Impacts of active tectonics on geogenic arsenic enrichment in groundwater in the Hetao Plain, Inner Mongolia

High arsenic (As) groundwater has become a serious environmental problem threatening the health of 300 thousand people in the Hetao Plain (HP) of Inner Mongolia, China, where the microscopic mechanisms of As releasing in groundwater have been widely studied. However, the macro-behaviors of As under the large-scale geological conditions, particularly the impact of geogenic arsenic enrichment in groundwater by active tectonics is still uncertain. Here, we combine in-situ measurements of As concentration in groundwater and ore bodies, the locations of ore bodies, the active faults patterns and chronology, and the paleo-earthquake age sequences to explore the transport processes of As from source to groundwater controlled by the geological forces. Results show that the spatial pattern of groundwater As concentration exhibits large heterogeneity in the HP, particularly high in two areas: the Hangjin Rear Banner and Wuyuan area. Langshan, western part of the Western Yinshan Mountains, is more enriched in As-laden ore bodies and black rocks (dark carbonaceous sedimentary rocks and their metamorphic rocks). In the south of HP, reducing groundwater does not contain high As contents because the sediment (i.e., Yellow River alluvial deposit) itself bears little As. The larger vertical uplifting rate made a larger erosion rate of Langshan during the 50 ka B.P to 20 ka B.P, causing higher As background values in sediments. Differential uplifts, piedmont active faults, and earthquakes act as the internal dynamics, whereas the external effects of weathering, transport, and deposition play important roles in driving As-bearing bedrock debris to the foreland basin. The younger with more frequent seismotectonic events renew the As-bearing bedrock debris in sediments and the organic matter reactivity, causing higher As released to groundwater. This study can guide future identification of the geogenic high As groundwater.