Combined use of δ13C, δ15N, and δ34S tracers to study anaerobic bacterial processes in groundwater flow systems

We present an approach for determining the major anaerobic bacterial processes in aquifers, using the combined stable isotope ratios of major elements (C, N, and S) as net recorders of the biogeochemical reactions. The Kumamoto groundwater is constituted of two major flow systems, A-A' and B-B', within 10(3) km-scale area. Previous study [Hosono, T., Tokunaga, T., Kagabu, M., Nakata, H., Orishikida, T., Lin, I-T., Shimada, J., 2013. The use of delta N-13 and delta O-18 tracers with an understanding of groundwater flow dynamics for evaluating the origins and attenuation mechanisms of nitrate pollution. Water Res. 47, 2661 -2675.] investigated the nitrate sources and extent of denitrification using delta N-15(NO3) and delta O-18(NO3) tracers. In the present study, we studied a type of denitrification (heterogenic vs. autotrophic) and occurrence of sequential anaerobic processes along the flow systems with newly measured delta C-13(DIC) and delta S-34(SO4). In A-A' flow system, C, N, and S isotopic compositions did not change along the flow direction. However, in B-B' flow system significant sulfate reduction (with a maximum delta S-34(SO4) enrichment of +55 parts per thousand) occurred along with denitrification (with a maximum delta N-15(NO3) enrichment of +38 parts per thousand) as the groundwater flowed down-gradient. Depletions in delta S-34(SO4) (-8 parts per thousand maximum) were found only sporadically. These observations imply that autotrophic denitrification could occur in very limited parts of the study area. Moreover, the occurrence of methanogenic reactions was suggested by the enriched delta C-13(DIC) signature (+8 parts per thousand maximum) at a denitrification hotspot. By characterizing C, N, and S isotope compositional changes in wide redox range (from aerobic oxidation of organic carbon, through denitrification, to sulfate reduction, until methanogenesis), we could develop the model of C, N, and S isotopic evolutional patterns under different redox scenarios. Proposed model is useful in obtaining a comprehensive understanding of the major anaerobic bacterial processes in aquifer systems, including distinguishing between heterotrophic and autotrophic denitrification. (C) 2014 Elsevier Ltd. All rights reserved.