Collaborative Research: Groundwater Dynamics and Arsenic Contamination in the Ganges Delta: Irrigated Agriculture, Subsurface Chemical Transport, and Aquifer Flushing

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0510750HarveyA-1We are now in a position to solve the puzzle of why dissolved arsenic concentrations aredangerously high in the groundwater of the Ganges Delta. Over the last five years several researchgroups have provided detailed characterizations of the static geochemical characteristics of groundwaterand sediments in arsenic-contaminated aquifers. The challenge now is to determine how groundwaterflow transports chemicals in and out of the subsurface, and hence controls subsurfacebiogeochemistry. We propose to develop novel hydrologic methods to characterize the complex spatialand temporal patterns of groundwater flow, and then to employ hydrogeologic models to study theevolution of groundwater geochemistry. By combining dynamic hydrological models with geochemicalcharacterization we intend to answer key scientific questions that have thus far eluded us:Why has arsenic not been flushed from some aquifers? Combined estimates of groundwaterresidence times and arsenic retardation factors indicate that arsenic residence times are only decades tocenturies, implying that arsenic should be flushed from the aquifers that are thousands of years old. Isdissolved arsenic supplied by a continuous source, or are high concentrations transient?Will arsenic concentrations change in the future? Our field injection-withdrawal experiments show thatarsenic concentrations respond within days to biochemical perturbations. The adoption of dry-seasonrice cultivation has dramatically altered geochemical input and outputs from aquifers. How does thischange affect subsurface geochemistry and dissolved arsenic concentrations?Why do arsenic concentrations differ between neighboring wells? Arsenic concentrations at nearbylocations in grey-colored anoxic aquifers often differ greatly, despite similar sediment characteristics. Dothese dramatic gradients result from the pattern of groundwater flow and recharge?What are the intellectual merits of the proposed activity?These questions can only be resolved by determining how groundwater dynamics controlchemical input and output to aquifers over two timescales: (1) Seasonal cycle: The hydrology of Bangladeshannually cycles between Monsoon flooding and dry-season arid conditions when evapotranspirationgreatly outstrips precipitation and irrigation water is pumped from aquifers to meet the transpirationdemands of crops. This cycle drives water table oscillations that create seasonally varying oxic/anoxicconditions in soils and also drives water exchange between aquifers and surface water (rice paddies,ponds and rivers). (2) Anthropogenic changes over decades: The Ganges Delta has beendramatically altered over the last three decades by population growth and the advent of irrigatedagriculture. Groundwater irrigation has changed the location, timing and chemical content of recharge.Anoxic irrigation water is ponded in rice fields over much of the land, thereby changing both thehydrologic budget and the biogeochemistry of recharge, potentially mobilizing arsenic from soil layers thatmay be rich in arsenic and iron (oxy)hydroxides. Furthermore, pumping changes flow-paths deep inaquifers, affecting both the rates and locations of recharge as well as groundwater exchange with surfacewater bodies that now receive much higher loads of untreated waste.We propose to: (1) Build on our successful field program in Bangladesh by extending our fieldcharacterization from vertical geochemical profiles at one location to three dimensional flow around thislocation; (2) Characterize recharge and discharge and map transient flow-paths through the aquifer byapplying novel combination of natural isotope data and numerical inverse methods for groundwater flow;(3) Conduct a detailed study of geochemical fluxes through the bottom of a rice field, now a principlesource of groundwater recharge at our site, and a very likely source of dissolved arsenic; (4) Constructpredictive numerical models that couple groundwater flow and recharge with the biogeochemicaltransformations that control arsenic.What are the broader impacts of the proposed activity?This collaborative project is built on our successful and productive partnership over the last fiveyears. We will continue to place a significant emphasis on the education and transfer of technology, withfurther exchange of students between BUET, MIT and Tufts. We also will continue to collaborate withother research groups including Stanford, EAWAG in Switzerland, UBC in Vancouver and UCLA. Ourresearch findings should answer some key scientific questions and also help evaluate alternative arsenicmitigation strategies and better manage water resources in Bangladesh.