Millions of people consuming groundwater in Bangladesh and other affected South Asian countries are exposed to As levels that are predicted to cause death and disease in the coming decades. The sequence of steps that leads to elevated groundwater As concentrations in these regions remains poorly understood, despite intense efforts by scientists throughout the world over the past several years. This is a serious problem because major policy decisions hinge on accurate prediction of the impact of groundwater pumping for irrigation on groundwater As concentrations. We propose here to explore with a series of simple incubation experiments a new mechanism for As accumulation in groundwater that could have important implications for the response of shallow Bangladesh aquifers to irrigation.
On the basis of spatial patterns of local recharge constrained by 3H-3He dating and groundwater As profiles established at four sites in Bangladesh, we propose that weathering of primary minerals is the rate-limiting step of As mobilization in shallow aquifers. The available data suggest that As release by this process is of similar magnitude across a wide range of environments, relatively insensitive to redox conditions, and therefore possibly entirely abiotic in nature. The implication is that the distribution of As is determined primarily by spatial variations in the rate of flushing of shallow aquifers by local recharge. This is a direct challenge to the widely-held view that the distribution of As in groundwater is controlled primarily by microbially-mediated reductive dissolution of Fe oxyhydroxides.
The available field data indicate a relatively consistent rate of As release of ~20 ug/L per year for a wide range of settings. A flux of this magnitude should therefore be detectable directly by isolating freshly collected aquifer material and sampling the groundwater periodically. In this pilot-study, we propose to conduct such incubations anaerobically for 18 months with aquifer material from 4 depth intervals at a well-documented location. The role of microorganisms 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 treatments such as aerobic incubation and replacement of in situ groundwater with low-As groundwater or pond water. In addition to As concentrations, the major cations Na, Mg, K, and Ca will be monitored as indicators of chemical weathering. Redox-sensitive solutes such as Fe, Mn, and S will also be measured.
Results from these simple experiments could lead to a radical change in our understanding of the acquisition of solutes by groundwater in the case of a troublesome constituent such as As (NSF Criterion 1). The new understanding derived from this work could influence policies that affect the lives and health of millions of people in South Asia (NSF Criterion 2). If our new explanation for As accumulation holds, continued irrigation might actually reduce As concentrations in shallow aquifers over time. This contrasts with the recent attribution of elevated As levels in Bangladesh groundwater to irrigation.
