Reactive Organic Carbon: The Big Unknown In Groundwater Arsenic Mobilization In Southeast Asia
Mladenov, Natalie 1 ; Zheng, Yan 2 ; Nemergut, Diana 3 ; Simone, Bailey 4 ;Legg, Teresa 5 ; McKnight, Diane 6
1 INSTAAR, University of Colorado
2 UNICEF
3 INSTAAR, University of Colorado
4 INSTAAR, University of Colorado
5 INSTAAR, University of Colorado
6 Dept of Civil, Environmental, and Architectural Engineering and INSTAAR, University of Colorado
Human consumption of groundwater with elevated arsenic concentrations in the Ganges-Brahmaputra Delta and other deltaic regions of southeast Asia has resulted in a human health catastrophe. A process-level understanding of the mechanisms by which arsenic is mobilized is a priority, as it can influence government policy on the future use of groundwater resources. The prevailing hypothesis to explain the liberation of arsenic from Himalayan-derived sediments is that dissolved organic matter (DOM) stimulates anaerobic iron-reducing and arsenic-reducing bacteria. The reduction of arsenic-bearing iron minerals and the reduction of arsenic itself then lead to desorption of arsenic from the sediments. In the recent literature it has been emphasized that the source and bioavailability of organic carbon “is possibly the most prominent outstanding issue limiting our ability to predict the distribution of As in groundwater” (Fendorf et al. 2010). Measures of DOM quality have been central to advancing our understanding of the sources of reactive organic carbon in arsenic-laden groundwater. Here, we highlight recent findings from co-workers along with findings from our research site in Araihazar, Bangladesh. Our work has shown that fluorescence spectroscopy is a robust technique for assessing both DOM sources and the redox state of humics that consistently identifies DOM in arsenic hotspots as sediment-derived and distinct from DOM in surface water. We have also found that the biogeochemical role of DOM is more complex than previously thought. In addition to serving as a subsidy for bacteria, DOM may: 1) complex with iron and keep arsenic in solution via bridging mechanisms, 2) compete with arsenic for sorption sites, and 3) act as an electron shuttle (via humic quinones in DOM) to enhance microbial Fe-reduction (Mladenov et al. 2010).
Fendorf, S. et al., 2010, Spatial and Temporal Variations of Groundwater Arsenic in South and Southeast Asia: Science, v. 328, p. 1123-1127.
Mladenov, N. et al., 2010, Dissolved organic matter sources and consequences for iron and arsenic mobilization in Bangladesh: Environmental Science & Technology, doi: 10.1021/es901472g.