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

Title
Assessing redox zones and seawater intrusion in a coastal aquifer in South Korea using hydrogeological, chemical and isotopic approaches
Authors
Duk-Min KimSeong-Taek Yun권만재Bernhard MayerKyoung-Ho Kim
Keywords
Bacterial sulfate reduction (BSR); Methanogenesis; Seawater intrusion; Shallow alluvial groundwater; Hydrochemistry; Sulfur and oxygen isotopes of sulfate
Issue Date
2014-12
Publisher
Chemical geology
Citation
VOL 2014, 119-134
Abstract
A shallow (b25 m), coastal alluvial groundwater systemunderneath 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 depthswith high sedimentary organicmatter 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 sulfateoxygen 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 SO4 2− 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 an
URI
http://pubs.kist.re.kr/handle/201004/48759
ISSN
00092541
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