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

0510750 HarveyA-1我们现在能够解决为什么恒河三角洲地下水中溶解的砷浓度高得危险的难题。在过去的五年中，几个研究小组提供了详细的静态地下水和沉积物的砷污染含水层的地球化学特征的特点。现在的挑战是确定地下水流如何运输化学物质进出地下，从而控制地下地球化学。我们建议开发新的水文方法来表征地下水流的复杂时空模式，然后采用水文地质模型来研究地下水地球化学的演化。通过将动态水文模型与地球化学特征相结合，我们打算回答迄今为止一直困扰我们的关键科学问题：为什么砷没有从一些含水层中被冲走？地下水停留时间和砷阻滞因子的综合估计表明，砷的停留时间只有几十年至几个世纪，这意味着砷应该从含水层是几千年的冲洗。溶解的砷是由一个连续的来源提供的，还是高浓度是短暂的？未来砷浓度会发生变化吗？我们的现场注入-取出实验表明，砷浓度在几天内响应生化扰动。旱稻种植的采用极大地改变了地下蓄水层的地球化学输入和输出。这种变化如何影响地下地球化学和溶解砷浓度？为什么相邻威尔斯井的砷浓度不同？尽管沉积物的特征相似，但灰色缺氧含水层附近地点的砷浓度往往差别很大。这些巨大的梯度是由地下水的流动和补给模式造成的吗？拟议活动的智力价值是什么？这些问题只能通过确定地下水动力学如何控制化学输入和输出到含水层的两个时间尺度来解决：（1）季节循环：孟加拉国的水文每年在季风洪水和旱季干旱条件之间循环，当蒸发蒸腾大大超过降水和灌溉水从含水层中抽出以满足作物的蒸腾需求。这一循环驱动了地下水位的波动，从而在土壤中产生季节性变化的氧化/缺氧条件，并驱动了含水层和地表水（稻田、池塘和河流）之间的水交换。(2)几十年来的人为变化：在过去的三十年里，由于人口增长和灌溉农业的出现，恒河三角洲发生了巨大的变化。地下水灌溉改变了补给的位置、时间和化学成分。灌溉水在大部分土地上的稻田中淤积，从而改变了补给的水文收支和地球化学，可能从富含砷和铁（氧）氢氧化物的土壤层中动员砷。此外，抽水改变了深层含水层的流动路径，影响了补给的速率和位置，以及地下水与地表水体的交换，现在地表水体接受了更高的未经处理的废物负荷。我们建议：（1）建立在我们在孟加拉国成功的现场计划，将我们的现场表征从一个位置的垂直地球化学剖面扩展到这个位置周围的三维流动;（2）利用天然同位素数据和地下水流数值反演方法相结合的新方法，确定地下水的补给和排泄特征，绘制地下水的瞬时流动路径：（3）详细研究了稻田底部的地球化学通量，稻田底部是我们现场地下水补给的主要来源，也是溶解砷的很可能来源;（4）建立预测性的数值模型，将地下水的流动和补给与控制砷的地球化学变化联系起来。这个合作项目是建立在我们过去五年成功和富有成效的伙伴关系之上的。我们将继续把重点放在教育和技术转让上，进一步加强BUET，MIT和Tufts之间的学生交流。我们还将继续与其他研究小组合作，包括斯坦福大学，瑞士的EAWAG，温哥华的UBC和UCLA。我们的研究结果应该回答一些关键的科学问题，也有助于评估替代砷减排战略和更好地管理孟加拉国的水资源。

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.