THE HETEROGENEITY OF ARSENIC IN THE CRUST: A LINKAGE TO OCCURRENCE IN GROUNDWATER

The occurrence of elevated concentrations of As in groundwater (> 50 ug/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 accepted view is that high-As ground waters are not necessarily related to areas of high-As in source rocks. On the other hand, the heterogeneous As distribution in the crust 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 < 1 mg/kg to > 100 mg/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 for that crust. Beyond the upper crust of Japan, 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 have elevated groundwater As. 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.