A shallow (<25m), coastal alluvial groundwater system underneath a paddy field in the Yangyang area of South Korea was investigated to examine the occurrence of redox processes. The aquifer is affected by seawater intrusion, and is characterized by a highly reducing environment facilitated by high organic matter in the sediments. Hydrochemical data with δ34S and δ18O of sulfate were examined for depth-specific groundwater from two multilevel samplers that were installed at seaward (YY2) and landward (YY1) locations. Shallow groundwater showed distinct patterns of redox zoning. Evidence of significant bacterial sulfate reduction (BSR) was observed throughout the nearly entire depths of the two boreholes, while at the depths of active seawater intrusion in YY2, conditions suitable for methanogenesis were never reached. Thus, at YY2 the deep zone of intense BSR was overlain by a zone in which methanogenesis occurred in a low-sulfate environment. In contrast, concurrent BSR and methanogenesis in YY1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive BSR. Considerable BSR in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of δ34Ssulfate up to 99.9‰. The inferred sulfur isotopic enrichment factor (ε) for BSR in the lower part of YY2 was -12.3‰, while ε at YY1 was much higher (-45.9‰). In addition, the observed trends of δ18Osulfate at YY1 indicated significant oxygen isotope exchange of sulfate-oxygen with ambient water, likely because of lower cell-specific rates of BSR and higher sulfur isotope fractionation as indicated by the δ34S. In contrast, there was little evidence of oxygen isotope exchange between water and SO42- at YY2. This study indicates that in coastal aquifers with sulfate-reducing activity, δ34S and δ18O of sulfate can reveal zones of active seawater intrusion and of trapped seawater. This study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic-rich coastal environment.
Bibliographical noteFunding Information:
This work was financially supported by a research fund ( R01-2007-000-20964-0 ) from the Korea Science and Engineering Foundation (KOSEF). Partial supports were provided by the National Research Foundation of Korea Grant (2013, University-Institute Cooperation Program) funded by the Korean Government ( Ministry of Science, ICT & Future Planning ) and by the Mine Reclamation Corporation funded by the Ministry of Trade, Industry and Energy . The third author (MJK) was partially supported by the Korea Ministry of Environment under “The GAIA Project-2013000540005”. Many students of Korea University helped the authors during well installation and field works.
© 2014 Elsevier B.V.
- Bacterial sulfate reduction (BSR)
- Seawater intrusion
- Shallow alluvial groundwater
- Sulfur and oxygen isotopes of sulfate
ASJC Scopus subject areas
- Geochemistry and Petrology