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

[510429 .伊斯兰]我们现在可以解开为什么恒河三角洲地下水中溶解的砷浓度如此之高的谜团了。在过去五年中，几个研究小组提供了砷污染含水层中地下水和沉积物的静态地球化学特征的详细描述。现在的挑战是确定地下水是如何将化学物质运进和运出地下，从而控制地下生物地球化学。我们建议发展新的水文方法来表征地下水流动的复杂时空格局，然后利用水文地质模型来研究地下水地球化学的演变。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？对地下水停留时间和砷滞留因子的综合估计表明，砷的停留时间只有几十年到几百年，这意味着砷应该从有几千年历史的含水层中被冲走。溶解的砷是由连续来源提供的，还是高浓度是暂时的？将来砷的浓度会改变吗？我们的现场注射-提取实验表明，砷浓度在几天内对生化扰动作出反应。旱季水稻种植的采用极大地改变了含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？为什么相邻井的砷浓度不同？尽管沉积物特征相似，但灰色缺氧含水层附近地点的砷浓度往往差异很大。这些戏剧性的梯度是地下水流动和补给模式的结果吗？所提议的活动在智力上有什么优点？这些问题只能通过确定地下水动力学如何在两个时间尺度上控制含水层的化学输入和输出来解决：(1)季节性循环：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，此时蒸散量大大超过降水，灌溉用水从含水层抽出以满足作物的蒸腾需求。这种循环驱动地下水位振荡，在土壤中产生季节性的缺氧/缺氧条件，也驱动含水层和地表水（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30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。大部分土地上的稻田里都有缺氧灌溉水，因此改变了水文收支和补给的生物地球化学，潜在地从富含砷和铁（氧）氢氧化物的土层中调动砷。此外，抽水改变了含水层深处的流动路径，既影响了补给的速率和位置，也影响了地下水与地表水体的交换，而地表水体现在接收了大量未经处理的废物。我们建议：(1)以我们在孟加拉国成功的油田项目为基础，将我们的油田特征从一个地点的垂直地球化学剖面扩展到该地点周围的三维流动；(2)采用自然同位素数据与数值反演方法相结合的方法，对含水层的补给和流量进行表征，绘制含水层的瞬态流动路径；(3)对稻田底部的地球化学通量进行详细研究，稻田现在是我们场地地下水补给的主要来源，也是很可能的溶解砷的来源；(4)建立地下水流动和补给与控制砷的生物地球化学转化耦合的预测数值模型。建议的活动有何更广泛的影响？这个合作项目建立在我们过去五年成功和富有成效的伙伴关系的基础上。我们将继续把重点放在教育和技术转让上，进一步在北工大、麻省理工学院和塔夫茨大学之间交换学生。我们还将继续与其他研究小组合作，包括斯坦福大学、瑞士EAWAG、温哥华UBC和加州大学洛杉矶分校。我们的研究结果应能回答一些关键的科学问题，也有助于评估替代的砷缓解战略，并更好地管理孟加拉国的水资源。

0510429IslamWe are now in a position to solve the puzzle of why dissolved arsenic concentrations are dangerously high in the groundwater of the Ganges Delta. Over the last five years several research groups have provided detailed characterizations of the static geochemical characteristics of groundwater and sediments in arsenic-contaminated aquifers. The challenge now is to determine how groundwaterflow transports chemicals in and out of the subsurface, and hence controls subsurface biogeochemistry. 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 the evolution 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 groundwater residence times and arsenic retardation factors indicate that arsenic residence times are only decades to centuries, 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 that arsenic concentrations respond within days to biochemical perturbations. The adoption of dry-seasonrice cultivation has dramatically altered geochemical input and outputs from aquifers. How does this change affect subsurface geochemistry and dissolved arsenic concentrations? Why do arsenic concentrations differ between neighboring wells? Arsenic concentrations at nearby locations 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 Bangladesh annually cycles between Monsoon flooding and dry-season arid conditions when evapotranspiration greatly outstrips precipitation and irrigation water is pumped from aquifers to meet the transpiration demands 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 been dramatically altered over the last three decades by population growth and the advent of irrigated agriculture. 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 the hydrologic 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 in aquifers, 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 field characterization from vertical geochemical profiles at one location to three dimensional flow around this location; (2) Characterize recharge and discharge and map transient flow-paths through the aquifer by applying 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 principle source of groundwater recharge at our site, and a very likely source of dissolved arsenic; (4) Construct predictive numerical models that couple groundwater flow and recharge with the biogeochemical transformations 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 five years. 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 with other research groups including Stanford, EAWAG in Switzerland, UBC in Vancouver and UCLA. Our research findings should answer some key scientific questions and also help evaluate alternative arsenicmitigation strategies and better manage water resources in Bangladesh.