| Jianghan Lake Group,the largest freshwater lakes in China,locates in the middle and lower reaches of Yangtze River,which is rich in resources rich and belongs to the economic developed area of China.With the continuous interference of human activities,the lake area is shrinking,the degree of eutrophication of lakes is accelerated,and lake classification is more refined.Currently,the study of organic pollutants such as n-alkanes(n-A),polycyclic aromatic hydrocarbons(PAHs),organochlorine pesticides(OCPs)and other pollutants in this area are mostly limited in exploring the distribution characteristics of total organic pollutants concentration and their source apportionment.The study based on the different types of lakes with comprehensive and comparative analysis has not been reported yet.In the present study,sediment cores in three representative lakes in the middle and lower reaches of the Yangtze River basin(East Lake,Liangzi Lake,Tangxun Lake)were sampled in the fall of 2013,to investigate the current pollution status of the Jianghan Lake groups,and supply a technical support for corresponding countermeasures.The concentration of n-alkanes,polycyclic aromatic hydrocarbons and organic carbon in the sampled sediment cores were analyzed.Their potential sources were identified based on corresponding diagnostic ratios.The ecological risk assessment caused by the contamination of PAHs was also investigated.The results indicate that the concentrations of n-alkanes in East Lake range from 2880 to 19906 ng/g;the total PAHs span the range of 184-1125 ng/g;the total organic carbons are in the range of 0.65%-5.01%.In Liangzi Lake,n-alkanes are in the range of 1157-23811 ng/g,the concentrations of total PAHs locate between 44 and 324 ng/g,and the total organic carbons are from 0.79% to 4.21%.In Tangxun Lake,n-alkanes range from 4580 to 28502 ng/g,the total PAHs range from 169 to 545 ng/g,and the total organic carbons are in the range of 0.76%-4.18%.Compared to the previous reports,the concentration of n-alkanes in lake sediments are high.PAHs in the East Lake range from moderate to high pollution level;PAHs in Liangzi Lake and Tangxun Lake are at moderate pollution level.The vertical distribution profile analysis of n-alkanes indicates that n-alkanes in Liangzi Lake and Tangxun Lake are almost constant under from the depth of 10 cm to the bottom(around 50-60 cm).However,they keep constant form the depth of 20 cm in East Lake.It is obvious that East Lake has suffered from significant disturbance degree compared with the other two lakes.The distribution profiles of n-alkane homologues exhibit with the characteristic of double peaks,where n-C27,n-C29,n-C31 and n-C33 are dominant,followed by the peak shape of n-alkanes with low molecular weight(n-C10 to n-C19).The carbon preference index(CPI)analysis indicates that n-alkanes in surface sediment are originated from terrestrial plants and the release of petroleum hydrocarbons.The CPI values in the three lakes decrease with the increase of the sediment depth,which suggests that the some of the biogenic alkanes have been biodegraded.Wax plant and natural n-alkanes ratios also show the mixed contribution of terrestrial plants and petroleum hydrocarbons to n-alkanes in the three lakes,with the predominance of terrestrial plants input.Moreover,the ratios of pristane / phytane are less than 1,indicating the anoxic sedimentary environment and terrestrial organic matter origin in the three lakes.Both ratios between pristane / n-C17 and phytane / n-C18 are small,indicating the high evolution degree.All other diagnostic ratios analysis also suggests that n-alkanes in the three lakes are dominantly derived from terrigenous organic matter.Parent PAHs and their alkylated congeners were identified and quantified in the present study.For parent PAHs,4-6 ring parent PAHs are dominant compared with 2-3 rings congeners,suggesting the dominant pyrogenic contribution;perylene is dominant among the identified parent PAHs,especially in the deep depth of sediment core,suggesting the dominant biogenic source in the deep sediment.For APAHs,fluoranthenes,pyrenes and phenanthrenes are dominant compared with other families.Parent PAH is dominant in each APAH family,and the level of APAHs in each family decrease with the increased alkylated degree,which suggests the dominant contribution of pyrogenic source combined with the direct release of fossil fuel.The total concentrations of parent PAH are generally higher than alkyalted congeners in the different depth of sediment cores in the three lakes.Similar to n-alkanes the vertical distribution analysis of PAHs shows that PAHs in Liangzi Lake and Tangxun Lake are almost constant under 10 cm depth,while East Lake after 20 cm depth.Generally,the levels of APAHs and most of parent PAHs decrease with the increase of the sediment depth,which suggests the direct release of petroleum hydrocarbons is significant in the surface sediment.However,perylene,a typical biogenic PAH,increases with the increase of the sediment depth.Therefore,the contamination of three lakes have similar characteristic.However,the vertical distribution profiles of PAHs vary with specific environment,because the different types of lakes have suffered from the varied impacts of human activities.For example,East Lake,locating in the center downtown of Wuchang,has suffered from longest term impacts from human activities since 1970 s.This history was recorded by the altered levels of n-alkanes and PAHs in the depth of around 20 cm.Reversely,the other three lakes are relatively undisturbed,their historical contamination records are relative stable till the depth of 10 cm.Similarly,the isomer ratio analysis also demonstrates the variation of PAH source in the different segments of sediment core.In all,biogenic source is dominant for PAHs in the deep depth of sediment core.The upper sediments are highly impacted by the direct release of fossil fuels,but pyrogenic source is still dominant.The ecological risk assessment demonstrates that no negative biological effects will be caused in the three lakes.Similar to n-alkanes and PAHs,total organic carbon(TOC)decrease rapidly with the increase of sediment depth,and then almost un-alter in the following deep depth.It is clear that organic matters have suffered from biodegradation process in the deep depth of sediment core.The linear regression of TOC and n-alkanes(PAHs)in the three lakes demonstrates that the linear correlation coefficients are 0.92,0.90,and 0.89(for n-alkanes),and 0.60,0.91,and 0.62(for PAHs),respectively,in East lake,Liangzi Lake and Tangxun Lake.This suggests that the study of n-alkanes and PAHs can reveal the source of organic matters in these lakes. |
PDF Full Text Request