Biogeochemical Characteristics Of Trace Rare Elements And Environmental Change In The Bohai Bay And The Changjiang Estuary

The studies on the biogeochemical behaviors of trace rare elements in marine environments are of important scientific and practical significance in evaluating marine eco-environment and studying marine environmental evolution. In this study, the trace rare elements in seawaters and sediments from Bohai Bay and Changjiang Estuary, typical bay and estuary, were analyzed. The systemic study on the geochemical distributions, transport processes, controlling factors, potential ecological risks, sources and sedimentary environmental evolution of trace rare elements and the coupling relationships between trace rare elements and environmental parameters in seawaters and sediments of Bohai Bay and Changjiang Estuary were helpful to develop the researches on the marine biogeochemical processes of trace rare elements. A series of results and viewpoints were presented as follows:1. The biogeochemical characteristics and behaviors of dissolved trace rare elements in Bohai Bay seawaters indicated their strong"nutrient-type"behaviors. The main sources for dissolved trace rare elements were the riverine input and atmospheric deposition.Se in Bohai Bay seawaters mainly existed as selenite (Se(IV)) and selenate (Se(VI)) and the predominant speciation was Se(VI), accounting for 66.7% of total Se; Sb mainly existed as antimonite (Sb(III)) and antimonate (Sb(V)) and the predominant speciation was Sb(V), accounting for 89% of total Sb; Te mainly existed as tellurite (Te(IV)) and tellurate (Te(VI)) and the predominant speciation was Te(VI), accounting for 71.4% of total Te. The dissolved trace rare element concentrations in the surface seawaters showed decreasing gradients from the coastal waters to the central bay with the higher concentrations near the Haihe Estuary. This distribution pattern was closely related to the riverine inputs, environmental conditions and biological activities. The concentrations of Se(VI), Sb(V) and Te(VI) at the surface waters were higher than those at the bottom waters in the regions with high phytoplankton biomasses, which was attributed to the biological oxidation of Se(IV), Sb(III) and Te(IV) at the surface waters, suggesting that biological process was a key factor controlling the speciation and concentrations of Se, Sb and Te in these regions. Besides, the riverine input was a main contributor to the trace rare elements in Bohai Bay. According to our estimate, about 5.67, 45.2, 33.4 and 4.89 t/yr of Se, Sb, As and Te reached Bohai Bay via rivers. However, the dissolved Sn concentration at the surface waters showed an increasing gradient from the coastal waters to the central bay with the higher concentration near the Haihe Estuary. Besides the riverine inputs, environmental conditions and biological activities, this distribution pattern was closely related to atmospheric input. About 2 and 0.85 t/yr of Sn reached Bohai Bay via rivers and atmosphere.The concentrations of trace rare elements (Se, As, Sb, Te and Sn) in Bohai Bay seawaters were present at the level of trace concentrations, which had no obvious harm on marine organisms. All of the trace rare elements had the significant relationships with nutrients, chlorophyll a concentrations and phytoplankton biomass, suggesting that they seemed to be involved in the biological cycling. Among these elements, Se was required for the normal growth and development of phytoplankton; Sb and As were assimilated by phytoplankton via the phosphate uptake pathway; Te and Sn could inhibit the growth of phytoplankton. Thus, these elements played key roles in regulating the phytoplankton productivity.2. The horizontal distribution pattern of trace rare elements in the surface sediments of Bohai Bay and Changjiang Estuary was coincident with the sedimentation pattern and controlled by the mode of"sediment type-transport process-sources". Results of the chemical speciation, enrichment factors and geoaccumulation indexes of trace rare elements suggested that the potential ecological risks and polluted levels were different for each element and these two areas were from unpolluted to moderately polluted level.Trace rare element concentrations increased with the sediment particle size decreasing and the clay content increasing. Besides, trace rare element concentrations had the significant positive correlations with Fe, Mn and Al oxide contents, suggesting that the distributions of trace rare elements were controlled by the physical transport and hydrodynamic processes, the dominant driving power affecting the sediment transportation and distribution. The chemical speciation of trace rare element concentrations changed with the change of sedimentary environmental conditions (e.g., pH, salinity, redox condition), suggesting that the distributions of trace rare elements also were controlled by chemical transport process. Besides, trace rare element concentrations increased with the total organic carbon (TOC) content and chlorophyll a concentration increasing, suggesting that biological transport process also influenced the distributions of trace rare elements. The environment factors (EFs) of Sb, Te and Bi were higher than 1.5, suggesting anthropogenic inputs; however, the EFs of other elements were lower than 1.0, revealing the natural inputs. Result of sequential extraction analyses showed that Sn, Ge and Tl were of no risk, Ge, Sb, In, W and V were at low risk, Te, Bi, Mo and U were at medium risk, and Cd was at high risk. The geoaccumulation index (Igeo) suggested that Ga, W, U, Ge, Mo, In, Tl and V could be regarded as unpolluted, Cd and Sn as unpolluted to moderately polluted, Sb as a moderately to strongly contaminated level, Bi as a strongly contaminated level and Te as a extremely contaminated level. The average Igeo (0.46) of all trace rare elements contained in the surface sediments of Bohai Bay reflected that this area was from unpolluted to moderately polluted level.Trace rare element concentrations in the surface sediments of Changjiang Estuary presented a decreasing trend from the coastal waters to the central sea with the highest concentrations near the Changjiang River and Hangzhou Bay. This distribution pattern was attributed to the sediment type, riverine input, atmospheric deposition and environmental conditions. The high concentrations of trace rare elements appeared at the high clay region, suggested that trace rare element concentrations increased with the clay content increasing. The riverine and atmospheric inputs were main contributors to trace rare elements in Changjiang Estuary. According to our estimate, about 4426, 37, 194, 92, 383, 19, 1614 and 599 t/yr of As, Hg, Bi, Se, Sb, Te, Sn and Ge reached the Changjiang Estuary via rivers and about 45.0, 31.1, 10.0, 3.5 and 1.8 t/yr of As, Se, Sb, Sn and Ge reached Changjiang Estuary via atmosphere. Result of sequential extraction analyses showed that As, Hg, Sb, Sn and Ge were of no risk, Bi and Se were at medium risk and Te was at high risk. The geoaccumulation indexes suggested that Hg, Sn and Ge contained in the sediments of Changjiang Estuary could be regarded as unpolluted, Se as unpolluted to moderately polluted, As, Sb and Te as a moderately contaminated level and Bi as a moderately to strongly contaminated level. The average Igeo (0.78) of the eight trace rare elements contained in the surface sediments of Changjiang Estuary reflected that this area was from unpolluted to moderately polluted level.3. Results of chemical speciation of trace rare elements in core sediments of Bohai Bay and Changjiang Estuary suggested the translation among the different fractions. The concentrations, enrichment factors and burial fluxes of trace rare element in core sediments of Bohai Bay and Changjiang Estuary varied significantly in recent one hundred years, so that the past century was divided into three periods.The distribution patterns of trace rare elements in core sediments of Bohai Bay and Changjiang Estuary were closely related to the sediment type, sources and diagenesis. The concentrations of trace rare elements increased with clay and TOC content increasing. There were significant native relationships among the different chemical speciation of trace rare elements, suggesting the translation among the different fractions. Based on the concentration, enrichment factor and burial flux changes of trace rare elements, the environmental evolution of Bohai Bay and Changjiang Estuary was divided into three periods. Due to the different occurring time of natural events and anthropogenic activities, the environmental evolution history between Bohai Bay and Changjiang Estuary was slightly different.Environmental evolution of Bohai Bay was divided into three periods, before 1970s, from 1970s to 1990s and after 1990s. These three periods for temporal changes of trace rare elements in Bohai Bay in recent one hundred years were attributed to the natural and anthropogenic activities. In the first period, the concentration, enrichment factor and burial flux changes of trace rare elements displayed an increasing trend with depth. This period was characteristic of relatively high runoff and sedimentation rate, the trace rare elements was mainly from natural sources. The second period was the time when the Bohai Rim Economic Circle developed quickly, as well as the pollution treatment and protection measures were inappropriate. During this time, Bohai Bay had been subjected to various pollutant sources and its environment became worsen. Thus, the influence of this period by anthropogenic activities was significant. The third period was the time when sediment environment had been improved.The environmental evolution of the Changjiang Estuary was divided into three periods, before 1960s, from 1960s to 1990s and after 1990s. These three periods for temporal changes of trace rare elements in Bohai Bay in recent one hundred years were attributed to the natural and anthropogenic activities. In the first period, the burial fluxes of trace rare elements had a slight fluctuation with a low level, suggesting that there was no obvious environmental change and the natural input was a main source in this period. The second period was the time when industries and agricultures along the coast of East China Sea developed quickly, as well as the pollution treatment and protection measures were inappropriate. During this time, the Changjiang Estuary and its adjacent areas had been subjected to various pollutant sources and its environment became worsen. Thus, the influence of this period by anthropogenic activities was most significant. The third period was the time when the sediment environment had been improved.