Controls on the δ 34S and δ 18O of dissolved sulfate in the Quaternary aquifers of the North China Plain

Abstract

► New data of δ 34 S SO4 and δ 18 O SO4 in large-scale Quaternary aquifers are reported. ► Possible sulfate sources are identified and biogeochemical processes are addressed. ► Specific δ 34 S SO4 and δ 18 O SO4 patterns are presented along the groundwater flowpaths. ► Variable contributions are implied from dissimilatory microbial sulfate reduction. The North China Plain (NCP) is a typical Cenozoic sedimentary basin and one of the largest alluvial plains in eastern Asia. It consists of piedmont alluvial plain, central alluvial-lacustrine plain, and littoral alluvial plain from west to east, where Quaternary aquifer systems developed in different depths. Groundwater from these aquifers is the most important water resource and supports a population of 200 million in the NCP. Currently the Quaternary groundwater is depleting fast due to both human and natural factors. Characterizing biogeochemical processes in groundwater is central to understanding groundwater chemistry. To identify the sources of dissolved sulfate ( SO 4 2 - ) and address subsequent biogeochemical processes in groundwater, we conducted a study of spatial distributions of δ 34 S SO4 and δ 18 O SO4 in the Quaternary aquifers in the northern part of the NCP. The [ SO 4 2 - ], δ 34 S SO4 , and δ 18 O SO4 along the west–east groundwater flowpaths in the three alluvial plains are analyzed and examined. The Quaternary groundwater (∼20 °C) has a [ SO 4 2 - ] of 37.25–177.41 mg/L with [ SO 4 2 - ]/[Cl − ] ratios ranging from 0.12 to 2.79. The δ 34 S SO4 and δ 18 O SO4 range from 4.9‰ to 30.9‰ and from 4.7‰ to 14.0‰, respectively. Supported by other geochemical and isotopic parameters such as total dissolved solids (TDS) concentrations, δD and δ 18 O of groundwater (δD and δ 18 O H2O ), and 14 C contents in groundwater, our study concludes that: (1) The shallow groundwater in the piedmont area is characterized by higher SO 4 2 - concentration of 153.22 mg/L with δ 34 S SO4 of 10.0–14.5‰ and δ 18 O SO4 of 4.7–9.0‰, whereas the groundwater in the piedmont area is characterized by lower SO 4 2 - concentration of 37.25 mg/L with δ 34 S SO4 of 4.9–7.3‰ and δ 18 O SO4 of 5.8–7.5‰. Therefore, the SO 4 2 - in the shallow groundwater is dominated by weathered sulfate minerals from the Taihang mountains and sulfate derived from oxidation of sulfides in infiltration paths, whereas the SO 4 2 - in the deep groundwater is dominated by atmospheric sulfate from precipitation and sulfate derived from decomposition of organic matter during its recharge processes; (2) Dissimilatory microbial sulfate reduction (DMSR) occurred in the deep confined aquifers in both central and littoral plains, where δ 34 S SO4 and δ 18 O SO4 increase along the groundwater flowpaths and present specific patterns for each aquifer which imply variable contributions from DMSR processes; (3) The deep confined aquifer in the littoral plain has a lower DMSR rate than the other aquifers, probably due to the effect of its lower permeability on physical exclusion of bacteria from the aquifer matrix. This study can provide a biogeochemical background for assessing groundwater evolution and its quality in the NCP.