The Sedimentary Record Of N-alkanes, PAHs And OCPs In The Sediment Core From Songhua Lake, Jilin Province

Songhua Lake is the largest artificial lake in Northeast China as part of the upper Songhua River.The developments of resources involve hydroelectric power generation and flood control,etc.The reservoir plays an important role in ecological protection,economic development and resident's life in area of the basin.The catchment is a stable collection area,by the means of degradation and production of bacteria and phytoplankton in the lacustrine ecosystem,as well as the load of the sediment from upper stream and coastal erosion,the extraneous refractory organic compounds can be preserved in the lake sediments.In this study,the geochemical characteristics of n-alkanes,PAHs and OCPs in a sediment core collected in Songhua Lake were measured through the calculation of conventional geochemical parameters,combined with the application of UNMIX multivariate source receptor model and BP neural network model.In this way,the main source types of sedimentary organic matter?OM?,the PAHs compounds and OCPs compounds in the core were analysed quantitatively,which provides implications on the changes of depositional environments and human activities spanning the last 60 years in this region.The content of total n-alkanes(C₁₃-C₃₆)in the natural sediments ranged from20.39?g/g to 168.35?g/g,averaged at 72.58?g/g,and the mean concentrations of PAHs and OCPs in the surface sediment were 7119.19ng/g and 71.14ng/g,respectively.The sedimentary rates of n-alkanes initially decreased and then increased from the bottom up.Molecular distribution patterns included unimodal distribution and bimodal distribution maximizing at C₂₉?C₃₁ and C₁₇?C₂₉?C₃₁,respectively,which were in conjunction with CPI,LMW/HMW,Paq and ACL values varying with depth.The content differences of PAHs compounds between core intervals at the bottom section were not remarkable.The input of Bicyclic-Tricyclic PAHs,tetracyclic PAHs and pentacyclic-hexacyclic PAHs fluctuated in the early early sedimentary history and then decreased.Pentacyclic-hexacyclic PAHs compounds accumulated significantly in recent core intervals to the surface,however the content of tetracyclic PAHs and pentacyclic-hexacyclic PAHs remained insignificant.Among the OCPs compounds,DDTs and HCHs accumulated relatively higher than other compounds in this class.The short-chained n-alkanes and middle-chained ones are primarily sourced from lacustrine algae and aquatic macrophyte,respectively.The input flux mainly fluctuated visibly owing to the intense variation of temperature and precipitation before mid-1960s but enhanced slightly since then.The OM having abundant long-chained molecules(>C₂₇)and strong odd/even predominance are interpreted to be derived from higher terrestrial vascular plants.It is concluded that terrestrial input accumulated extremely high in the primitave soil before reservoir filling at 1943,and a steady decrease occurred afterwards in response to the change of hydrodynamic conditions.A shift to relatively high proportion of terrigenous sediment occurred at mid-1960s and is explained by the increasing frequency of heavy floods and wanton destruction of vegetation-covered land which induced severe soil loss.In recent 20 years,the primary productivity of planktonic algae became active to some degree;however,terrestrial sources still dominated the input of OM in the sediment.Before the 1960s,the exhaust emissions of coal plants were the main source of PAHs into the Songhua Lake,which contributed approximately 36%⁶3%of the total content of PAHs and increased with time.Entering the 1960s,the main input sources of PAHs changed from of coal plants exhausts to gasoline combustion source and diesel combustion source,and since the late 1960s,gasoline combustion source contributed most PAHs compounds in the sediment.However,the PAHs of the traffic tunnel source began to make a significant contribution to the sediment input in the mid-1970s,and generally showed an increasing trend.Since the 1990s,the PAHs of the traffic tunnel source has occupied half of the total PAHs sedimentary input,followed by gasoline combustion emission source emission products.The organochlorine pesticides?OCPs?,such as chlordane,endosulfan and chlordane,began to be imported into the lake in the 1960s,but the contents of other compounds fluctuated in most sediment layers,except for the continuous input of DDTs throughout the whole sedimentary history.The DDTs detected in the sediment samples were mainly industrial products residues,and the HCHs were inherited directly from lindane.In contrast,dieldrin and endrin were flowed into the lake at low content level before the1970s,indicating the migration distance was remote.Before the 1960s and after the mid-1980s,the sedimentation process in Songhua Lake tended to be in an oxidation condition,while during this 20 years the sedimentary environment in Songhua Lake tended to be under reducing condition.