Assessing redox zones and seawater intrusion in a coastal aquifer in South Korea using hydrogeological, chemical and isotopic approaches
- Assessing redox zones and seawater intrusion in a coastal aquifer in South Korea using hydrogeological, chemical and isotopic approaches
- Duk-Min Kim; Seong-Taek Yun; 권만재; Bernhard Mayer; Kyoung-Ho Kim
- Bacterial sulfate reduction (BSR); Methanogenesis; Seawater intrusion; Shallow alluvial groundwater; Hydrochemistry; Sulfur and oxygen isotopes of sulfate
- Issue Date
- Chemical geology
- VOL 2014, 119-134
- 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
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