Assessing redox zones and seawater intrusion in a coastal aquifer in South Korea using hydrogeological, chemical and isotopic approaches

A shallow (<25 m), 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 delta S-34 and delta O-18 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 delta S-34(sulfate) up to 99.9 parts per thousand. The inferred sulfur isotopic enrichment factor (epsilon) for BSR in the lower part of YY2 was - 12.3%, while epsilon at YY1 was much higher (-45.9 parts per thousand). In addition, the observed trends of delta O-18(sulfate) 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 d34S. 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, delta S-34 and delta O-18 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. (C) 2014 Elsevier B.V. All rights reserved.