Insights into paleoenvironment and chemical weathering as controls on arsenic accumulation in Late Pleistocene to Holocene aquifers

Abstract

High As groundwater has been reported in aquifers comprising Late Pleistocene and Holocene sediments worldwide, and could be due to the depositional environment and sedimentological processes. To investigate the paleoenvironment and chemical weathering as mechanisms controlling the fate of As, this study characterized the elemental, clay mineral, and Li isotopic compositions of sediments of an As-contaminated aquifer in the Datong Basin, China. Indices for reconstructing chemical weathering conditions, for example, the Chemical Index of Alternation (CIA), indicate overall incipient to intermediate chemical weathering conditions in the Datong Basin during the Late Pleistocene to Holocene. The higher CIA values at depths of ∼20–∼85 m indicate stronger chemical weathering and a shift in climatic regime towards warm and humid conditions in Stage 2 compared to Stage 1 and Stage 3. Under such climatic conditions, intense chemical weathering favors the leaching of silicate-bound Fe and thus the production of various secondary Fe-oxide minerals. Besides, the positive relationship between CIA values and easily reducible Fe oxides further shows the enrichment of more weakly crystalline Fe-oxides during the depositional condition; this finding is also supported by the results of chemical extractions performed on the sediments. Moreover, the good correlations between total organic carbon (TOC) and both Fe/Na and As concentrations suggest the enrichment of As and OM along with Fe enrichment during chemical weathering; this is attributed to the adsorption or co-precipitation of As and OM on Fe minerals. Under warm and humid conditions, intense chemical weathering released a large amount of Fe minerals; As was also released due to stronger weathering of the terrestrial bedrock and sequestered by Fe oxides and organic matter, thus forming complexes involving As, Fe, and OM. FTIR and chemical extraction results confirm the formation of As–Fe-OM complexes during the geological history contributes to As enrichment and accumulation in the late Pleistocene sediments. These findings provide new insights into the interaction between the paleoenvironment and the accumulation of ferric oxides/hydroxides and OM, which serve as controlling mechanisms with respect to As enrichment in Late Pleistocene to Holocene sediments, and thus shed light on the sedimentogenesis of high As aquifers.