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Defining organic matter reactivity and its influence on arsenic release to groundwater in the Mekong Delta

English title Defining organic matter reactivity and its influence on arsenic release to groundwater in the Mekong Delta
Applicant Dittmar Jessica
Number 129691
Funding scheme Fellowships for prospective researchers
Research institution Department of Applied Earth Sciences Stanford University
Institution of higher education Institution abroad - IACH
Main discipline Pedology
Start/End 01.05.2010 - 30.04.2011
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All Disciplines (3)

Discipline
Pedology
Hydrology, Limnology, Glaciology
Geochemistry

Keywords (10)

arsenic; organic matter; redox; aquifers; sediments; biogeochemistry; soil; groundwater; Mekong Delta; Southeast Asia

Lay Summary (English)

Lead
Lay summary
Naturally occurring arsenic (As) in groundwater aquifers is widely recognized as a threat to water resources. An estimated 100 million people worldwide are exposed to hazardous levels of As in their drinking water, predominately in South and Southeast Asia.
Biologically-driven reductive dissolution of As-bearing Fe(III)-(hydr)oxides and As(V) are generally accepted to be the dominant mode of As release, resulted from microbial degradation of organic matter (OM) under limited oxygenation within soils/sediments. As the release process is governed by OM and its decomposition mechanisms and rates, it is crucial to understand the nature and reactivity of OM.
Within the Mekong Delta of Cambodia, As release to groundwater was recently identified to occur mainly in the shallow clay layers of permanently saturated wetlands. In contrast, lesser amounts of As are released in the surficial clay layers below seasonally saturated wetlands. The two locations therefore provide an ideal opportunity to study the nature of OM and compare its reactivity during decomposition in similar environments under different redox conditions. Accordingly, we seek to determine how different geomorphic features result in different quantities and type of OM, and to link the characteristics of OM to the quantity of As release.
During a field trip in July 2010 two 1 m deep soil profiles from two different locations (permanently and seasonally saturated wetlands), were sampled for OM characterization. Visual inspection of the seasonally saturated wetlands profile showed two different OM morphologies with leaf-type OM in the upper and amorphous OM in the lower part of the profile, while no leaf-type OM was found in the permanently saturated wetlands. ATR-FTIR spectroscopy revealed significant differences between the two materials, especially in the areas of C=O vibrations of carboxylates and of CH and NH (amide II) bending motions.
Two high-resolution sediment sequences up to 6 m depth in both seasonally and permanently saturated wetlands are presently being examined by ATR-FTIR spectroscopy. Further, synchrotron-based FTIR spectra of transition zones and of the organic matter-mineral assemblages are being investigated. The specific chemical states are then be related to reactivity through incubation studies. Collectively, the results of this study will provide valuable insights into the role and heterogeneity of OM driving As liberation.
Direct link to Lay Summary Last update: 21.02.2013

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