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Applied Geochemistry 22 (2007) 2777–2794

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AppliedGeochemistry

Transport of trace elements under different seasonal conditions:Effects on the quality of river water in a Mediterranean area

Rosa Cidu *, Riccardo Biddau

Dipartimento di Scienze della Terra, via Trentino 51, I-09127 Cagliari, Italy

Received 29 September 2006; accepted 20 June 2007Editorial handling by R.B. WantyAvailable online 14 August 2007

Abstract

Concentrations of total and dissolved elements were determined in 35 water samples collected from rivers in Sardinia, aMediterranean island in Italy. The overall composition did not change for waters sampled in both winter and summer (i.e.,January at high-flow condition and June at low-flow condition), but the salinity and concentrations of the major ionsincreased in summer. Concentrations of elements such as Li, B, Mn, Rb, Sr, Mo, Ba and U were higher in summer withonly small differences between total and dissolved (i.e., in the fraction <0.4 lm) concentrations. The fact that these elementsare mostly dissolved during low flow periods appears to be related to the intensity of water–rock interaction processes thatare enhanced when the contribution of rainwater to the rivers is low, that is during low-flow conditions. In contrast, theconcentrations of Al and Fe were higher in winter during high flow with total concentrations significantly higher than dis-solved concentrations, indicating that the total amount depends on the amount of suspended matter. In waters filteredthrough 0.015 lm pore-size filters, the concentrations of Al and Fe were much lower than in waters filtered through0.4 lm pore-size filters, indicating that the dissolved fraction comprises very fine particles or colloids. Also, Co, Ni, Cu,Zn, Cd and Pb were generally higher in waters collected during the high-flow condition, with much lower concentrationsin 0.015 lm pore-size filtered waters; this suggests aqueous transport via adsorption onto very fine particles. The rare earthelements (REE) and Th dissolved in the river waters display a wide range in concentrations (

PREE: 0.1–23 lg/L; Th:

<0.005–0.58 lg/L). Higher REE and Th concentrations occurred at high flow. The positive correlation betweenP

REEand Fe suggests that the REE are associated with very fine particles (>0.015 and <0.4 lm); the abundance of these particlesin the river controls the partitioning of REE between solution and solid phases.

Twenty percent of the water samples had dissolved Pb and total Hg concentrations that exceeded the Italian guidelinesfor drinking water (>10 lg/L Pb and >1 lg/L Hg). The highest concentrations of these heavy metals were observed athigh-flow conditions and they were likely due to the weathering of mine wastes and to uncontrolled urban wastes dis-charged into the rivers.� 2007 Published by Elsevier Ltd.

0883-2927/$ - see front matter � 2007 Published by Elsevier Ltd.doi:10.1016/j.apgeochem.2007.06.017

* Corresponding author. Tel.: +39 070 6757724; fax: +39 070282236.

E-mail address: [email protected] (R. Cidu).

1. Introduction

The chemical composition of surface waterdepends on several factors including: abundanceof elements in the drainage and susceptibility of

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2778 R. Cidu, R. Biddau / Applied Geochemistry 22 (2007) 2777–2794

source minerals to weathering; climatic, morpholog-ical and hydrological conditions; speciation, sorp-tion, and other chemical processes affecting themobility of elements (Smith and Huyck, 1999). Inaddition, human activities may have an importanteffect on the source and aqueous transport of ele-ments. One or more factors may control the mobil-ity and dispersion of elements under a given set ofconditions.

For reasons unrelated to human activity, watersmay contain natural (background) concentrationsof toxic or harmful elements that exceed the stan-dards established by various national governmentsfor drinking water and for the protection of aquaticlife (USEPA, 1995; W.H.O., 2004). The assessmentof background-water quality is fundamental for reg-ulatory purposes in restoring polluted waters totheir pristine state (Langmuir, 1997), although esti-mating background conditions in populated regionsis a challenge.

The rare earth elements (REE) comprise the ele-ments from La to Lu in the periodic table, and usu-ally include Y due to its similar chemical behavior.In recent years, medical, industrial and agriculturalapplications of REE have been increasing rapidly(Haxel et al., 2002; Verplanck et al., 2005). Thereis a tendency for REE to accumulate in the soil,and eventually, be released to the aquatic system.It has been observed that several plants are able touptake and accumulate the REE (Wyttenbachet al., 1996; Chua et al., 1998; Pang et al., 2002).Prolonged environmental and dietary exposure toREE may adversely affect human health and a num-ber of diseases have been associated with exposureto REE (Sabbioni et al., 1982; Ding and Ma,1984; Zhang et al., 2000), although how they affecthuman health via the food-chain or directly throughthe water or the air remains unclear. Drinking waterguidelines are currently not available for REE.

This study investigates the influence of differentseasonal conditions on the mobility of elements,especially those usually present at trace (6lg/L) lev-els in natural waters. The study focused on riverwaters from Sardinia in Italy. Sardinia representsan ideal location for this study due to climatic con-ditions with arid summers, and mild and humidwinters, its position at the centre of the Mediterra-nean area, and the variety of rocks that outcrop ina relatively small area (20,000 km2). In addition,Sardinia contains several mineral deposits exploitedintensively from 1880 to 1980 that are potentialsources of several trace elements, including Pb,

Zn, Cd and Cu. Most mining activities ceasedbetween 1980 and 1990, and the residues of miningexploitation and processing have been left on site,commonly without adequate disposal (Cidu andFanfani, 2002).

2. Study area

Sardinia is located in the Mediterranean Seabetween 38�40 0N and 41�40 0N and between 7�50 0Eand 9�40 0E. The highest elevations of 1000–1800 ma.s.1. generally are located in the eastern part ofthe island. Sardinia has variable climatic conditions,from semiarid to semihumid. From 1922 to 1992,mean rainfall ranged from <500 mm/a in the plainsand coastal areas to 1100 mm/a in the mountains,with a mean of <50 and 90 rainy days/a, respec-tively (R.A.S., 1998). Climatic conditions in theregion are characterized by long periods of heatand drought interrupted by relatively short rainyperiods, with occasional heavy rain events. Accord-ingly, the flow of rivers and streams is stronglydependent on rainfall, and may vary by severalorders of magnitude in a year, as well as from oneyear to another. In Sardinia, surface waters consti-tute an important resource. The river water col-lected in artificial basins represents about 70% ofthe water supply for agricultural, industrial anddomestic uses for a population of approximately1.6 million people, although tourism in July andAugust may increase the population up to 3 million.

The main geologic features are shown in Fig. 1(Carmignani et al., 200lb). A Paleozoic basement,consisting of granitic and metamorphic rocks,extends nearly continuously from north to south inthe eastern part, and it is interrupted by Tertiary sed-iments and volcanic rocks in the western part of theisland. Remains of Mesozoic carbonate rocks lay onthe basement. A prominent structure, marked byimportant regional faults and known as the Sardi-nian Rift, extends from north (the Gulf of Asinara)to south (the Gulf of Cagliari). The Sardinian Rifthas been filled by volcanic rocks (Oligo-Mioceneandesite, and Plio-Pleistocene basalt), marine andlacustrine sediments (Miocene conglomerate, sand-stone, carbonate and marl sequences) and recentcontinental sediments (Carmignani et al., 2001a).

3. Sampling and analytical methods

Water sampling of 11 sites was carried out in win-ter (January 2003) under high-flow conditions and

R. Cidu, R. Biddau / Applied Geochemistry 22 (2007) 2777–2794 2779

in early summer (June 2003) under low-flow condi-tions. Sampling was performed in June becausemany rivers are nearly dry at the end of summer.Water sampling of 24 sites was carried out in Janu-ary 2003 only. The locations of water samples areshown in Fig. 1. Waters were collected from bridgesavoiding sampling where stagnant water or abun-dant vegetation was observed. At the sampling sites,the physical–chemical parameters and alkalinitywere measured; waters were filtered through0.4 lm pore-size Nuclepore polycarbonate filtersinto pre-cleaned high-density polyethylene bottles,and acidified with HNO3 (Suprapure grade, CarloErba) for metal analyses by quadrupole ICP-MS

Fig. 1. Map showing the main geological features of Sardinia (Carm

(Perkin–ElmerSCIEX ELAN 5000 and ELANDRC-e) using Rh as an internal standard. In addi-tion, B, Mn, Fe, Zn, Sr and Ba were determinedby ICP-OES (ARL Fisons 3520). An aliquot of fil-tered sample was acidified with HCl (Suprapuregrade, Carlo Erba) for the determination of Asand Sb by hydride generation ICP-MS (Cidu,1996). Dissolved components refer to those concen-trations measured in the filtered sample, that is inthe fraction <0.4 lm. At two sites (No. 15A and16A) in February 2006, an additional aliquot filteredfirst through 0.4 lm then through 0.015 lm pore-size Nuclepore polycarbonate filters and acidifiedwith HNO3 was collected for determination of

ignani et al., 2001a,b) and the locations of river water samples.

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Fig. 2. Box plots showing Sr (a), B (b), Mn (c), Li (d), U (e) and Mo (f) concentrations for filtered (F) and non-filtered (nF) waterssampled under high-flow (January) and low-flow (June) conditions. Each box includes the 25th and 75th percentiles with the mediandisplayed as a line.


elements in ‘‘true solution’’. Aliquots of non-filteredand acidified water were collected for determinationof total concentrations of trace elements. To checkpotential contamination during sampling and analy-ses, blank solutions were prepared in the field withultra-pure water (MILLI-Q, specific conductance:0.06 lS/cm) and processed using the same procedureused for the water samples.

High concentrations of Ba may cause interfer-ence, via Ba-oxide formation, for the determinationof REE (especially Eu) by ICP-MS. To minimize theinterferences, the level of oxides was maintainedbelow 2% in the DRC-ICP-MS and no correlationbetween dissolved Ba and dissolved Eu wasobserved. In addition, potential interferences werechecked by the standard addition method (Cidu,


1999) on water samples with different compositionsand also by high-resolution ICP-MS; significantinterferences were not found.

Detection limits were derived as follows: the meanvalue and standard deviation (SD) of a significantnumber of blank solutions analyzed within an analyt-ical sequence were calculated; the value correspond-ing to 5 * SD was taken as the limit. Both precisionand accuracy were evaluated by analyzing randomlyduplicated samples, certified multi-standard solu-tions (Merck ICPVI, Spectrascan REE and Th),and standard reference solutions (NIST1643c, d).Analytical errors were estimated at610% for concen-trations higher than 1 lg/L and close to 20% for con-centrations in the range of 0.01–0.1 lg/L.

Speciation calculations were carried out withPHREEQC (version 2) using the MINTEQ data-base (Parkhurst and Appelo, 1999). For the purposeof statistical analyses, data below the detection limitwere set to half the value of that limit. Data are pre-sented in Figs. 2 and 4 as box plots (Synergy Soft-ware, 2005). Each box includes the 25th (lowerquartile LQ) and 75th (upper quartile UQ) percen-tiles with the median displayed as a line. The linesextending from the top and bottom of each boxmark the minimum and maximum values that fallwithin an acceptable range; any value outside of thisrange is an outlier. Outliers are points whose valueis either lower than LQ-1.5 * IQD or greater thanUQ + 1.5 * IQD, where IQD is the interquartile dis-tance UQ–LQ.

4. Results and discussion

The physical–chemical parameters and the dis-solved concentrations of major and minor elementsin waters from Sardinian rivers are reported else-where (Cidu et al., 2007) with relevant data summa-rized below. The pH values were near neutral toslightly alkaline and ranged from 7.1 to 8.3 andthe redox potential (Eh values: 0.42–0.51 V) indi-cated oxidizing conditions. Neither pH nor Eh val-ues showed significant seasonal variations.

The specific conductivity, which ranged from 0.2to 1.8 mS/cm at high-flow, varied significantlyamong samples and depends on the dominant lithol-ogy in the watershed and the areal extent of the riverbasin. The lowest values were observed in riversdraining mainly granitic and volcanic rocks in smallbasins. The highest values were observed in riversdraining mainly sedimentary formations. Conduc-tivity generally increased from the headwaters to

the mouth of rivers; it was generally higher at lowflow.

Those waters draining silicate-rich bedrock weredominantly Na–Cl in composition and had lowtotal dissolved solids (TDS: 0.1–0.4 g/L). Thosewaters draining sedimentary-rich bedrock weredominantly Na–Ca–Cl–HCO3 in composition andhad higher TDS (0.6–1.2 g/L). Sulfate was the dom-inant anion in water from Rio San Giorgio (No.17), located downstream of a large abandoned min-ing area. In waters sampled in different seasons, theTDS and dissolved major cations and anionsshowed higher concentrations in summer.

4.1. Dissolved and total trace elements

Concentrations of minor and trace elements mea-sured in filtered (F) and non-filtered (nF) watersamples collected in January under high-flow condi-tions are reported in Table 1, and those collected inJune under low-flow conditions are reported inTable 2. Among elements not reported in Tables1and 2, a concentration of 1.2 lg/L Tl was detectedin the Rio San Giorgio (No. 17), which also con-tained high values of dissolved Zn. Dissolved Agconcentrations (up to 0.20 lg/L) were detected inthe No. 5, 11, 15 and 16 waters sampled in January,and were associated with relatively high values ofdissolved Pb (Table 1). A concentration of 12 lg/LSb was measured in the water from Flumendosa(No. 9). This sample is located downstream of thesignificant Sb deposits of Villasalto where miningactivities ceased in 1970 (Carmignani et al., 1978).

Dissolved (F) and total (nF) concentrations of B,Li, Rb, Sr, Ba, Co, Ni, Mo and U in Tables 1 and 2are similar with variations often close to the esti-mated analytical error. For waters sampled duringdifferent seasons, the range of dissolved and totalconcentrations of Sr, B, Mn, Li, U and Mo areshown in the box plots of Fig. 2a–f with outlier sam-ples shown as open circles. Similar median valuesfor dissolved and total concentrations indicate thatthese elements are transported primarily in the dis-solved load. This seems confirmed by tests carriedout using 0.015 lm and 0.4 lm pore-size filters(Table 2) that indicate very similar B, Li, Rb, Sr,Mo and U concentrations in both the <0.015 lmand <0.4 lm fractions.

Dissolved concentrations of B, Li, Rb, Sr, Ba,Mn and Mo were higher in samples collected inJune under low-flow conditions, when the riversmay receive significant inputs from groundwater.

Table 1Dissolved (filtered through 0.4 lm filter) and total (unfiltered) concentrations of trace elements in Sardinian river waters sampled in January 2003

No. Flow TDSa Al B Li Rb Sr Ba Fe Mn Zn Cd Cu Co Ni Mo Pb U As Hg(m3/s) (g/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L)

1 0.8 0.19 Fb 99 16 1.8 1.3 75 12 230 12 29 0.76 3.0 0.22 <1 <0.1 10 0.25 <1nFc 280 17 2.0 1.6 79 13 530 17 30 1.24 1.7 0.24 <1 0.11 28 0.32 <0.7

2 4 0.30 F 540 29 3.1 2.9 110 21 620 58 19 0.29 2.8 2.2 <1 0.33 5.2 0.71 <1nF 1100 33 3.5 3.9 110 23 1200 66 27 0.38 3.0 2.6 1.6 0.36 8.8 0.83 <0.7

3 80 0.26 F 240 30 2.5 1.2 120 30 290 30 13 0.45 1.9 0.28 <1 0.3 7.5 0.56 2nF 480 31 2.5 1.4 129 31 560 33 21 0.65 1.9 0.30 1.2 0.33 13.7 0.63 0.7

4 7 0.29 F 840 36 4.5 2.8 140 45 1100 121 21 0.64 3.2 0.83 2.1 0.31 18 0.57 3nF 1900 37 5.4 4.3 150 44 1900 130 30 0.77 4.2 1.0 3.1 0.29 22 0.64 <0.7

5 2 0.50 F 2700 80 5.4 6.9 350 71 2700 93 30 0.56 6.2 1.6 5.6 1.1 18 1.41 <1nF 4600 78 6.0 9.5 360 75 4400 100 40 0.6 6.7 1.9 6.7 0.88 20 1.45 3

6 7 0.08 F 51 4 0.9 0.5 35 21 95 11 11 0.25 1.6 0.15 <1 0.12 4.5 0.07 2nF 100 4 1.4 0.6 39 21 200 15 28 0.29 1.7 0.18 <1 0.18 7.4 0.11 <0.7

7 0.1 0.11 F 68 5 1.3 0.6 39 20 98 14 20 0.24 2.5 0.18 <1 0.26 1.2 0.10 2nF 170 5 1.7 0.7 44 20 280 21 20 0.31 2.3 0.25 <1 0.35 1.3 0.14 <0.7

8 0.1 0.53 F 57 20 9.5 1.5 280 30 68 23 8 <0.1 2.7 0.21 <1 0.44 <1 1.00 <1nF 11 24 9.2 1.4 290 32 82 21 10 <0.1 2.8 0.22 1.6 0.53 <1 1.11 <0.7

9 3 0.27 F 22 30 4.3 0.6 120 44 81 3.2 35 0.57 2.1 0.09 <1 0.68 16 0.39 2nF 22 30 4.5 0.6 140 53 230 4.2 38 0.8 2.5 0.13 1 0.69 27 0.40 0.7

10 2 0.17 F 29 20 2.4 0.5 62 44 110 15 15 0.21 1.5 0.17 <1 0.53 4.7 0.04 2nF 97 27 2.7 0.6 68 46 210 16 15 0.35 1.1 0.17 <1 0.60 10.9 0.08 0.7

11 0.5 0.24 F 1200 40 1.7 3.3 83 53 1100 290 3300 21 11 2.7 7.8 0.14 57 0.27 <1nF 2700 40 2.3 4.6 86 54 2400 290 3300 21 12 3.2 8.7 0.11 69 0.29 <0.7

12 0.1 0.78 F 87 200 20.4 2.4 740 62 160 12 29 0.49 2.3 0.38 3.3 3.0 6.0 2.9 1.2nF 270 210 20.8 2.7 820 69 360 16 36 0.57 2 3 0.49 4.2 3.3 11.5 3.3 0.9

13 0.2 1.17 F 9 190 14.2 1.3 880 65 15 6.3 14 0.16 2.0 0.16 2.5 2.1 1.0 3.4 <1nF 50 190 13.9 1.4 930 67 90 11.6 13 0.19 1.8 0.21 2.8 2.2 4.3 3.7 1.2

14 5 0.93 F 52 175 15.1 4.1 460 49 110 11 23 0.24 3.1 0.28 2.2 2.0 4.5 1.7 <1nF 180 170 14.8 4.2 490 51 250 16 22 0.26 2.6 0.37 2.4 2.0 8.8 1.9 <0.7

15 1 0.46 F 790 49 3.6 2.7 140 98 1100 116 180 0.95 8.3 1.04 3.3 0.29 45 1.1 <1nF 1940 52 5.1 3.8 150 108 1900 130 200 1.09 9.3 1.4 4.8 0.22 53 1.1 1.5

16 1 0.58 F 660 90 4.3 3.7 190 130 870 256 365 2.5 10.7 2.3 5.1 0.45 33 1.9 <1nF 1490 93 4.7 4.4 200 150 1450 260 400 3.0 12.0 2.6 6.2 0.34 46 2.1 1.3

17 0.1 0.75 F 28 110 7.5 7.8 160 33 60 65 2000 15 4.9 0.47 2 2 0.62 35 0.36 1.1nF 42 112 7.6 8.1 170 33 100 65 2000 16 4.9 0.53 2.9 0.67 45 0.38 2.1

18 20 0.36 F 65 39 1.9 2.0 170 23 120 190 19 0.15 1.5 0.14 1.0 0.34 2.3 0.45 <1nF 140 39 2.1 2.1 1.90 24 190 140 18 0.15 1.9 0.17 1.9 0.36 2.8 0.50 <0.7

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19 0.7 0.43 F 93 39 4.8 3.3 300 28 170 30 21 0.23 1.9 0.15 <1 0.70 5.5 1.72 2nF 230 43 4.8 3.5 310 29 250 30 21 0.24 2.0 0.2 1.6 0.77 7.7 1.87 <0.7

20 0.3 0.21 F 130 22 1.2 2.5 67 20 170 17 14 0.13 1.0 0.10 <1 0.1 4.7 0.04 <1nF 350 25 1.1 2.6 72 251 310 24 14 0.15 0.9 0.16 <1 0.1 7.3 <0.04 <0.7

21 0.3 0.60 F 42 70 8.0 4.2 610 30 77 14 17 0.14 2.1 0.35 2.4 2.4 3.6 3.9 3nF 95 71 7.6 4.3 620 30 120 14 17 0.15 2.0 0.35 2.8 2.4 4.6 4.2 <0.7

22 0.5 0.64 F 69 110 19 8.0 630 30 110 28 13 0.10 2.7 0.25 1.8 2.5 3.2 3.5 5nF 110 113 19 7.9 630 30 170 28 14 0.12 3.0 0.26 2.2 2.4 3.8 3.9 <0.7

23 10 0.25 F 500 35 4.1 3.2 90 16 480 34 15 0.13 1.9 0.21 1.1 0.52 5.4 0.65 <1nF 1000 38 4.1 4.0 100 17 790 4.0 14 0.13 1.9 0.28 1.1 0.51 5.8 0.70 <0.7

24 11 0.27 F 590 37 10.1 4.3 110 21 560 50 11 <0.2 2.2 0.26 <1 0.55 3.4 0.74 <1nF 1200 39 10.5 5.2 120 21 970 53 13 <0.2 2.2 0.33 1.2 0.54 4.5 0.83 <0.7

25 8 0.63 F 94 115 9.0 5.4 460 28 94 27 13 <0.1 2.6 0.30 2.1 1.73 0.8 2.51 2nF 210 117 9.1 5.5 470 29 190 30 12 <0.1 2.2 0.32 2.3 1.74 <1 2.86 <0.7

26 0.3 0.17 F 40 10 2.4 0.8 50 9 98 4.6 5 <0.1 <1 0.10 <1 <0.1 2.2 0.19 <1nF 70 12 2.4 0.9 55 9 140 5.8 6 0.11 <1 0.10 <1 0.14 3.0 0.22 <0.7

27 5 0.27 F 108 30 3.3 0.7 83 20 220 26 15 <0.1 1.2 0.12 <1 0.20 2.9 0.16 <1nF 280 30 3.4 1.0 85 25 370 29 15 <0.1 1.1 0.17 <1 0.19 5.1 0.20 0.7

28 1 0.23 F 170 24 3.8 1.3 90 14 270 81 14 <0.1 1.3 0.17 <1 0.48 2.8 0.61 <1nF 380 25 3.7 1.6 100 15 400 35 18 <0.1 1.6 0.23 <1 0.50 3.8 0.62 7.4

29 8 0.35 F 350 39 5.1 2.8 150 19 220 16 14 0.13 2.1 0.17 <1 0.32 5.3 0.98 2nF 450 42 5.2 2.9 150 19 360 17 21 0.15 2.1 0.22 1.2 0.36 6.4 1.12 <0.7

30 1 0.12 F 56 8 2.6 0.7 39 5 50 4.9 7 <0.1 0.9 0.09 <1 <0.1 <0.5 0.39 <1nF 120 8 2.8 0.8 45 5 100 6.6 8 <0.1 1.1 0.10 <1 0.13 <0.5 0.40 <0.7

31 2 0.13 F 106 12 2.6 0.9 46 10 180 11 7 <0.1 1.3 0.16 <1 0.37 1.2 0.37 <1nF 230 13 2.9 1.1 49 10 320 16 10 <0.1 1.2 0.19 <1 0.39 1.4 0.40 1.3

32 10 0.23 F 28 30 1.3 0.8 74 16 70 7.8 12 0.11 1.3 0.10 <1 0.30 3.4 0.75 <1nF 67 32 1.2 0.8 81 16 120 11 13 0.15 1.3 0.14 <1 0.33 5.4 0.75 <0.7

33 1 0.22 F 240 27 2.8 1.4 89 25 400 46 16 0.1 3.0 0.26 <1 0.47 2.6 1.15 <1nF 690 29 2.8 1.9 95 26 880 54 22 0.1 3.1 0.39 1.0 0.48 3.1 1.18 0.7

34 0.2 0.35 F 18 35 4.4 1.5 140 29 16 10.1 9 0.10 2.5 0.10 <1 0.83 <1 0.19 <1nF 27 37 4.6 1.4 150 29 32 2.2 9 <0.1 2.2 0.11 <1 0.93 <1 0.19 0.7

Concentrations of Ag, Be, Bi, Te and Tl were below detection limit (0.1 lg/L) with few exceptions reported in the text.a Total dissolved solids.b Filtered through 0.4 lm pore-size filters.c Non-filtered.

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Table 2Dissolved (filtered through 0.4 lm filter) and total (unfiltered) concentrations of trace elements in Sardinian river waters sampled in June 2003 (except 15A and 16A sampled inFebruary 2006)

No. Flow TDSa Al B Li Rb Sr Ba Fe Mn Zn Cd Cu Co Ni Mo Pb U As Hg(m3/s) (g/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L) (lg/L)

2 0.5 0.54 Fb 25 56 4.6 4.1 130 13 55 140 13 0.05 1.2 14 2.1 1.1 0.4 1.1 2nFc 52 59 4.8 4.0 140 15 100 167 12 0.06 1.5 14 2.2 1.1 0.9 1.2 <0.5

3 10 0.33 F 320 50 2.8 2.5 130 30 480 88 20 0.12 2.6 2.0 2.9 0.6 3.5 0.86 2nF 680 53 3.2 3.2 150 36 990 113 19 0.12 4.1 2.3 3.4 0.5 4.5 0.89 <0.5

4 1 0.48 F 34 67 5.9 1.8 270 42 44 146 10 0.07 1.8 0.3 1.9 0.9 0.7 1.8 4nF 230 71 6.1 2.1 290 49 370 270 9 0.08 2.1 0.4 2.0 0.9 2.7 1.9 <0.5

5 0.05 0.71 F 11 144 6.8 5.8 420 47 15 10 6 0.06 2.4 0.6 2.5 1.7 0.5 1.0 2nF 19 148 6.7 5.9 450 52 28 13 6 0.05 2.1 0.6 2.4 1.8 0.7 0.99 <0.5

11 0.1 0.47 F 9 173 4.4 5.4 180 35 127 75 230 0.88 1.8 0.6 2.4 0.8 9.5 0.48 3nF 50 170 4.7 5.0 180 35 160 77 250 1.25 2.6 0.7 2.5 0.7 14.0 0.39 <0.5

12 0.05 0.98 F 11 223 29 1.9 1160 68 16 5.8 20 0.06 3.1 0.3 3.8 3.2 0.6 3.6 1nF 18 229 28 1.9 1260 72 29 6.8 21 0.06 2.1 0.3 3.6 3.3 <0.5 3.7 <0.5

13 0.1 1.11 F 4 200 16 1.3 990 67 29 10 10 0.13 2.0 0.2 2.8 2.4 0.1 2.8 <1nF 100 210 15 1.4 1030 72 107 25 10 0.11 3.0 <0.3 2.8 2.3 <0.5 2.8 <0.5

14 1 1.18 F 40 236 17 4.5 700 65 61 39 20 0.27 2.1 0.6 3.0 2.0 1.3 2.1 1nF 125 244 17 5.0 760 71 155 66 20 0.21 2.5 0.7 3.1 2.3 1.8 2.2 1

15 0.3 0.59 F 32 147 4.6 4.9 170 79 148 150 27 0.13 1.9 0.4 2.7 0.9 2.9 0.84 3nF 190 156 4.8 5.5 180 96 380 200 32 0.15 2.4 0.4 2.9 1.0 7.1 0.86 4

16 0.5 0.62 F 120 167 4.4 7.0 160 80 290 94 85 0.43 5.2 0.6 3.3 0.8 7.0 0.75 1nF 170 172 4.3 7.4 180 95 350 1701 104 0.41 5.6 0.6 3.2 0.8 10.4 0.75 1.5

17 0.1 1.49 F 6 312 15 14.9 300 29 34 330 2960 17.4 3.5 1.4 5.0 1.7 13.3 0.66 <1nF 8 322 14 15.2 310 39 80 360 3330 17.8 4.7 1.5 5.0 1.7 33.4 0.60 2.5

35 0.1 0.35 F 8 39 14 3.1 140 120 50 6.9 6 0.07 1.0 <0.1 <1 1.4 0.6 0.70 1nF 28 40 13 3.2 150 130 67 6.7 6 0.06 0.8 <0.3 <1 1.4 1.5 0.70 <0.5

15A 5 0.37 FFd 70 50 5.2 1.5 130 42 <20 1 8 0.04 3.1 0.12 <1 0.62 0.2 0.76 nd ndF 330 52 5.3 1.8 140 65 450 34 23 0.15 3.3 0.38 2 0.66 8.6 0.83 0.9 nd

16 A 0.05 0.83 FF 18 73 11 1.7 240 40 <20 4 21 0.2 3.7 0.24 2.7 0.37 0.1 2.2 nd ndF 190 74 11 1.8 250 60 250 1200 1600 11 4.8 8 18 0.39 2.7 2.5 0.6 nd

a Total dissolved solids.b Filtered through 0.4 lm pore-size filters.c Non-filtered.d Filtered through 0.015 lm pore-size filters.

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Dissolved concentrations of U and Mo are corre-lated with HCO3 (r2 = 0.82 and r2 = 0.75, respec-tively). Dissolved concentrations of B, Li, Rb andSr increase with salinity. These elements behavedsimilarly to the major cations (Cidu et al., 2007)and appear related to the degree of weathering.On the basis of speciation computation, B occursalways as H3BO0

3, and Li, Rb, Sr and Ba as freeions. The dominant Mn species were Mn2+ (70–100%) followed by MnCO0

3 (15–30%), whileMnSO0

4 (up to 17%) occurred in waters with highSO4 concentrations (e.g., Rio San Giorgio No.17). Dissolved Mo and U occur as anion species:MoO2�

4 (100%); UO2ðCO3Þ4�3 and UO2ðCO3Þ2�2 ,80–20% and 65–35%, respectively, under high-and low-flow conditions. As expected under oxidiz-ing condition (Hodge et al., 1996), Mo and Uexhibit a conservative behavior in the studiedwaters.

Fig. 3 shows the correlation between Fe and Alconcentrations determined in both filtered andnon-filtered aliquots (r2 = 0.98, number of sam-ples = 92). The highest concentrations of both ele-ments were in samples collected in January at highflow. The most abundant aqueous species of Feand Al were FeðOHÞþ2 , FeðOHÞ03 and AlðOHÞ03,AlðOHÞ�4 , respectively. The ranges in dissolvedand total concentrations of Fe and Al in waterssampled both in January and June are shown inthe box plots of Fig. 4a and b. The median valuefor the total concentration is significantly higherthan the median value for the dissolved concentra-tion. Total concentrations of Fe and Al depend onthe amount of material in suspension that increases

1

10

100

1000

10000

1 10 100 1000 10000Al (µg/L)

Fe (

µg/L

)

January nFJune nFJanuary FJune F

r2= 0.98n = 92

Fig. 3. Plot showing concentrations of Fe versus Al in theSardinian river waters.

in winter as flow increases. The filtration tests car-ried out with different pore-size filters showed thatFe and Al concentrations are much lower in the<0.015 lm fraction as compared with the<0.4 lm fraction (see Table 2). This result indicatesthat the dissolved fraction of Fe and Al comprisesvery fine particles (>0.015 lm and <0.4 lm indiameter, e.g., clay and Fe oxides) and/or colloids,which is consistent with the pH and Eh valuesobserved in the water. Significant amounts ofFe(III) and Al colloids within a size range of0.1–0.3 lm may form at near-neutral to alkalinepH in oxidized environments (Zanker et al.,2003). Moreover, secondary Fe oxides in riverwater may occur as nano-size particulate spheres(0.01–0.02 lm in diameter) that are associated withthe edges of clay grains (Poulton and Raiswell,2005). Significantly lower dissolved concentrationsof Fe and Al were observed under low-flow condi-tions when low turbulence allows fine particles tosettle. This process can be enhanced by increasesin salinity: as the ionic strength increases, the inter-action and aggregation of colloids is faster, result-ing in the settling of larger aggregates (Ouddaneet al., 1999; Ranville and Schmiermund, 1999).

The behavior of Pb was similar to that observedfor Fe and Al (Fig. 4c) with median concentrationshigher at high flow. Also, Zn, Cu, Ni, Cd and Coconcentrations are generally higher under high-flow conditions and relatively small differencesare observed between dissolved and total concen-trations (Table 1, Fig. 4d–f). These elementsappear to be confined to the fraction between0.015 and 0.4 lm (Table 2). The decrease in theconcentrations of Pb, Zn, Cu, Ni, Cd and Coobserved at low flow may be due to adsorptiononto a variety of metal-bearing (especially Al, Feand Mn) oxide and hydroxide minerals at near-neutral pH (Smith, 1999). This process causes theremoval of these trace elements from solutionand was more marked at low flow when theamount of fines in the water is less.

It is quite common for people to assume that theaqueous fraction passing through 0.4 or 0.45 lmpore-size filters is dissolved. Saturation index (SI)values for some mineral phases were calculatedusing dissolved concentrations determined in the15A and 16B water samples filtered through 0.4and 0.015 lm. The results are reported in Table 3;it can be seen that SI values of mineral phases con-taining Al, Ba, Fe, Mn, Pb and Zn are overesti-mated in the 0.4 lm pore-size filtered waters.

0

1000

2000

3000

Fe (μ

g/L

)

Fe

Jan F Jan nF June nFJune F0

1000

2000

3000

Al (

μg/L

)

Al

Jan F Jan nF June nFJune F

0

20

40

60

80

Pb (

μg/L

)

Pb


2

4

6

8

10

Ni (

μg/L

)

Ni


0

1

2

3

4

Cd

(μg/

L)

Cd


1

2

3

4

Co

(μg/

L)

Co


Fig. 4. Box plots showing Fe (a), Al (b), Pb (c), Ni (d), Cd (e) and Co (f) concentrations for filtered (F) and non-filtered (nF) waterssampled under high-flow (January) and low-flow (June) conditions. Each box includes the 25th and 75th percentiles with the mediandisplayed as a line.


4.2. Rare earth elements and thorium

Concentrations of Th and the REE in the Sardi-nian river waters are listed in Table 4. These ele-ments show a wide range in concentrations(<0.005–0.58 lg/L Th; 0.1–22.8 lg/L

PREE) for

either rivers draining similar rock types (e.g., the

rivers of Vignola No. 26 and Liscia No. 27 draingranite-dominant catchments; Fig. 1), or in thesame river along the flow path (e.g., the Tirso riverNo. 1–4). The concentration of Th in natural watersat low temperature is usually very low due to itsscarce abundance, its strong hydrolysis which limitsits solubility, the slow solution rate of Th-bearing

Table 3Saturation index (SI) values for some mineral phases in 0.4 lm and 0.015 lm pore-size filtered waters (samples 15A and 16A)

Mineral phase 15A 16A

0.4 lm 0.015 lm 0.4 lm 0.015 lmSI SI SI SI

Barite BaSO4 0.28 0.08 0.77 0.55Calcite CaCO3 0.24 0.25 0.28 0.28Cerussite PbCO3 �0.32 �1.87 �0.80 �2.24Chalcedony SiO2 �0.34 �0.34 �0.11 �0.11Fe(OH)3 0.47 �1.14 0.27 �0.80Gibbsite Al(OH)3 2.65 1.99 2.40 1.80Gypsum CaSO4:2H2O �2.22 �2.22 �1.38 �1.38Hydrozincite Zn5(OH)6(CO3)2 �3.73 �6.02 2.14 �6.86Illite K0.6Mg0.25Al2.3Si3.5O10(OH)2 6.07 4.56 5.97 4.59Kaolinite Al2Si2O5(OH)4 5.79 4.47 5.73 4.54Manganite MnO(OH) 2.91 1.51 4.06 1.59Rhodochrosite MnCO3 �0.94 �2.33 0.33 �2.14Smithsonite ZnCO3 �1.91 �2.36 �0.40 �2.20Strontianite SrCO3 �0.16 �0.17 �0.22 �0.23


minerals, and sorption processes. The average con-centration of Th in seawater is 0.00064 lg/L asreported by Langmuir and Herman (1980). A med-ian concentration of 0.006 lg/L Th has beenreported for 476 groundwater samples from bore-holes in Norwegian crystalline bedrocks (Frengstadet al., 2000). Thorium concentrations in the range of0.019–0.088 lg/L have been observed in surfacewaters draining U-rich mineralized areas (JuradoVargas et al., 1995).

There was a distinct difference among waterssampled during different seasons with Th andREE concentrations much higher in filtered waterssampled in January compared to those in filteredwaters sampled in June (Table 4). This behavior isconsistent with that observed for dissolved Al andFe concentrations. Fig. 5 shows that river watershigh in dissolved Fe were also enriched in dissolvedREE; this correlation (r2 = 0.89, number of sam-ples = 48) suggests an association of the REE withFe-rich very fine particles or colloids. Since theabundance of fine materials in any particular river,as well as along the river course, may vary signifi-cantly, a large variation in REE content wasobserved. At low flow (June), concentrations ofREE in the non-filtered water were always higherthan those in the filtered water. This was alsoobserved in low-flow (<10 L/s) stream waters ofCentral Sardinia collected in the spring of 1999and having similar concentrations of REE (Biddauet al., 2002). In samples 15A and 16A, the REE con-centrations in the <0.015 lm fraction were at leastone order of magnitude lower than those in the

<0.4 lm fraction. These observations highlight theimportance of the fine fraction in the aqueous trans-port of REE.

Fig. 6 shows the abundances of REE normal-ized to Post-Archean average Australian Shale(PAAS; McLennan, 1989) in river waters filteredthrough 0.4 lm pore-size filters. In January(Fig. 6a), the REE patterns are characterized bya weak convexity centered on the intermediateREE. Similar patterns have been previouslyreported by Hoyle et al. (1984) for coarsely filteredriver waters, and by Elderfield et al. (1990) forriver waters filtered through 0.4 lm but known tosupport relatively high concentration of colloidsand dissolved organic matter. In June (Fig. 6b),the REE patterns are characterized by a positiveanomaly in Eu and by an enrichment in the heavyREE. Considering the oxidizing conditions of thewaters, the Eu anomaly may be due to dissolutionof Eu-enriched minerals, such as Ca-feldspar, cal-cite (Lee et al., 2003) and sulfides (Leybourneet al., 2000). This effect was more apparent in Junesamples because the contribution of groundwaterto the river was higher than in January. The lightREE are adsorbed more easily than heavy REE,and therefore are preferentially removed from solu-tion (Elderfield et al., 1990). This effect was morepronounced under low-flow conditions when lessfine material was suspended in the water. Impor-tantly, waters draining different rock types havesimilar REE patterns, which suggests that thedominant composition of the rocks in thewatershed probably plays only a minor role in

Table 4Yttrium, REE and Th concentrations in Sardinian river waters

No. DateP

REEa Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th(lg/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L)

1 Jan03 Fb 3.27 470 540 1000 140 570 120 30 130 19 100 18 45 7 44 7 282 Jan03 F 10.00 1100 1800 3600 460 1600 340 53 320 45 240 44 110 18 110 17 1402 June03 F 0.46 75 70 140 15 60 11 6 13 2 11 3 12 3 26 5 8

nFc 0.81 100 130 260 30 120 24 7 27 3 21 4 15 3 30 7 333 Jan03 F 3.54 430 650 1200 150 590 110 30 130 17 90 17 42 7 39 6 303 June03 F 6.70 920 1100 2200 260 1000 210 46 200 31 170 38 110 17 110 18 270

nF 9.10 1200 1500 3100 370 1400 290 64 300 43 230 50 140 19 130 22 2404 Jan03 F 13.93 1700 2300 4900 600 2200 480 120 480 70 370 70 180 25 140 23 2704 June03 F 0.54 88 86 160 20 80 15 11 21 3 14 3 9 2 17 3 12

nF 2.83 320 480 980 110 450 100 27 100 13 63 14 40 5 37 6 805 Jan03 F 22.80 2900 3700 7400 1000 2900 840 170 800 120 630 120 300 42 260 40 5805 June03 F 0.31 70 36 71 8 30 8 9 11 1.4 10 4 17 3 26 6 <5

nF 0.35 74 40 79 10 44 9 10 14 1.6 11 4 18 3 28 6 <56 Jan03 F 1.38 220 228 400 58 240 60 14 56 8 40 8 20 2.7 17 3 67 Jan03 F 1.77 230 20 600 78 300 60 17 63 8 40 7 19 3 18 2 98 Jan03 F 0.31 40 60 120 15 40 8 8 9 1.1 6 0.9 1.3 0.3 2 0.2 <59 Jan03 F 0.92 130 170 290 38 140 35 21 42 5 25 3.7 9 1 7 1.4 <510 Jan03 F 1.13 160 190 340 46 190 45 15 50 6 27 5 14 1.5 12 2 2511 Jan03 F 16.60 2400 2800 5100 700 2700 590 170 620 90 470 92 250 37 220 35 33011 June03 F 0.43 90 66 110 14 52 10 7 20 2 14 3 13 2 21 4 <5

nF 0.82 130 150 240 31 110 26 12 30 4 26 5 15 3 23 5 712 Jan03 F 2.02 280 330 680 80 310 67 31 81 10 55 9 24 3 23 3 3412 June03 F 0.20 40 30 53 6 23 6 10 13 0.4 4 1 3 0.6 4 1 <5

nF 0.21 41 30 55 7 26 7 12 12 0.6 5 1 3 0.6 5 1 <513 Jan03 F 0.22 48 32 50 7 30 7 13 10 1.2 6 1.3 2 0.7 5 1.1 <513 June03 F 0.12 34 15 23 3 15 2 11 3 1 3 1 2 0.2 3 1 <5

nF 0.81 120 130 260 30 130 28 16 27 4 22 4 12 1.5 10 2 1514 Jan03 F 0.73 100 120 230 30 110 23 16 40 4 21 4 12 2 12 2 814 June03 F 0.53 100 73 140 17 70 15 13 40 2 16 3 14 1 13 2 10

nF 1.31 190 200 400 52 200 46 21 70 7 36 7 20 3 23 4 3215 Jan03 F 8.76 14,900 1170 2570 1490 1480 350 95 400 59 280 54 140 19 110 16 19015 June03 F 1.11 66 240 460 52 170 15 16 50 2 13 2 7 1 8 1 7

nF 1.66 200 250 540 65 270 80 27 87 8 48 7 21 3 19 3 28

2788R

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16 Jan03 F 7.35 1400 980 2000 290 1240 300 96 340 51 260 49 120 16 100 15 9016 June03 F 0.39 64 63 110 13 60 12 14 30 2 11 2 4 0.7 6 1.0 <5

nF 0.54 88 90 150 19 80 18 17 32 3 16 3 8 0.9 8 1.4 <517 Jan03 F 0.33 65 50 80 13 50 13 9 19 2 11 2 5 1 4 0.9 <517 June03 F 0.09 18 12 17 2 10 3 5 14 0.6 2 0.5 1 0.2 2 0.2 <5

nF 0.20 30 30 56 7 30 5 8 20 1.0 5 1.0 2 0.3 3 0.5 <518 Jan03 F 1.52 190 340 500 67 220 40 16 50 7 35 6 19 2 19 3 519 Jan03 F 2.51 250 630 930 110 310 56 20 75 9 48 9 25 4 23 3 720 Jan03 F 3.25 600 470 900 130 540 120 24 140 19 110 23 68 9 65 10 1921 Jan03 F 1.16 180 240 370 46 130 25 12 34 4 28 7 24 5 40 9 922 Jan03 F 1.45 210 280 490 65 200 41 13 40 6 35 5 20 3 20 4 1623 Jan03 F 6.39 1050 1100 1900 270 1000 200 36 200 30 180 37 100 16 100 16 15024 Jan03 F 7.98 1200 1400 2500 340 1200 260 45 270 40 230 40 130 18 120 19 17025 Jan03 F 1.04 180 180 320 40 140 29 11 37 5 28 6 22 3 24 4 1326 Jan03 F 1.26 290 170 270 47 190 50 7 56 10 50 10 36 5 41 5 1827 Jan03 F 4.24 470 870 1400 190 700 130 23 140 18 93 18 48 6 49 7 8028 Jan03 F 2.94 570 430 800 110 450 100 16 110 19 100 21 68 9 78 12 4429 Jan03 F 2.98 420 540 930 120 490 100 24 100 14 81 16 43 6 45 7 4630 Jan03 F 1.70 450 170 350 57 240 67 9 72 14 83 17 57 9 70 11 2031 Jan03 F 2.62 500 360 730 98 400 91 16 110 17 98 19 58 8 61 9 4832 Jan03 F 1.64 130 360 580 81 300 49 10 49 6 25 5 13 2 13 2 1733 Jan03 F 7.50 680 1500 2700 360 1300 260 43 260 32 140 24 70 9 60 8 5534 Jan03 F 0.42 60 84 140 18 56 10 6 14 2 9 2 6 1.1 7 1.8 <535 June03 F 0.27 38 43 86 9 34 8 20 10 1.1 6 1.0 3 0.3 2.7 0.5 5

nF 0.35 54 57 100 13 58 11 20 15 1.8 8 1.2 4 0.4 3.0 0.5 715A Feb06 FFd 0.16 40 20 30 5 27 6 7 8 1.1 6 1.6 3 0.3 4 0.9 6

F 4.20 730 590 1200 180 700 190 50 190 30 150 28 77 10 63 9 1316A Feb06 FF 0.05 14 7 7 1 6 1 5 1 <1 2 1.4 <1 <1 <1 <1 <5

F 3.32 890 340 820 110 540 130 40 140 24 127 25 65 8 53 7 11

a Y + La � Lu.b Filtered through 0.4 lm pore-size filters.c Non-filtered.d Filtered through 0.015 lm pore-size filters.

R.

Cid

u,

R.

Bid

da

u/

Ap

plied

Geo

chem

istry2

2(

20

07

)2

77

7–

279

42789

0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000

Dissolved Fe (µg/L)

Dis

solv

ed Y

+R

EE

(µg

/L)

January

June

r2 = 0.89

Fig. 5. Plot showing theP

REE (Y + La � Lu) versus Feconcentrations in 0.4 lm pore-size filtered waters.

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Nor

mal

ized

RE

E c

once

ntra

tion

REE/0.4 µm

REE/PAAS

Fig. 7. Patterns of REE in 0.015 lm pore-size filtered water (No.15A). Concentrations normalized to the Post-Archean averageAustralian Shale (PAAS) and to the 0.4 lm pore-size filteredwater.


the partitioning of each REE in the river waters.The variation in REE concentrations observed inJanuary compared to June may be due to the dif-ferent hydrological conditions that control theamount of fine particles present in the water.

Fig. 7 compares the REE patterns in the watersample No. 15A; REE concentrations determinedin the 0.015 lm pore-size filtered water are normal-ized to PAAS and to the corresponding concentra-tion in the 0.4 lm pore-size filtered water. Bothnormalizations show similar patterns indicating thatREE in PAAS represent a good analog of fine par-

1

10

100

1000


3451115

RE

E /

PAA

S (x

10^

6)

a b

Fig. 6. Plots showing the distribution of REE abundance normalizedpore-size filtered waters sampled in (a) January and (b) June.

ticles in the river. REE partitioning between the<0.4 lm pore-size and <0.015 lm pore-size aqueousfraction is consistent with patterns observed underlow-flow condition (Fig. 6b), i.e. positive Eu anom-aly and slightly higher mobility of heavy REE undernear-neutral to alkaline pH conditions observed inthe studied waters.

4.3. Water quality

Table 5 reports the World Health Organizationguidelines and Italian regulations for drinkingwater, along with the water samples with dis-solved concentrations above the Italian limitsunder high- and low-flow conditions. Dissolved

0.01

0.1

1

10

100


24511

RE

E /

PAA

S (x

10^

6)

to the Post-Archean average Australian Shale (PAAS) in 0.4 lm

Table 5WHO and Italian guidelines for drinking water, and water samples with dissolved concentrations above the Italian limits under high-flow(January) and low-flow (June) conditions

WHOa Italyb Waters above limit

Samples collected both in January and June Samples collected only in January

High-flow Low-flow

As lg/L 10 10 None None NoneB lg/L 500 1000 None None NoneBa lg/L 700 –c None None NoneCu lg/L 2000 1000 None None NoneNi lg/L 20 20 None None NoneCr lg/L 50 50 None None NoneV lg/L – 50 None None NoneAl lg/L – 200 2, 3, 4, 5, 11, 15, 16 3 23, 24, 29, 33Fe lg/L – 200 2, 3, 4, 5, 11, 15, 16 3, 16 23, 24, 27, 28, 29, 33Mn lg/L 400 50 2, 4, 5, 11, 15, 16, 17 2, 3, 4, 11, 15, 16, 17Pb lg/L 10 10 4, 5, 11, 15, 16, 17 17 9Hg lg/L 1 1 5, 13, 15, 16, 17 15, 16, 17 28, 31Cd lg/L 3 5 11, 17 17Sb lg/L 20 5 2 9U lg/L 15 – None None NoneZn lg/L – – None None None

a World Health Organization guidelines (W.H.O., 2004).b Italian regulations (G.U.R.I., 2001).c Not regulated.


As, a highly toxic element, and other regulatedelements such as B, Cu, Ni, Cr and V, werealways below the limits established for drinkingwater. A significant number of water sampleshad dissolved Al, Fe, Mn, Pb and Hg above theItalian limits. For sites that were sampled in bothJanuary and June, samples exceeded the guidelinesmore commonly at high flow than at low flow.Dissolved Mn exceeded the Italian regulation in20% of samples, under either high- or low-flowconditions.

Among the highly toxic heavy metals, dissolvedCd was found above the Italian limit of 5 lg/L intwo samples (Rio San Giorgio No. 17 and SitzerriNo. 11), both of which include drainage from majormining districts. In these waters, Cd2+ and CdCO0

3

were calculated to be the dominant aqueous species.Twenty percent of waters sampled at high flow hadPb and Hg concentrations above the limits. Thedominant Pb species was PbCO0

3 (80–100%). Themost abundant Hg species was HgðOHÞ02 (74–95%), but as the Cl concentration increased the cal-culated proportion of HgClOH0 and HgCl0

2 speciesincreased.

The distribution of dissolved Pb and total Hgin the Sardinian river waters under high-flow con-

ditions is shown in Fig. 8 together with the loca-tions of the most relevant industrial sites andabandoned mining districts. The petrochemicaland chemical plants at Porto Torres, Portoscuso,and Sarroch, all located close to the coast, shouldhave very low influence on the quality of riverwaters, while the chemical plants at Ottana mayhave some influence on the water quality in theTirso River. Fig. 8 shows that the highest Pb con-centrations, as well as high Hg, were found in theSitzerri No. 11, Rio San Giorgio No. 17, and Cix-erri No. 15 and 16 waters draining large Pb–Znmining districts. However, high concentrations ofPb and/or Hg also occurred in waters (e.g., Mog-oro No. 5, Mannu Oschiri No. 28 and PosadaNo. 31) that do not drain mining areas or knownmineralized areas (De Vivo et al., 1998), and thatare located far away from industrial plants. Thesource of Pb and Hg at these sites is probablydue to urban wastes discharged close to the rivers.Indeed, the presence of abandoned electric appli-ances, car batteries and car frames was observedduring sampling in several areas. The fact thatthese wastes underwent flooding in winter, might,in part, explain the higher concentrations at highflow than at low flow.

Tem

o

Liscia

Coghinas

FlumendosaFlum

inim

annu

Tirso

PicoccaCixerri

Mannu P. Torres

Tirs

o

Vign

ola

Posada

Cedrino

Mogoro

Sitz erri

Cagliari

Oristano

Nuoro

Sassari

As, Pb

Cu

Pb, Zn

San Giorgio

Pb, AgLeni

Flu

min

iman

nu

Ozieri

Osc

hiri

Porto Torres

Ottana

Portoscuso Sarroch

Tem

o

Liscia

FlumendosaFlu

min

iman

nu

Tirso

Picocca

Mannu P. Torres

Vig

n ol a

Posada

Cedrino

Cagliari

Oristano

Nuoro

Sassari

Tirs

o

As, Pb

Cu

Cixerri

MogoroSi tzerr i

Pb, Zn

San Giorgio

Leni

Coghinas

Ozieri

Osc

hiri

Porto Torres

Ottana

SarrochPortoscuso

µg/L Pb

<1-5

6-10

11-20

21-60

µg/L Hg

<0.7-0.91.0-2.0

2.1-7

Abandoned mining districts

Main industrial sites

0 30 km

Fig. 8. Map showing dissolved Pb and total Hg concentrations in the river waters sampled in January.


5. Conclusions

In this study 35 water samples were collectedfrom 19 rivers in Sardinia, Italy; 11 of the riverswere sampled during both high and low flow condi-tions. The river waters were variable in chemicalcomposition and salinity depending on the composi-tion of rocks drained. The relative abundances ofmajor ions did not change in water sampled in dif-ferent seasons, but the salinity and the concentra-tion of major dissolved ions increased in the

summer due to the low contribution of rainwater,evaporation processes, and the input of groundwa-ter to the rivers.

The concentrations of trace elements varieddepending on flow. A group of elements, namelyB, Li, Rb, Sr, Ba, Mn, Mo and U, that are mainlyderived from water–rock interaction processes,showed higher dissolved concentrations in the sum-mer when the rainfall was very low. Another groupof elements, which include Al, Fe, Pb, REE and Th,had higher concentrations in winter when the


amount of fine materials was higher due to the run-off from rain events. A significant number of watersshowed Pb and Hg concentrations above the Italianregulations for drinking waters. The highest concen-trations of these heavy metals were observed underhigh flow conditions. The weathering of minewastes, and the discharge of urban wastes into riv-ers, was likely the main sources of metals.

Many trace elements dissolved in the watersappear to be associated with particles ranging from0.015 to 0.4 lm in diameter, onto which they areprobably adsorbed. Considering that the load ofthis fraction in a river may vary significantlydepending on the amount of rain and runoff, andthat this fraction may host toxic elements, the sam-pling season should be carefully considered in stud-ies devoted to the assessment of the quality of riverwater. It appears that commonly used filters withpore sizes between 0.2 and 1.2 lm including the con-ventional filter with a pore size of 0.45 lm (Ranvilleand Schmiermund, 1999) used to separate dissolvedcomponents from the suspended matter, are notadequate to distinguish the aqueous amount of ele-ments hosted in the very fine fraction. Also, the useof dissolved amounts determined in 0.4 lm pore-size filtered waters may result in overestimated satu-ration index values, especially for Al- and Fe-bear-ing minerals.

Acknowledgements

This study was partially funded by the Italian‘‘Ministero Universita Ricerca Scientifica Tecnolog-ica’’ (ex 60%) and the Fondazione Banco di Sarde-gna. The authors wish to thank F. Podda forcontributing to ICP-MS analyses. Many thanks toNadine Piatak, Richard B. Wanty and an anony-mous reviewer for the useful comments andsuggestions.

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