[M-IS11] Biogeochemistry

[MIS11-04] Investigation on the sources of arsenic and the formation process of arsenic-rich peat in groundwater level in Asia

*Thinh Van NGUYEN¹, Kuwahara Yoshihiro¹, Masaru Matsumoto², Thanh Dinh Xuan³, Nguyen Dinh Nguyen³, Myo Zaw⁴ (1.Department of Environmental changes, Faculty of Social and Cultural Studies, Kyushu University, Fukuoka, 819-0395, Japan, 2.Institute of Tropical Agriculture, Kyushu University, Fukuoka, 819-0395, Japan, 3.Faculty of Geology, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam., 4.Graduate School of Integrated Sciences for Global Society, Kyushu University, Fukuoka, 819-0395, Japan)

Keywords:Arsenic contamination, Holocene groundwater, ancient plant, DNA sequencing, Microorganism, ICP-MS

An estimation of more than 130 million people worldwide is exposed to drinking water with an excessive arsenic (As) content. High As concentrations in groundwater (for drinking purpose) were reported in the deltas in Bangladesh and Vietnam, such as Mekong delta – one of the biggest plain in Asia. Here, arsenic (As) concentration and major characteristics of the sediment cores in a shallow aquifer in Bangladesh and Vietnam were investigated using ICP-MS, XRD and ¹⁴C-AMS chronology techniques. The results of the present study indicated that the peak concentrations of As (54.7 to 127 µg/g) were in some peat layers (at a depth of 7.5 – 8.0 m in Bangladesh and ~23 m in Vietnam). Several types of iron (oxyhidr)oxides and framboidal pyrite, which contain As also were found in the peat samples (Figure 1-A:G). The high concentrations of As were in an exchangeable form, As-bearing iron crystalline and As-bearing organic materials. The isotope carbon analysis results revealed that As-rich peat layers in Bangladesh were formed in this area from 3170 to 3901 cal yrs BP, while those in Vietnam were formed from 5962 to 8361 cal yrs BP.

In addition, the original source and initial ecological condition of the As-rich peat sediments in the investigated areas will be clarified using molecular biological techniques, and biodiversity classification of diatoms. I will show the preliminary result of the DNA extraction from the As-rich peat samples, as well as the PCR results using primers of chloroplast loci, which will be used to identify the original plant of the peat. By using the ancient DNA templates of the samples, we expect to reveal the formation condition of the As-rich peat layers in the aquifer, which has not been clarified yet.

The microbial community of the sediment in Bangladesh was analyzed using PCR-DGGE technique. The 16S rRNA gene-based phylogenetic analysis revealed that the bacterial strains in the peats were mainly affiliated with genera Acinetobacter, Enterobacter, Escherichia, Bacillus, Clostridiaceae and Acinetobacter (Figure 1-H). Under the permanent saturated condition, the As-rich peat layers should be considered as an important potential driver of the groundwater As in this area.
In the future, the diversity of the bacterial community and their roles on the As mobility under the differential condition of the aquifer, such as high salinity, will be discovered. The pilot experiment using real As-rich sediment and varied levels of intrusion salt will be expected to predict the changes of As concentration in groundwater under effects of sea-level rise in the investigated region.