Abiotic arsenic mobilization in Bangladesh groundwater aquifers?

点击翻译按钮获取中文摘要

Abiotic Mobilization of Arsenic in Bangladesh Groundwater Aquifers?AbstractMillions of people consuming groundwater in Bangladesh and other affected South Asiancountries are exposed to As levels that are predicted to cause death and disease in the comingdecades. The sequence of steps that leads to elevated groundwater As concentrations in theseregions remains poorly understood, despite intense efforts by scientists throughout the worldover the past several years. This is a serious problem because major policy decisions hinge onaccurate prediction of the impact of groundwater pumping for irrigation on groundwater Asconcentrations. We propose here to explore with a series of simple incubation experiments anew mechanism for As accumulation in groundwater that could have important implications forthe response of shallow Bangladesh aquifers to irrigation.On the basis of spatial patterns of local recharge constrained by 3H-3He dating andgroundwater As profiles established at four sites in Bangladesh, we propose that weathering ofprimary minerals is the rate-limiting step of As mobilization in shallow aquifers. The availabledata suggest that As release by this process is of similar magnitude across a wide range ofenvironments, relatively insensitive to redox conditions, and therefore possibly entirely abiotic innature. The implication is that the distribution of As is determined primarily by spatial variationsin the rate of flushing of shallow aquifers by local recharge. This is a direct challenge to thewidely-held view that the distribution of As in groundwater is controlled primarily bymicrobially-mediated reductive dissolution of Fe oxyhydroxides.The available field data indicate a relatively consistent rate of As release of ~20 ug/L peryear for a wide range of settings. A flux of this magnitude should therefore be detectabledirectly by isolating freshly collected aquifer material and sampling the groundwaterperiodically. In this pilot-study, we propose to conduct such incubations anaerobically for 18months with aquifer material from 4 depth intervals at a well-documented location. The role ofmicroorganisms in As mobilization will be tested by additions of an antibiotic and acetate,respectively. Aquifer material from one depth interval will be subjected to additional treatmentssuch as aerobic incubation and replacement of in situ groundwater with low-As groundwater orpond water. In addition to As concentrations, the major cations Na, Mg, K, and Ca will bemonitored as indicators of chemical weathering. Redox-sensitive solutes such as Fe, Mn, and Swill also be measured.Results from these simple experiments could lead to a radical change in ourunderstanding of the acquisition of solutes by groundwater in the case of a troublesomeconstituent such as As (NSF Criterion 1). The new understanding derived from this work couldinfluence policies that affect the lives and health of millions of people in South Asia (NSFCriterion 2). If our new explanation for As accumulation holds, continued irrigation mightactually reduce As concentrations in shallow aquifers over time. This contrasts with the recentattribution of elevated As levels in Bangladesh groundwater to irrigation.