Groundwater Dynamics and Arsenic Mobilization in Bangladesh Assessed Using Noble Gases and Tritium STEPHAN KLUMP,* ,†,‡ ROLF KIPFER, †,§ OLAF A. CIRPKA, † CHARLES F. HARVEY, ⊥ MATTHIAS S. BRENNWALD, † KHANDAKER N. ASHFAQUE, ⊥ ABU BORHAN M. BADRUZZAMAN, | STEPHAN J. HUG, † AND DIETER M. IMBODEN ‡ Department of Water Resources and Drinking Water, Eawag: Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, 8600 Du ¨ bendorf, Switzerland, Department of Environmental Sciences and Department of Earth Sciences, Swiss Federal Institute of Technology Zu ¨ rich (ETH), 8092 Zu ¨ rich, Switzerland, Ralph M. Parsons Laboratory for Water Resources and Hydrodynamics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, and Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh The contamination of groundwater by geogenic arsenic is the cause of major health problems in south and southeast Asia. Various hypotheses proposing that As is mobilized by the reduction of iron (oxy)hydroxides are now under discussion. One important and controversial question concerns the possibility that As contamination might be related to the extraction of groundwater for irrigation purposes. If As were mobilized by the inflow of re-infiltrating irrigation water rich in labile organic carbon, As-contaminated groundwater would have been recharged after the introduction of groundwater irrigation 20-40 years ago. We used environmental tracer data and conceptual groundwater flow and transport modeling to study the effects of groundwater pumping and to assess the role of re- infiltrated irrigation water in the mobilization of As. Both the tracer data and the model results suggest that pumping induces convergent groundwater flow to the depth of extraction and causes shallow, young groundwater to mix with deep, old groundwater. The As concentrations are greatest at a depth of 30 m where these two groundwater bodies come into contact and mix. There, within the mixing zone, groundwater age significantly exceeds 30 years, indicating that recharge of most of the contaminated water occurred before groundwater irrigation became established in Bangladesh. Hence, at least at our study site, the results call into question the validity of the hypothesis that re-infiltrated irrigation water is the direct cause of As mobilization; however, the tracer data suggest that, at our site, hydraulic changes due to groundwater extraction for irrigation might be related to the mobilization of As. Introduction Groundwater contamination by geogenic As is a widespread problem in south and southeast Asia. One of the most severely affected countries is Bangladesh (1, 2). The alluvial aquifers of the Ganges, Brahmaputra, and Megna river floodplains are intensively used for water supply. Tube wells have been installed since the 1970s in order to provide pathogen-free drinking water instead of the biologically contaminated surface water used before. The As concentrations in ground- water from about a third of the recently installed 6-10 million drinking water wells exceed the Bangladesh standard of 50 μg/L (World Health Organization standard is 10 μg/L). Millions of people are threatened by arsenicosis and by increasing cancer rates (3, 4). From a geochemical point of view, the principal chemical mechanisms of As mobilization are understood on a con- ceptual level, whereas the processes resulting in the erratic, patchy distributions of As contaminations in Bangladesh are still unknown. Different hypotheses have recently been formulated, which propose different spatial and hydro- geochemical origins of As in groundwater. Most authors assume that As is mobilized by the reduction of iron (oxy)- hydroxides (3, 5-8). Despite the geochemical studies, coupling of As contamination with groundwater hydraulics was never rigorously addressed. Up to now, little work has been done to evaluate groundwater hydrology in relation to the origin and distribution of As contamination. In rural Bangladesh, groundwater is extracted for irrigation from the same aquifers that are used for drinking water production, and pumping volumes for irrigation are larger, by far, than those for drinking water. The impact of water extraction for irrigation on groundwater flow and As mo- bilization is controversially discussed. Some researchers argue that irrigation pumping has had little effect on groundwater flow (9), whereas others argue that pumping has significantly affected flow patterns (6, 10). It is conceivable that pumping could either increase or decrease As levels. Arsenic levels could be increased by the transport of organic carbon (C org) into regions of the aquifer where Corg mobilizes As, or simply by transporting As from regions with high aqueous As concentrations. It has been speculated that Corg may come (i) directly from surface waters (permanent ponds and rivers) which receive large inputs of organic waste (agricultural and anthropogenic), (ii) from sediments beneath these recharge areas, or (iii) from irrigated rice fields (6, 11). Scenario three assumes irrigated paddy fields, being local sources of re-infiltrated irrigation water rich in labile Corg, which causes the mobilization of As in the sediments. The subsequent transport of the As-contaminated groundwater produces an As plume, resulting in the observed bell-shape of the vertical profile of the As concentration (hypothesis A, Figure 1A). Other papers assume that irrigation pumping may decrease As concentrations by flushing As from the system (e.g., ref 7, hypothesis B, Figure 1B), a process that may explain why irrigation wells at our site have lower As concentrations on average than drinking water wells (10). Pumping could also lower As concentrations by increasing the oxic recharge that immobilizes As. However, at our field site dissolved oxygen concentrations are <0.35 mg/L through- out the depth profile (12). In this paper, we suggest that * Corresponding author phone: +41-(0)44-823 5531; fax: +41- (0)44-823 5210; e-mail: stephan.klump@eawag.ch. † Swiss Federal Institute of Aquatic Science and Technology. ‡ Department of Environmental Sciences, Swiss Federal Institute of Technology Zurich. § Department of Earth Sciences, Swiss Federal Institute of Technology Zurich. ⊥ Massachusetts Institute of Technology. | Bangladesh University of Engineering and Technology.