Using dual isotopic data to track the sources and behaviors of dissolved sulfate in the western North China Plain

This paper investigated the sources and behaviors of sulfate in groundwater of the western North China Plain using sulfur and oxygen isotopic ratios. The groundwaters can be categorized into karst groundwater (KGW), coal mine drainage (CMD) and pore water (subsurface saturated water in interstices of unconsolidated sediment). Pore water in alluvial plain sediments could be further classified into unconfined groundwater (UGW) with depth of less than 30 m and confined groundwater (CGW) with depth of more than 60 m. The isotopic compositions of KGW varied from 9.3 parts per thousand to 11.3 parts per thousand for delta S-34(SO4) with the median value of 10.3 parts per thousand (n = 4) and 7.9 parts per thousand to 15.6 parts per thousand for delta O-18(SO4) with the median value of 14.3 parts per thousand (n = 4) respectively, indicating gypsum dissolution in karst aquifers. delta S-34(SO4) and delta O-18(SO4) values of sulfate in CMD ranged from 10.8 parts per thousand to 12.4 parts per thousand and 4.8 parts per thousand to 8.7 parts per thousand respectively. On the basis of groundwater flow path and geomorphological setting, the pore water samples were divided as three groups: (1) alluvial-proluvial fan (II1) group with high sulfate concentration (median values of 2.37 mM and 1.95 mM for UGW and CGW, respectively) and positive delta S-34(SO4) and delta O-18(SO4) values (median values of 8.8 parts per thousand and 6.9 parts per thousand for UGW, 12.0 parts per thousand and 8.0 parts per thousand for CGW); (2) proluvial slope (II2) group with low sulfate concentration (median values of 1.56 mM and 0.84 mM for UGW and CGW, respectively) and similar delta S-34(SO4) and delta O-18(SO4) values (median values of 9.0 parts per thousand and 7.4 parts per thousand for UGW, 10.2 parts per thousand and 7.7 parts per thousand for CGW); and (3) low-lying zone (II3) group with moderate sulfate concentration (median values of 2.13 mM and 1.17 mM for UGW and CGW, respectively) and more positive delta S-34(SO4) and delta O-18(SO4) values (median values of 10.7 parts per thousand and 7.7 parts per thousand for UGW, 20.1 parts per thousand and 8.8 parts per thousand for CGW). In the present study, three major sources of sulfate could be differentiated as following: sulfate dissolved from Ordovician to Permian rocks (delta S-34(SO4) = 10-35 parts per thousand and delta O-18(SO4) = 7-20 parts per thousand), soil sulfate (delta S-34(SO4) = 5.9 parts per thousand and delta O-18(SO4) = 5.8 parts per thousand) and sewage water (delta S-34(SO4) = 10.0 parts per thousand and delta O-18(SO4) = 7.6 parts per thousand). Kinetic fractionations of sulfur and oxygen isotopes as a result of bacterial sulfate reduction (BSR) were found to be evident in the confined aquifer in stagnant zone (II3), and enrichment factors of sulfate-sulfur and sulfate-oxygen isotopes calculated by Rayleigh equation were 12.1 parts per thousand and 4.7 parts per thousand respectively along the flow direction of groundwater at depths of 60-100 m. The results obtained in this study confirm that detailed hydrogeological settings and identification of anthropogenic sources are critical for elucidating evolution of delta S-34(SO4) and delta O-18(SO4) values along with groundwater flow path, and this work also provides a useful framework for understanding sulfur cycling in alluvial plain aquifers. (C) 2014 Elsevier Ltd. All rights reserved.