论地壳中砷丰度的不均一性与高砷地下水分布的关系

在过去的二十多年以来,世界各地越来越多的地区不断地发现高砷＞50ug/L地下水.饮用高砷水危害到几千万人口的健康,尤其在亚洲.学术界普遍认为区域性高砷水的分布不一定与沉积物源区的岩石的砷含量有关.文章对此观点提出疑问和修正.这主要是基于砷在地壳中的分布与地壳的构造历史有着较明显的空间系统关系.在岩浆岩、沉积岩和变质岩体中,砷的浓度范围为＜1mg/kg到＞100mg/kg.并且砷浓度的分布不为正态,平均值与中值有差异.砷丰度在晶岩中分布的不均一性主要是由于富砷矿物的含量高.目前岩石中砷浓度的数据较少,不足以用于研究砷异常分布,如异常分布图的制定.但是,高砷的硫化物矿物普遍存在于中国的金矿中,而且其分布与板块碰撞的造山带有密切关系.在日本新近的上地壳中,砷(与锡)的丰度均超出地球上地壳的平均值.这里的砷异常已被解释为源于海相沉积物.因此文章提出地壳中的砷高异常与不均一性成因的假说:即海洋-大陆型碰撞之后再加上大陆-大陆型碰撞的不断富集过程.富砷矿物包括硫化物、氧化物及硅酸盐(较少见).这些富砷矿物在风化之后,被下游的快速沉降的盆地积累,使这些地区较易产生高砷地下水.文章指出两个未来研究方向:源区岩石砷的浓度及矿物学研究,和风化作用及沉积过程的研究,以便确证地表中砷的丰度异常与高砷地下水分布的关系.

The Heterogeneity of Arsenic in the Crust: A Linkage to Occurrence in Groundwater

The occurrence of elevated concentrations of As in groundwater (＞50μg/L) has been reported at an increasing number of locations around the world in the last 2 decades, affecting the health of tens of millions humans and mostly in Asia. A widely held view is that high-As ground waters are not necessarily related to areas of high-As in source rocks. This paper challenges this view by presenting evidence for the heterogeneous As distribution in the crust that was surprisingly systematic in relation to the plate tectonic evolution of the crust. The concentrations of As in igneous, sedimentary and metamorphic rocks can range from＜1mg/kg to＞100mg/kg, and are not normally distributed because the geometric mean and median values are different for each rock type. This heterogeneity of As in crystalline rocks is tentatively attributed to the mixing of dispersed As-rich common minerals such as sulfides and oxides into a matrix of other rock-forming minerals. Data are too few to construct a geographic distribution of rocks with high-As anomaly but As-rich sulfides in gold deposits in China are geographically associated with the orogeny belts formed by collisions of two cratons. Because the enrichment of As (and Sb) in the young upper crust of Japan relative to the representative values of As in the upper crust can be traced back to subducting marine sediments, a conceptual model of oceanic-continental collision followed by continental-continental collision is proposed to explain the systematic geographic patterns of As anomaly. The As anomalies are mainly in the ophiolitic complexes that host As-rich minerals either as sulfide minerals in ore deposits mined for more valuable metals, or as oxide or silicate minerals in the Himalayan serpentinites. It is further hypothesized that rapid and perhaps selective weathering of As-bearing minerals allows As to be trapped in rapidly accumulating sedimentary basins or deltas down-gradient, making these locations more prone to develop groundwater As problems. Further investigations of As concentrations and mineralogy in source rocks and its re-distribution, transformation and deposition in sedimentary aquifers are required to determine the significance of As heterogeneity in the crust to groundwater As.