Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

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Controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the Red River flood plain, Vietnam Flemming Larsen a, *, Nhan Quy Pham b , Nhan Duc Dang c , Dieke Postma a , Søren Jessen d , Viet Hung Pham e , Thao Bach Nguyen b , Huy Duc Trieu b , Luu Thi Tran e , Hoan Nguyen b , Julie Chambon d , Hoan Van Nguyen b , Dang Hoang Ha b , Nguyen Thi Hue e , Mai Thanh Duc e , Jens Christian Refsgaard a a Geological Survey of Denmark and Greenland (GEUS), Østervoldgade 10, DK-1350 Copenhagen K, Denmark b Hanoi University of Mining and Geology, Viet Nam c Institute for Nuclear Science and Technology, Viet Nam d Institute of Environment and Resources, Technical University of Denmark, Denmark e Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, VNU, Viet Nam article info Article history: Available online 4 July 2008 * Corresponding author. E-mail address: flar@geus.dk (F. Larsen). abstract 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.06.014 Applied Geochemistry 23 (2008) 3099–3115 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem Geological and hydrogeological processes controlling recharge and the mobilization of As were investigated in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The geology was investigated using surface resistivity methods, geophysical borehole logging, drilling of boreholes and installation of more than 200 piezometers. Recharge processes and surface–groundwater interaction were studied using (i) time-series of hydraulic head distribution in surface water and aquifers, (ii) the stable isotope composition of waters and (iii) numerical groundwater modeling. The Red River and two of its distributaries run through the field site and control the groundwater flow pattern. For most of the year, there is a regional groundwater flow towards the Red River. During the monsoon the Red River water stage rises up to 6 m and stalls the regional groundwater flow. The two distributaries recharge the aquifer from perched water tables in the dry season, whilst in the flooding period surface water enters the aquifer through highly permeable bank sediments. The result is a dynamic groundwater flow pattern with rapid fluctuations in the groundwater table. A transient numerical model of the groundwater flow yields an average recharge rate of 60–100 mm/a through the confining clay, and a total recharge of approximately 200 mm/a was estimated from 3 H/ 3 He dating of the shallow groundwater. Thus in the model area, recharge of surface water from the river distributaries and recharge through a confining clay is of the same magnitude, being on average around 100 mm/a. The thickness of the confining clay varies between 2 and 10 m, and affects the recharge rate and the transport of electron acceptors (O2, NO3and SO 2 4 ) into the aquifer. Where the clay layer is thin, an up to 2 m thick oxic zone develops in the shallow aquifer. In the oxic zone the As concentration is less than 1 lg/L but increases in the reduced zone below to 550 lg/ L. In the Holocene aquifer, As is mobilized at a rate of around 14 lg/L/a. An As mass balance for the field site shows that around 1100 kg of As is annually leached from the Holocene sand and discharged into the Red River, corresponding to 0.01% of the total pool of As now present in the Holocene sand. Ó 2008 Elsevier Ltd. All rights reserved.
3100 F. Larsen et al. / Applied Geochemistry 23 (2008) 3099–3115 1. Introduction Groundwater enrichment by geogenic As is a widespread problem in the flood plain aquifers of Southeast Asia. Ultimately the source of As is the transport of As-containing sediments from the Himalayas down to the flood plains (Stanger, 2005; Charlet and Polya, 2006). On a regional scale, the distribution of high-As waters in West Bengal and Bangladesh shows great variation due to differences in sediment distribution, diagenesis and variations in abstraction depths (Smedley and Kinniburgh, 2002). On a smaller scale, the patchy distribution of As in groundwaters of Bangladesh is influenced by the complex hydrology of paddy rice fields, irrigation channels and boreholes and waste water ponds. These produce a mosaic of recharge and discharge areas resulting in complex groundwater flow paths and variable As distribution (Harvey et al., 2006). The more detailed coupling of As enrichment to groundwater hydraulics has only recently been addressed (e.g. Klump et al., 2006; Stute et al., 2007). Klump et al. (2006) made a rigorous analysis of how irrigation pumping is affecting the flow system in the upper 30 m of the aquifer. Stute et al. (2007) studied the hydrological control on the groundwater As concentration in Bangladesh and reported an increase in the As concentration with increasing groundwater age for the upper 20 m of the aquifer. Although the mechanism of As release from the sediments is still uncertain, the relationship between the As concentration and groundwater age suggests the kinetics of As release from the sediments and the groundwater residence time to be important factors (Stute et al., 2007). High groundwater As concentrations have also been reported from aquifers of the Red River flood plain, Vietnam (Berg et al., 2001, 2008). The objective in this study is to clarify the hydrogeological and geochemical processes controlling the mobilization of As in a Red River Holocene aquifer. The field site is near the village Dan Phuong, 30 km NW of Hanoi. The geochemical results of the study have been published by Postma et al. (2007). The field site (Fig. 1) is located between the dyke and the Red River and is intersected by two minor distributaries to the Red River. Between the dyke and the river there is no groundwater abstraction and therefore the natural interaction between the surface waters and the shallow aquifer can be studied. The study addresses the hydrogeology and As mobilization in a Holocene aquifer to a depth of 22 m below the surface. Fig. 1. Location of the Dan Phuong field site, 30 km NW of Hanoi on the Red River flood plain (UTM coordinates: 565695; 2338909). The location of boreholes is indicated by dots except for those contained in the H- and the K-transects which each contain up to 100 piezometers. The orientation of the geological cross-section in Fig. 4 is shown as the line A–B, and the model domain for the numerical groundwater model is also indicated.
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Groundwater arsenic in the Red Rive
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Søren Jessen Groundwater arsenic i
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organic matter may prevent As mobil
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Resumé Naturligt frigivet arsen (A
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efterfølgende forsøgt at opstille
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Preface This PhD thesis deals with
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Table of contents 1 INTRODUCTION: T
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The PhD research presented in this
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educed form as As(III), which is al
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A more complex and intimate relatio
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sedimentary As content (Postma et a
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(Anawar and Mihaljevi, 2009), and g
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3 Regional As distribution in the R
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The mobility of arsenic in a Red Ri
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