Copper And Its Isotopic Compositions In A Large Anthropogenically Impacted River

Copper（Cu）is a transition metal element widely hosted in rocks,minerals,and fluids,which is usually involved in diagenesis and minerogenetic processes.Moreover,Cu is also an essential micronutrient element for life,playing an important role during processes for example enzyme formation and protein regulation.However,Cu in high concentration usually acts as a toxic pollutant,and thus threating to the ecosystem and human health once released into the environment in excess.It has been reported that the amount of Cu discharging to the natural environment is estimated as high as 3.5×10⁷kg each year due to human activities such as mining,industry,food,and biomedicine.This additional source of Cu may significantly affect its biogeochemical cycle.Fluvial systems carry not only abundant information related to elemental biogeochemical cycle but also human activities.In the Earth’s“critical zone”,rivers are an important pathway for the transfer of the weathering products of rocks and minerals from continents to oceans.In densely inhabited and agricultural/industrial regions,rivers act as not only water supply for regional development but also sinks and carriers of various pollutants produced by human activities.Therefore,the clarification of sources,migration and transformation of Cu in large rivers may provide us crucial clues and implications for further understanding the biogeochemical cycle of Cu,and constraining the flux of Cu from continents to the oceans.As pointed out in recent studies,copper isotope(δ⁶⁵Cu)is a potentially powerful proxy for distinguishing the anthropogenic and natural sources of Cu in natural ecosystem as well as tracing biogeochemical processes.However,previous studies mainly focus on natural background and mining-affected rivers,while few systematic studies are carried out in large river systems impacted by multiple anthropogenic activities.The Pearl River is the second largest river in China in terms of annual discharge,with geologically significant differences from upstream to downstream.In the upstream,the surface outcrop is mainly covered by karst areas,whereas in the middle-lower reaches,silicate sedimentary rocks and Quaternary sediments are widely spreading.In the catchment,population density,industrialization and economic development display obvious imbalance between the upstream and downstream.Therefore,the Pearl River basin is ideal to be chosen to study the geochemical behavior of Cu in rivers.River samples（dissolved load,suspended load,bed sands and bank silt sediments）were collected from upstream to downstream for the whole Pearl River basin during the dry and wet seasons.In addition,depth-profile suspended sediments were also collected in wet seasons.In order to establish a spectrum of Cu isotope compositions,potential sources of Cu in rivers were also collected.Through the analysis of both Cu concentrations and Cu isotope compositions,combined with other geochemical parameters（e.g.hydrochemical parameters,major and trace elements,and organic carbon content）,the major results and conclusions were summarized as follows:1)Our study provided a systematic evaluation of the temporal-spatial evolution of dissolved Cu and its isotopes over the whole Pearl River basin,and with its sources being constrained as well.The influence of internal processes inside the river on the migration and transformation of dissolved Cu were also explored.The surface dissolved Cu concentration varies from 3.1 to 49.6 nmol/L in the Pearl River,with an average of12.2 nmol/L,which is lower than the worldwide average Cu concentration of river waters（23.3 nmol/L）.The highest river dissolvedδ⁶⁵Cu values were observed in the Pearl River basin,which are all positive and ranging from+0.20 to+1.83‰,and being much higher than that of soils and primary rocks.In general,the dissolved Cu concentration andδ⁶⁵Cu values both increased from upstream to downstream.The surface dissolvedδ⁶⁵Cu value was higher in the downstream Pearl River delta region compared with the fractionation values of Cu isotope resulted from chemical weathering,possibly indicating that dissolved Cu at here was mainly controlled by other sources.In the up-middle reaches,the dissolved Cu was mainly derived from natural sources,whereas in the lower reach,the major sources were human activities including mine wastewater and effluent from sewage treatment plants rather atmospheric,agricultural and traffic emission.In contrast with the increasing contribution to dissolved Cu from carbonate dissolution in the upstream karst areas in the dry season,the dissolved Cu in the river delta was mainly affected by mine wastewater during the wet season.2)The temporal-spatial evolution of Cu contents and its isotopes of suspended sediments were also determined,and with the sources of suspended sediments being also constrained.The Cu contents of surface suspended sediments ranged from 21.5 to361.2μg/g in the Pearl River,with an average value of 62.1μg/g.In general,the highest contents of Cu were observed in the Beijiang River and the Pearl River Delta but there was no regular trend along the river channel.Enrichment analysis suggested a slight enrichment of Cu in suspended sediments and bed load,with an enrichment factor（EF）of 0.75～9.3（average value,1.93）.As similar with contents,the degree of enrichment of Cu significantly increased in the Beijiang River and the Pearl River Delta.The mass balance calculation and the partition coefficient of Cu both suggested that Cu was mainly transported by suspended forms in the Pearl River Basin.Theδ⁶⁵Cu values of suspended sediments and bedload ranged from-0.03 to+0.35‰,which was close to that of soils and primary rocks.Combined with the EF and the Cu isotope composition,we concluded that the Cu in suspended sediments and bedload was mainly derived from natural sources.Besides,the decreasing Cu contents and increasingδ⁶⁵Cu value after the confluence of the Nanpanjiang and Beipanjiang in the upper reaches might be affected by biological activities,industrial and mining activities.The increase of Cu contents andδ⁶⁵Cu values in the Beijiang River and Pearl River Delta were mainly controlled by mining and municipal sludge.The disagreement between the isotopic fractionation induced by the transformation of mineralogy and the spatial change of mineralogy of bedload indicated that Cu contents and its isotope compositions in suspended sediments and bedload were not controlled by grain size and mineral phase in the Pearl River basin.3)We also found that the seasonal variations of dissolved and particulate Cu concentrations in large rivers should be taken into account for precise estimation of flux to oceans.In the Pearl River basin,dissolved Cu was mainly exists in the form of organic complexation.Correlations between organic carbon and Cu concentration/Cu isotope composition indicated that the in-situ organic complexing,biological adsorption and uptake had a little influence on the temporal-spatial changes of dissolved and particulate Cu and its isotopes in the Pearl River basin.Correlations between the concentration of suspended sediment and Cu content/Cu isotope compositions showed that the mineral adsorption was not the controlling factor for the temporal-spatial changes of dissolved and particulate Cu and its isotopes in the Pearl River Basin.And it’s worth noting that the concentration and isotope composition of dissolved and particulate Cu were significantly different before entering the estuaries between the dry and wet season,which indicated that the seasonal variation should be taken into account when estimating the flux of Cu and weighted Cu isotopic composition to oceans.