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PREAS Résumé de rapport

Project ID: 338972
Financé au titre de: FP7-IDEAS-ERC
Pays: Denmark

Mid-Term Report Summary - PREAS (Predicting the arsenic content in groundwater of the floodplains in SE Asia)

The occurrence of arsenic in groundwater of the floodplains of the Ganges-Brahmaputra-Meghna, Mekong and Red River, all draining the Himalayas, was first discovered in the early 1990’s. In these areas millions of shallow tube wells were installed to avoid drinking surface water contaminated with microbial pathogens. Unfortunately many of these wells have an arsenic content greatly exceeding the WHO guideline of 10 µg/L. Arsenic in drinking water significantly increases the risk for cancers of liver, bladder and lung, cardiovascular disease and hyperkeratosis, even at low concentrations of arsenic in the range of 10-50 µg/L. More than 100 million people in Bangladesh, China, India, Vietnam, Nepal, and Cambodia are now estimated to be exposed to drinking water with a harmful arsenic content.
Typically these floodplains show a high spatial variability between sites having a low or a high arsenic content in the groundwater. And although we have a reasonable qualitative understanding of the processes involved in the release of arsenic to the groundwater, a quantitative understanding which allows us to predict the arsenic concentration in the groundwater at a given location is completely lacking.
The objective of the present project is accordingly to create such a quantitative understanding of the spatial distribution of arsenic in the groundwater of these floodplains. The working hypothesis is that the burial age of the aquifer sediments has a large impact on the geochemical properties of these Holocene sediments. The youngest sediments are believed to have a high content of reactive minerals and organic constituents, leading to a high groundwater arsenic content, while as the sediment age increases, the reactivity of the sediments becomes less.
The task at hand is therefore to determine and quantify the geochemical processes involved in arsenic release and how they change over time. This should be coupled to a spatial understanding of the distribution of sediments of different ages and depositional environment within the floodplain. The third aspect to be considered is groundwater movement and residence time in these sediments. The intention is to include all three aspects into a 3D model which is able to predict the groundwater arsenic content over space and time.
As study site we have selected a field area of about 10x20 km on the floodplain of the Red River 30 km upstream of Hanoi, Vietnam. The groundwater in the field area has been sampled in detail and shows a highly variable arsenic concentration that many places exceed the drinking water limit. The geology of the field area has been elucidated using drilling, geophysical profiling and borehole logging and the analysis of satellite images. Using these methods it appears to be possible to reconstruct the previous courses of the main channel of the Red River over the last 6000 years. Work is on the way to combine the groundwater chemistry and groundwater flow patterns with the geological data. At the same time aquifer sediments taken from the field area are being used for ongoing laboratory experiments to quantify the geochemical processes affecting arsenic release. These data are also used to construct a 1D reactive transport model that may predict the evolution of the groundwater chemistry over Holocene time. The model is currently being tested and the first steps are done to incorporate the 1D model into the final 3D model
Here halfway the project period we may note with satisfaction that the project largely follows the time schedule and many promising results have already been obtained. In the second half of the project period we will slowly turn our main focus from data collection to evaluation, analysis, modeling and publishing.

Reported by

The Geological Survey of Denmark and Greenland