Origin and availability of organic matter leading to arsenic mobilisation in aquifers of the Red River Delta, Vietnam

Groundwater arsenic (As) concentrations in the Red River Delta (Vietnam) are often patchy and related to the microbially induced reduction of Fe oxy-hydroxides. In this study, we explored the influence of the origin, composition and availability of natural organic matter on the hydrochemical variability in the aquifers of Van Phuc. Carbon isotope signatures (d 13 Corg) and C/N ratios were assessed in combination with lithology, geochemistry, hydrochemistry, hydrology and the distribution of specific biomarkers. The elationship of C/N ratios andd 13 Corgdistinguished four groups of sediment types that differ in their organic carbon sources. This includes organic carbon originating predominantly from vascular C3plants (C/N: 15.4e21.0,d 13 Corg: 28.6 to 26.7‰), C4plants (C/N: 10.6;d 13 Corg: 14.8‰), freshwater derived particulate organic carbon (C/N: 8;d 13 Corg: 24‰) as well as mixtures incorporating both sources. At the high As sites, we found particulate organic carbon (POC) being 1e2‰less depleted ind 13 Corgthan at low As sites. More importantly, however, our assessment shows that, the availability of organic matter has to be considered decisive with regard to groundwater As contamination. Fine-grained clayey sedi-ments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer. However, at the sites that are high in dissolved As in Van Phuc, sediment layers rich in organic matter are hydraulically connected to the underlying aquifer. Here, soluble organic matter seeping into the aquifer can induce and/or enhance reducing conditions, thereby mobilising As from Fe oxy-hydroxides. Our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments.

Title: 

	Origin and availability of organic matter leading to arsenic mobilisation in aquifers of the Red River Delta, Vietnam
Authors:	Vi Mai Lan Pham Thi Kim Trang Pham Hung Viet
Keywords:	Arsenic Vietnam Organic matter d 13 Corg C/N-ratio
Issue Date:	2017
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND
Citation:	ISIKNOWLEDGE
Abstract:	Groundwater arsenic (As) concentrations in the Red River Delta (Vietnam) are often patchy and related to the microbially induced reduction of Fe oxy-hydroxides. In this study, we explored the influence of the origin, composition and availability of natural organic matter on the hydrochemical variability in the aquifers of Van Phuc. Carbon isotope signatures (d 13 Corg) and C/N ratios were assessed in combination with lithology, geochemistry, hydrochemistry, hydrology and the distribution of specific biomarkers. The elationship of C/N ratios andd 13 Corgdistinguished four groups of sediment types that differ in their organic carbon sources. This includes organic carbon originating predominantly from vascular C3plants (C/N: 15.4e21.0,d 13 Corg: 28.6 to 26.7‰), C4plants (C/N: 10.6;d 13 Corg: 14.8‰), freshwater derived particulate organic carbon (C/N: 8;d 13 Corg: 24‰) as well as mixtures incorporating both sources. At the high As sites, we found particulate organic carbon (POC) being 1e2‰less depleted ind 13 Corgthan at low As sites. More importantly, however, our assessment shows that, the availability of organic matter has to be considered decisive with regard to groundwater As contamination. Fine-grained clayey sedi-ments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer. However, at the sites that are high in dissolved As in Van Phuc, sediment layers rich in organic matter are hydraulically connected to the underlying aquifer. Here, soluble organic matter seeping into the aquifer can induce and/or enhance reducing conditions, thereby mobilising As from Fe oxy-hydroxides. Our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments.
Description:	TNS06993 ; APPLIED GEOCHEMISTRY Volume: 77 Special Issue: SI Pages: 184-193 Published: FEB 2017
URI:	http://repository.vnu.edu.vn/handle/VNU_123/28633 http://www.sciencedirect.com/science/article/pii/S0883292716300063
ISSN:	0883-2927
Appears in Collections:	Bài báo của ĐHQGHN trong Web of Science