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For their analysis,0.4 g sediment was treated with 8 ml of an oxidiz-ing mixture (6.0 ml HNO3:2 ml HCl) and 4.0 mLHF (40%) in a Telfon recipient put in amicrowaveoven (Milestone ETHOS E) (1,000 W, 160◦C,6 min; 1,000 W, 210◦C, 4 min; 1,000 W, 210◦C,20 min). After being cooled down, the recoveredsamples are diluted to 25 ml with double-distilleddeionized water.Flame atomic absorption spectrometry (FAAS,PEAA800) was used to determine heavy metallevels, including Cu, Ni, Cr, and major element Fein sediments, and graphite furnace atomic absorp-tion spectrometry (GFAAS, PEAA800) was usedto determine Pb and Cd. As and Hg levels weredetermined by atomic fluorescence spectrometry(AFS-2201). Quality control for heavy metals wasprovided by parallel analysis of certified referencematerial SOIL ESS-4 (certified values of Cu, Ni,Cr, As, Pb, Cd, Hg, and Fe are 26.3 ± 1.7, 32.8 ±1.3, 70.4 ± 4.9, 11.4 ± 0.7, 22.6 ± 1.7, 0.083 ± 0.008,0.021 ± 0.004, and 28,474 ± 216, their measuredvalues are 26.1 mg/kg, 33.8 mg/kg, 67.7 mg/kg,11.2 mg/kg, 21.8 mg/kg, 0.082 mg/kg, 0.018 mg/kg,and 27,679 mg/kg, respectively).Data analysisIn order to quantify possible anthropogenic in-puts, it ismore useful to calculate the degree of en-richment of an element by piding its ratio to thenormalizing element by the same ratio found inthe chosen baseline (Förstner and Whitman 1981;Li 1981), enrichment factors (EF) were calculatedaccording to:EF = XY sample XY baselinewhere X is the concentration of the potentiallyenriched element and Y is the concentration ofthe proxy element. The enrichment factor in sed-iments is widely used in environmental studiesand usually relies on the average composition ofshale as a baseline and Fe (4.72%) in averageshale is also as normalizing element (Turekian andWedepohl 1961; Trefry and Presley 1976;SinexandWright 1988; Slobodan et al. 1999; Loska et al.2003; Susana et al. 2005). Five contamination cat-egories are recognized on the basis of the enrich-ment factor (Sutherland 2000): EF < 2depletionto minimal enrichment, 2 ≤ EF < 5 moderateenrichment, 5 ≤ EF < 20 significant enrichment,20 ≤ EF < 40 very high enrichment, and EF > 40extremely high enrichment.The index of geoaccumulation originally usedby Müller since the late 1960s (Ji et al. 2008)andthe Potential Ecological Risk Index (Hakanson1980) are the two methods that mostly usedas quantitative measures to assess potential hazardous trace elements pollution and their po-tential ecological risk in aquatic sediment contam-ination, respectively (Covelli and Fontolan 1997;Loska et al. 1997, 2003; Muniz et al. 2004; Audryet al. 2004;Jietal. 2008).The Index of geoaccumulation (Igeo)wascalcu-lated according to:Igeo = Log2 Cn/ (K × Bn) Where Cn is the levels of targeted potential haz-ardous trace elements (e.g., Cu, Ni, etc.) in thesediment samples, Bn is the mean geochemicalbackground values of potential hazardous traceelements in average shale (Turekian and Wede-pohl 1961; Cu, 45.0 mg/kg; Ni, 68.0 mg/kg; Cr,90.0 mg/kg; As, 13.0 mg/kg; Pb, 24.0 mg/kg; Cd,0.30 mg/kg; and Hg, 0.40 mg/kg), K = 1.5 is intro-duced to include possible differences in the back-ground values due to lithological variations. Theinterpretation of the obtained results is as follows:rank 1: Igeo ≤ 0 practically uncontaminated, rank2: 0 < Igeo ≤ 1 uncontaminated to moderatelycontaminated, rank 3: 1 < Igeo ≤ 2 moderatelycontaminated, rank 4: 2 < Igeo ≤ 3 moderately toheavily contaminated, rank 5: 3 < Igeo ≤ 4 heav-ily contaminated, Rank 6: 4 < Igeo ≤ 5 heavily tovery heavily contaminated and rank 7: Igeo > 5very heavily contaminated.Hakanson (1980) introduced the concept of riskin the sediment analysis.He proposed a “potentialecological risk index” which takes into consider-ation a contamination factor, the toxicity of themetal, its abundance, etc. The value of the indexof potential ecological risk is described by thefollowing equation:Cif= Cis/Cin Eir = Tir × CifRI =n 1Eir =n 1Tir × Cif=n 1Tir × Cis CinWhere: Cifis the contaminative factor of element(i), Cis is the examined element (i) in the samples, Cin is the background value (here is hazardoustrace elements in average shale), Eir is the poten-tial ecological risk of element (i), Tir is toxicityparameter of element (Cu, 5; Ni, 2; Cr, 2; As, 10;Pb, 5; Cd, 30; and Hg, 30), and RI is the index ofpotential ecological risk of total hazardous traceelements (Table 3).Results and discussionDistribution and heavy metal levels in lakesedimentsTable 2 summarizes total potential hazardoustrace element levels in western Chaolu Lakesediments over the sampling period. As can beseen, the ranges of these potential hazardoustrace element levels vary much from differentsampling stations, the range of Cu vary from23.3 mg/kg to 48.4 mg/kg, Ni, 29.8∼65.7 mg/kg;Cr, 41.9∼106 mg/kg; As, 3.88∼12.7 mg/kg; Pb,72.0∼113 mg/kg; Cd, 0.38∼1.33 mg/kg; and Hgis 0.107 mg/kg to 0.218 mg/kg. The ranges ofthe selected potential hazardous trace elementlevels were within the world common range inlake sediment that Förstner and Whitman (1981)reported. To allmean heavymetal andAs concen-trations, only Pb and Cd, exceeded average shale(Turekian and Wedepohl 1961). In fact, Pb andCd are much higher than the background valuesat every sampling station from the lake, the meanvalues of Pb, Cd are 94.9 mg/kg and 0.92 mg/kg,which are 4.7 and 3.1 times than that of averageshale (Turekian andWedepohl 1961), respectively(Table 3).The present study, as can be seen from Fig. 2,the EF values of Cu, Ni, Pb, As, and Hg werealmost always <2.0, suggesting that these fivepotential hazardous trace elements were minimalenrichment in sediments of Chaohu Lake. But toPb and Cd, the value was approaching to 11 and 8, respectively, which means that there was signif-icant anthropogenic impact of Pb and Cd levels inthe lake sediments. For example, the higher EFvalues of Cd observed at station 204# and 206#of the lake, which are entrances of Shiwuli Riverand Paihe River into the lake, receiving untreateddomestic and industrial wastewater from HefeiCity and Feixi Country. Figure 2 also shows thatthe EF values at the entrance where Nanfei Riverentering into the lake are not high except Pb,the highest EF values of Pb and Cd occur at theentrance of Paihe River entering into the lake,indicating that Paihe River maybe also the othermain region of these potential hazardous traceelement pollution into the Chaohu Lake.Heavy metal contamination and potentialecological risk in sedimentsTo evaluate the selected potential hazardoustrace element pollution and obtain their potentialecological risk systematically, the present studycombines with the two methods of the index ofgeoaccumulation (Ji et al. 2008) and the PotentialEcological Risk Index (Hakanson 1980) to assessCu, Ni, Cr, As, Pb, Cd, and Hg in sediments ofwestern Chaohu Lake.Tables 4 and 5 are the values of geoaccumu-lation index and potential ecological risk index,respectively. As can be seen from Table 4,the Igeovalues of Cu, Ni, Cr, As, and Hg are less than 0in all sampling stations, applying the classificationby Ji et al. (2008), the tested sediments in westernChaohu Lake are practically uncontaminated forthese five hazardous trace elements, Table 5 alsoshows that the values of Cif< 1 and Eir < 40 ofCu, Ni, Cr, As, and Hg, suggesting that there islittle contamination of these potential hazardoustrace elements and their potential ecological riskare light, respectively.Among all sampling stations, the range ofIgeo(Pb) is from 1.26 to 1.91, and the average1.64, indicating that Pb in examined sedimentsare moderately contaminated, and the value ofEir (Pb) < 40, suggesting that Pb in sediments hasa light potential ecological risk.