北江沉积物中As和Tl污染历史演变规律研究

Historical evolution of As and Tl pollution in sediments of Beijiang River

为重建北江沉积物中As和Tl污染的历史演变过程,揭示人类活动对北江水体环境质量的影响,采集北江干流7个代表性的沉积柱,分析沉积物理化性质(如pH值、有机质、粒径等)、As和Tl质量比以及过剩^(210)Pb(^(210)Pb_(ex))活性。北江沉积物中As和Tl质量比平均值分别为109.6 mg/kg和1.7 mg/kg,是广东省土壤背景值的12.3倍和2.43倍。富集系数(E_(f))和地积累指数(I_(geo))评价结果表明:参照广东省土壤元素背景值,沉积物中As质量比处于显著或强烈富集水平,污染程度达到中强度水平;Tl处于轻度富集和污染水平。代表性沉积柱沉积年代和沉积速率计算结果显示:1998年前,年平均沉积速率约为0.35 cm/a;而1998年飞来峡水库建成后,年平均沉积速率增加到1.1 cm/a。沉积柱(S6)中As和Tl过剩金属通量经历了5个阶段的变化:受区域矿山开采活动影响,As和Tl过剩金属通量在1954—1988年增长缓慢,1990—1998年快速增加,1999—2002年出现下降,2003—2005年再次增长,2006年后逐渐下降。其中2003—2005年增长是矿山活动和“镉污染事件”共同作用所致,2006年后As和Tl过剩金属通量逐渐下降是政府干预的结果。沉积物中As质量比与铁、锰质量比呈现显著正相关关系,而Tl质量比受细颗粒组分的影响更显著。“洪水事件”改变了沉积物粒径特征,减小了沉积物中As和Tl质量比。多种污染评价方法和铅同位素年代学的有机结合,更清晰地揭示了水体环境质量的演变过程。

To reconstruct the historical evolution of As and Tl pollution in sediments of Beijiang River and reveal the influence of human activities on the water environment,we collected seven representative sedimentary cores in the mainstream of the Beijiang River and analyzed the sediment physical-chemical properties(pH,organic matter and grain size),the mass ratios of As and Tl,and the radioactivity of excess^(210)Pb(^(210)Pb_(ex)).Then,we determined the sedimentary age by lead-210 isotope dating technique and used enrichment factor,geoaccumulation index,and excess metal flux method to evaluate the pollution level of arsenic and thallium in sediments at different periods.The average mass ratios of As and Tl in Beijiang sediments are 109.6 mg/kg and 1.7mg/kg,respectively,which are 12.3 and 2.43 times the soil background values in Guangdong Province.The enrichment factor(E_(f))and Geoaccumulation index(I_(geo))indicate that referring to the background value of soil elements in Guangdong Province,sediments are significant or strong enrichment of As,and the pollution is serious.In general,about 88.6 percent of the sediment samples in the Beijiang River have an arsenic mass ratio above the moderately to strongly polluted level.Arsenic pollution in the sediments is worthy of attention.Tl is slightly enriched and belongs to the light pollution level.The results of^(210)Pb dating and deposition rate in the representative sediments show that the average sedimentation rate is approximately 0.35 cm/a before 1998.However,after the construction of the Feilaixia Dam,the sedimentation rate increased to 1.1cm/a.The excess As and Tl fluxes show five periodic characteristics over the past sixty years in the^(210)Pb dating-core(S6).Affected by regional mining activities,the excess metal fluxes of As and Tl increase slowly from 1954 to 1988,increase rapidly from 1990 to 1998 and decrease from 1999 to 2002,and then increases again from 2003 to 2005.The result is not only because of the mine activities but also the result of“cadmium pollution events”.After 2005,the excess metal fluxes of As and Tl decrease gradually due to government intervention.The mass ratios of As in the sediments have a significant positive correlation with the mass ratios of Fe and Mn,while the mass ratio of Tl is obviously affected by fine particle components.The"flood events"significantly change the grain size characteristics of sediments,thus decreasing the mass ratios of As and Tl.The integrated pollution assessment methods and lead isotope chronology provide a clearer understanding of the evolution process of aquatic environmental quality.