Hydrological and Mineralogical Factors Influencing Paradoxical Groundwater Arsenic Release in the Red River Delta, Vietnam
Abstract
In South and Southeast Asia, the widespread contamination of groundwater arsenic (As) via microbial reduction of As-bearing iron (Fe) minerals in the subsurface results in toxic levels of arsenic above the World Health Organization (WHO) drinking water standard of 10 ug/L. High groundwater arsenic levels are generally found in gray Holocene aquifers whereas orange-sanded Pleistocene aquifers are typically a safer, lower As alternative. In the Red River Delta of Vietnam and elsewhere, Pleistocene aquifers can also have elevated arsenic levels, often due to increased groundwater pumping from the growing Hanoi area drawing high As water from Holocene aquifers, or from reduction induced by advected groundwater and organic carbon from the Red River. To determine which factors threaten the Pleistocene aquifers, we critically examine the hydrological and geochemical factors that could influence arsenic levels in the area. Exploiting an asymmetry in the region just south of Hanoi, yearlong spatiotemporal measurements of dissolved arsenic levels reveals a paradox between a Pleistocene aquifer site in Yen My (west bank) with higher As concentrations than a Holocene site in Van Duc (east bank). We monitor the influence of local and regional hydrology via water table measurements, stable water isotopes and conservative anion concentrations linked to the release of aqueous As. Preliminary x-ray absorption spectroscopy (XAS) data point to As(V)/arsenic sulfide minerals in Yen My versus As(III) minerals in Van Duc. Coupled to hydrology, downcore Fe Extended X-Ray Absorption Fine Structure (EXAFS) and As X-ray Absorption Near Edge Structure (XANES) stratigraphy and spatiotemporal dissolved organic carbon data serve to narrow down the possible sources of carbon and reductive processes that affect As speciation and transport. Overall, understanding sources that endanger the Pleistocene aquifers may elucidate important As cycling mechanisms at play that threatens water quality for millions of people in the region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.H13J1533N
- Keywords:
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- 0463 Microbe/mineral interactions;
- BIOGEOSCIENCES;
- 0496 Water quality;
- BIOGEOSCIENCES;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1831 Groundwater quality;
- HYDROLOGY