This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Emphasis on the elevated level of arsenic in groundwater tapped by millions of shallow tubewells across southern Asia over the past two decades has tended to obscure that anoxic aquifers that are low in As are also widespread and often within reach of drilling in the affected regions. This proposal addresses two key issues for assessing the vulnerability of low-As aquifers: (1) whether dissolved or particulate organic carbon (DOC or POC) drives aquifers towards microbial dissolution of Fe oxyhydroxides and (2) to what extent adsorption and precipitation of As attenuate the impact of high-As groundwater drawn into low-As aquifers. The proposed field investigation takes advantage of the previously documented (a) juxt-apposition at the same depth of grey sands associated with high-As groundwater and orange sands with low-As groundwater in a village 15 km south of Hanoi and (b) large groundwater withdrawals by the city of Hanoi resulting in water from the former being drawn laterally through the latter. The configuration is particularly well-suited to shed light on the key processes that determine the sustainability of continued reliance on low-As aquifers for the provision of drinking water throughout the affected region.
Scientific merit ¡V An unprecedented combination of state-of-the-art analytical methods will be applied to a previously documented transition from orange to grey sands corresponding to low- to high-As groundwater, respectively: 14C-analysis of phospholipid fatty acids (PLFAs) constituting the membranes of living cells, probing of the redox state and bonding environment of As and Fe in aquifer sands by X-ray absorption spectroscopy, 3H-3He dating of groundwater, and optically-stimulated luminescence (OSL) dating of quartz grains. These measurements will provide the basis for testing the following suite of working hypotheses pertaining to the vulnerability of low-As aquifers in southern Asia. Whether the associated predictions hold up to closer scrutiny or not, the implications of the proposed research will extend beyond the As problem as they address the more general question of redox control in groundwater.
H1 Reductive dissolution of Fe oxyhydroxides associated with the release of As to groundwater is driven primarily by the microbial degradation of POC deposited at the same time as the aquifer sands. H2 DOC advected by groundwater flow does not result in significant reductive dissolution of Fe oxyhydroxides and therefore does not trigger conversion of orange sands to grey or the release of As. H3 Mixing of high-sulfate (SO4) groundwater with high-As groundwater results in precipitation of As-sulfides fueled by the oxidation of POC in the aquifer deposit. H4 Breakthrough of As through orange sands can be predicted by considering mainly As removal by precipitation of As-sulfides and competitive adsorption of As and phosphate (PO4).
Broader Impact - Drinking water distributed by several of Hanoi¡¦s municipal water supply plants contains >100 ?Ãg/L As, i.e. over ten-fold the WHO guideline for As in drinking water of 10 ?Ãg/L. This is due to insufficient treatment of groundwater drawn from orange aquifers but presumably originating in part from adjacent grey aquifers. Expressed as the last of our working hypotheses, the proposed research will help determine whether the As content of groundwater tapped by the municipal water supply of the city of Hanoi, and similarly situated groundwater pumping stations throughout southern Asia, could be lowered at the source by favoring entrainment of river water, herewith reducing the need for treatment:
H5 Municipal water supplies engineered to draw river water through a reducing aquifer would benefit from a reduction in As prior to abstraction due to the dual effect of increasing SO4 and lowering PO4.
Eawag, our Swiss partner in this project, has built an extensive network of contacts with city water managers over the course of a long-term collaboration with the Hanoi University of Science. Engineers at QEA, the environmental consultancy charged with modeling groundwater and As transport, in consultation with a senior hydrologist at MIT, will help ensure that the outcome of the research is translated into practical recommendations. Graduate research by one student in Vietnam will be covered over the duration of the project and partially covered for post-doctoral work at Lamont-Doherty.
