Pcomp1 (58.838 %)

Pcomp1 (58.838 %)

Pcomp2 (27.227 %)

NO3 Cr

Cu

0.2

Mn

Elsa

Cu

0.4

Ni Mo

Li Pb B

cAlk

0.2

COD

0.0

Si

U

Rb Na Cl

NO3

-0.2

V

Ca SO4 Mg Sr

K

Ba

-0.4

0

estuary

Era

-0.6

-1 FI

source

-0.8

B2

B1 -2 -1

0

1

2

3

4

5

Pcomp1 (44.675 %)

-1.0 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Pcomp1 (44.675 %)

Fig. 10. Unstandardized PCA-component (Pcomp) analyses of the Arno River and tributaries for 1996 (A) and 1996 + 1997 (B) samples and variables.

by computed ion speciation; (4) COD and NO3 are positively correlated due to their common origin mostly from anthropogenic sources; (5) the elements Pb, B and V plot together with major ions like Na and Cl, all possibly being substantially controlled by human activities. The above features match those recognized in the all inclusive 1996+1997 HCA cluster diagram (not shown), which outlines the following groupings of elements: Pb and V associated with Na and Cl, B coupled with Li and Rb; Ni, Cu and to a lower extent Ba, all related with COD and NO3, the presence of Ba in this group may be related to COD and Ba-organic complexes in solution; then the paired Al and Fe, and the Si, cAlk, U and As grouping. 5. Conclusions On the basis of two sampling campaigns carried out in November 1996 and June 1997, the geochemical behaviour of trace elements in the Arno River and its main tributaries was investigated. Main conclusions derived from this study are summarized below. 1. Comparing results on filtered (0.45 lm) and unfiltered water sampled in June, removal percentages were higher for Fe and Al, lower for Si, Mn, Cr, Cu, Pb, Ni, Co and V, and negligible for Li, B, Rb, Sr, Ba, Mo and U. On average, 70% Fe, 80% Al and 90%

Si should occur in the studied waters as colloidal particles of <0.45 lm size. 2. Iron and Al show significant positive correlations (r = 0.95–0.98 in the Arno River; r = 0.90 in overall tributaries). In the Arno River, they show perfectly synchronous downstream profiles with higher concentrations in the lower part, and are more abundant in summer than in autumn. The sharp increases of Fe and Al in the Arno River (and tributaries) passing from the Valdarno Medio to the Valdarno Inferiore (near to Florence) corresponds to an abrupt change in the lithology from siliciclastic dominated to clay-sand (lacustrine-marine)-dominated, in keeping with a predominant lithological control. The same behaviour is observed for other trace elements (e.g. Mn, Cr, Co, Ni, Cu, B, Rb), with Florence city acting as a divide between lower concentrations upstream and higher concentrations downstream. 3. The elemental relationships with Fe in the Arno River are generally good for Mn, Cu, Ni, Co, Li and Rb, as expected due to adsorption phenomena. The elements Sr and Ba do not show any correlation, along with those elements present in solution as oxy-anions (B, V, Mo, U, As, Se and Sb). Chromium correlates with Fe when CrO24 is the dominant species in solution. Pointsources (natural or anthropogenic) along the main stem and tributaries can alter the relationship, resulting in anomalously high element to Fe ratios.

G. Cortecci et al. / Applied Geochemistry 24 (2009) 1005–1022

4. Based on the Italian and EU regulations the concentrations of elements important for human health were lower than the currently acceptable values, some exceptions being Mn, Fe and Hg in the Valdarno Medio-Inferiore. The same is valid for fish life (salmonids and ciprinides), the unacceptable concentrations being Cr in the Usciana and Ombrone tributaries, and Hg at six sites along the main river and the tributaries. 5. Binary plots and trace element profiles do not simultaneously define the similarities between elements or samples. Otherwise, standardized and unstandardized multivariate analyses produce variables and samples clustering with similar results, discerning similarities among the variables and samples and helping to define the natural or anthropogenic sources of the elements. Finally, attention has been paid to the EU water policy aimed at achieving low pollution levels and ecosystem health for EU surface water and groundwater bodies by 2015 (Water Framework Directive; EU Directive 2000/60/EC). Because contamination can evolve to pollution and related adverse effects to humans and biota, its level must be assessed and the contaminant sources, as well as the short- and long-term contaminant trends, must be identified. Then, appropriate actions to protect and restore clean water must be undertaken. Although results from this investigation generally show compliance with the EU water policy, further work would be necessary to identify the water bodies at risk in the Arno River Basin. Specifically, a detailed chemical and isotopic characterization of geogenic and anthropogenic sources of elements at the basin scale would allow the natural baseline levels to be estimated and the anthropogenic contributions pinpointed. This requires enough funds to built up robust reference datasets for each source of contaminants. Another limitation is due to the poor Cd and Pb detection limits (DL values) being unable to comply with the EU recently established goals (i.e. Directive 2008/105/EC), but this should be overcome using ultra-pure instead of supra-pure reagents. Acknowledgments This paper is part of a sub-research project named ‘‘Concentration and speciation of heavy and toxic elements in waters from the Arno River watershed” (led by L. Fanfani), within the coordinated project ‘‘Geochemistry of surface waters and groundwater in riverine catchments undergoing environmental risk”, led by G. Cortecci and sponsored by the CNR of Italy. Some data on the November 1996 water samples were presented in a preliminary form by Cortecci et al., 1998 at the Water Rock Interaction Symposium (WRI-9), held at Taupo (New Zealand) under the splendid general secretaryship of Brian W. Robinson. The authors greatly appreciate the constructive comments and suggestions of W.M. Edmunds and an anonymous reviewer. Our gratitude to Dr. Pucci of the Centro di Monitoraggio Meteo-Idrologico di Pisa (Servizio Idrologico Regionale) for the discharge data of the Arno River at the San Giovanni alla Vena hydrometric station. References APAT-CNR IRSA 5130, 2003. Metodi Analitici per le Acque. Rapporto 29/2003. IGER, Roma; February 2004. Aquarno WWTP brochure, 2005. Impianto Aquarno-Depurazione dei reflui industriali e civili-Storia e Attualità. Centro Toscano Editoriale SRL. ARPAT, 2002. Indici di qualità dei corsi d’acqua significativi della Toscana (trend 1997–2001). Firenze; June 2002. ATSDR, 2000a. Toxicological profile for chromium. US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Atlanta, USA. (accessed 22.01.09).

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