Impact of sedimentation history for As distribution in Late Pleistocene-Holocene sediments in the Hetao Basin, China

Abstract

Purpose: To understand the impact of geochemical sedimentation history for arsenic (As) distribution in the sediment profiles of the Hetao Basin, we (1) evaluated sediments provenance and variations of weathering intensities, (2) attempted to reconstruct the depositional environments, and (3) explored the As and Fe speciation in the sediments. Combining the information above, different sedimentation facies were distinguished in the vertical profiles. Methods: Two sediments cores were drilled up to 80 m depth. Major and trace element compositions, including rare earth elements (REE), were analyzed. Carbon isotope ratios (δ13Corg) of embedded organic matter in the sediments were analyzed by isotope ratio mass spectrometry (IR-MS). Arsenic and Fe speciation of the sediments were determined by sequential extractions. Results and discussion: The similar REE geochemistry of rocks from the Lang Mountains and sediments in the Hetao Basin indicated that the sediments originated from the Lang Mountains. The C/N ratio (~ 4 to ~ 10) in combination with δ13Corg (− 27‰ to −2 4‰) suggested that sediments were mainly deposited in aquatic environments. The unconfined aquifer equaled the lacustrine deposit with less intensive weathering during last glacial maximum (LGM). Here, the As content (average, 5.4 mg kg−1) was higher than in the aquifer sediments below (average, 3.6 mg kg−1). Conclusion: Higher content of releasable As in combination with paleolake-derived organic matter aquifer sediments probably contributes to higher groundwater As concentration in the unconfined aquifer. This study provides the first insight into the impact of sedimentation history on As distributions in sediment profiles in the Hetao Basin.