Testing the Palaeosol Model of Arsenic Pollution in Groundwater

来自全球河流沉积含水层的地下水提供了世界上大部分的水供应；从亚洲三角洲的含水层中抽取的水为东南亚五分之一的人口提供了饮用水。自1995年以来，砷污染已明显影响到全世界三角洲含水层的地下水，并对人类健康造成严重后果。污染及其不利影响在西孟加拉邦和孟加拉国得到了最充分的记录。在所有受污染的三角洲，砷污染的分布是不均匀的，但我们不知道为什么。污染地区和未污染地区以一种似乎随机的方式并列在一起。这个提议试图检验一个解释砷污染随机性的假设。如果得到证实，这一假设将提供一个砷污染的预测模型，该模型将指导世界各地三角洲地区的含水层开发和修复。预测能力将有利于儿童基金会（促进孟加拉国的水井开发）等保健机构、与供水有关的政府和准政府当局（如孟加拉国公共卫生工程部）以及受砷污染影响的个人消费者。这个被检验的假设最好是在过去12.5万年海平面变化的背景下设定的。大约12.5万年前，海平面和现在差不多。随着时间的推移，气候变冷导致极地冰盖增加。进去的水是从海里出来的。结果，大约2万年前，当冰冠长到最大的时候，海平面下降了120米。从距今12万5千年到距今2万年，海平面的缓慢下降使海岸退缩，使以前被淹没的大片沿海土地暴露在风化和土壤形成中。虽然土壤在裸露的景观上广泛形成，但在活跃河流的河道中却没有形成。在2万年前到6000年前之间，冰盖部分融化，海平面恢复到以前的水平，淹没了古老的景观和表层土壤，并使它们被新的沉积所掩埋。这里要测试的假设被称为“古土壤模型”，即现在埋在三角洲中的许多旧土壤（称为古土壤）是不透水的，因此阻止了污染向下垂直迁移到古土壤现在覆盖的含水层。因此，下面的含水层受到保护，不受向下移动的污染的影响，就像帽子保护头部不受雨淋一样。这个假设正确吗？它真的能解释三角洲含水层中砷污染的不均匀分布吗？这个项目开始了，所以要找到答案。但还有更多。不幸的是，由于古土壤不是在河流流动的活跃河道中形成的，污染会使未受污染的含水层上具有保护作用的古土壤帽短路，并从受污染的古河道向深处横向移动进入这些含水层。这能多快发生？砷污染在地下的移动速度是多少？知道这个问题的答案对于确定未受污染的含水层能保持多久不受污染是很重要的。我们希望通过研究砷在地下的移动速度，利用沉淀物的实验室实验和长期监测水井中的地下水成分，得到一个答案。

Groundwater from riverine sedimentary aquifers across the globe provides much of the world's water supply; that drawn from such aquifers in Asian deltas provides a fifth of the SE Asia's population with its drinking water. Since 1995, it has become clear that arsenic-pollution affects groundwater in deltaic aquifers worldwide and that the consequence for human health has been severe. The pollution, and its adverse effects, are most fully documented in West Bengal and Bangladesh. In all polluted deltas, the distribution of arsenic pollution is patchy, but we don't know why. Polluted and unpolluted areas are juxtaposed in a manner that seems random. This proposal seeks to test an hypothesis that explains the randomness of arsenic pollution. The hypothesis will, if confirmed, provide a predictive model of arsenic pollution that will guide aquifer development and remediation in deltaic settings across the world. A predictive capacity would be of benefit to health agencies such as UNICEF (which promoted well development in Bangladesh), governmental and quasi-governmental authorities concerned with water supply (e.g. Department of Public Health Engineering, Bangladesh), and individual consumers affected by arsenic pollution. The hypothesis being tested is best set in the context of sea-level variations over the last 125,000 years. Around 125,000 years ago, sea-level was about the level it is today. As time passed, cooling climate caused the polar ice-caps to grow. The water that went into them came out of the sea. As a consequence, around 20,000 years ago when the ice-caps had grown to their largest size, sea level had fallen by 120 m. The slow decline in sea-level from 125,000 years before present to 20,000 years before present made the sea-shore retreat and exposed to weathering and soil formation large areas of coastal land that had previously been submerged. Although soils formed widely on the exposed landscape, they did not form in the chanells of active rivers. Between 20,000 years and 6,000 years ago the ice-caps partly melted and sea-level returned to its previous level, submerging the old landscape and its surficial soils and allowing them to be buried by renewed sedimentation. The hypothesis to be tested here, termed the 'palaeosol model', is that many of the old soils (termed palaeosols) that are now buried in deltas are impermeable, and so prevent the vertical migration of pollution downwards into the aquifers the palaeosols now cap. As a consequence, the underlying aquifers are protected from downward moving pollution, just as a hat protects the head from rain. Is this hypothesis correct? Can it really explain the patchy distribution of arsenic pollution in deltaic aquifers? The project sets out so find the answes. But there is more. Unfortunately, because the palaeosols did not form in the active river chanells where rivers flowed, pollution can short-circuit the protective palaeosol caps on unpolluted aquifers, and move into them by moving laterally at depth from polluted ancient river channels. How fast can that happen? What is the rate at which As-pollution can move in the subsurface? Knowing the answer to that question is important to establishing just how long the unpolluted aquifers will remain unpolluted. By investigating the rate at which arsenic moves in the subsurface, using both laboratory experiment with sediment and through monitoring groundwater composition in wells over time, we hope to obtain an answer.