Final 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 discovered in the early 1990's. Millions of tube wells were installed in these areas 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, cardiovascular disease and hyperkeratosis, even at low concentrations of 10-50 µg/L. More than 100 million people in Bangladesh, China, India, Vietnam, Nepal, and Cambodia are estimated as exposed to drinking water with a harmful arsenic content.
These floodplains show a large variation in arsenic concentration even over small distances, which is a big problem when setting-up water supplies. The origin of this variability has hitherto not been investigated systematically and a quantitative understanding of this phenomenon is the main objective of the present project. We investigated the variability of groundwater arsenic in the context of geological, geochemical and hydrogeological processes. Our field area, 10x20 km on the Red River floodplain 30 km NW of Hanoi, shows high and variable groundwater arsenic. First we analyzed the geological structure and its evolution over the last 6000 years reconstructing the location of the main channel of the Red River, using remote sensing, geophysics, sediment coring and dating of sediment burial ages using OSL. Results revealed a complex mosaic of older and younger sediments formed by migrating river channels cutting into older deposits. Second we constrained the hydrogeology with groundwater flow velocities, flow patterns and residence times in different parts of the aquifers. Thirdly, we mapped the spatial distribution of the groundwater chemistry and performed laboratory process studies to quantitatively describe the geochemical processes mobilizing arsenic from the sediments and its leaching from the deposits over time. To integrate geology, hydrogeology and geochemistry we used modeling. The geochemical processes were incorporated in a reactive transport model describing changes in water chemistry over 6000 years taking into account changes in the reactivity of sedimentary carbon and iron oxides over time as well other chemical processes. This model was developed on a limited dataset of hydrologically homogeneous data. Subsequently the model was tested on a larger dataset covering the whole field area with much larger hydrological variability. The total number of porevolumes of groundwater flushed through the aquifer since its burial emerged as a suitable parameter combining geochemical and hydrological changes over time. During the first 100 pore volumes flushing through the deposits, groundwater arsenic will be high. Thereafter the arsenic concentration decreases but first after 200 pore volumes sufficient arsenic is leached out of the aquifer to lower the arsenic concentration below the WHO limit. The porevolume concept indicates that the distribution of sand, silt and clay, will have an important influence on the arsenic concentration because groundwater is transported through them at different rates. Next we incorporated the 1-D reactive transport model into a 2-D model to analyze hydrogeological settings where high and low arsenic groundwaters are expected. Where sands outcrop to the surface high arsenic is rarely found because it becomes quickly flushed from the deposits. Reversely, a fine grained low permeable toplayer superimposing sandy aquifers is a prerequisite to develop shallow high arsenic groundwater. Another setting to develop high arsenic groundwater is below major river channels because here very little vertical flow occurs. High As groundwater is found on river islands and may become preserved as islands incorporate into floodplains. 2-D analysis also shows how sinking river basins with accumulating sediments and migrating river channels yield stacked aquifers with a high heterogeneity in groundwater arsenic. In this case we must also consider drainage of high arsenic groundwater from young reactive aquifers into older lower less reactive aquifers. The toolbox of combined geochemical and hydrological models can be used to recognize hydrogeological/geochemical situations in the field where high arsenic groundwater can be expected, also when detailed data allowing a quantitative analysis is not available.
These floodplains show a large variation in arsenic concentration even over small distances, which is a big problem when setting-up water supplies. The origin of this variability has hitherto not been investigated systematically and a quantitative understanding of this phenomenon is the main objective of the present project. We investigated the variability of groundwater arsenic in the context of geological, geochemical and hydrogeological processes. Our field area, 10x20 km on the Red River floodplain 30 km NW of Hanoi, shows high and variable groundwater arsenic. First we analyzed the geological structure and its evolution over the last 6000 years reconstructing the location of the main channel of the Red River, using remote sensing, geophysics, sediment coring and dating of sediment burial ages using OSL. Results revealed a complex mosaic of older and younger sediments formed by migrating river channels cutting into older deposits. Second we constrained the hydrogeology with groundwater flow velocities, flow patterns and residence times in different parts of the aquifers. Thirdly, we mapped the spatial distribution of the groundwater chemistry and performed laboratory process studies to quantitatively describe the geochemical processes mobilizing arsenic from the sediments and its leaching from the deposits over time. To integrate geology, hydrogeology and geochemistry we used modeling. The geochemical processes were incorporated in a reactive transport model describing changes in water chemistry over 6000 years taking into account changes in the reactivity of sedimentary carbon and iron oxides over time as well other chemical processes. This model was developed on a limited dataset of hydrologically homogeneous data. Subsequently the model was tested on a larger dataset covering the whole field area with much larger hydrological variability. The total number of porevolumes of groundwater flushed through the aquifer since its burial emerged as a suitable parameter combining geochemical and hydrological changes over time. During the first 100 pore volumes flushing through the deposits, groundwater arsenic will be high. Thereafter the arsenic concentration decreases but first after 200 pore volumes sufficient arsenic is leached out of the aquifer to lower the arsenic concentration below the WHO limit. The porevolume concept indicates that the distribution of sand, silt and clay, will have an important influence on the arsenic concentration because groundwater is transported through them at different rates. Next we incorporated the 1-D reactive transport model into a 2-D model to analyze hydrogeological settings where high and low arsenic groundwaters are expected. Where sands outcrop to the surface high arsenic is rarely found because it becomes quickly flushed from the deposits. Reversely, a fine grained low permeable toplayer superimposing sandy aquifers is a prerequisite to develop shallow high arsenic groundwater. Another setting to develop high arsenic groundwater is below major river channels because here very little vertical flow occurs. High As groundwater is found on river islands and may become preserved as islands incorporate into floodplains. 2-D analysis also shows how sinking river basins with accumulating sediments and migrating river channels yield stacked aquifers with a high heterogeneity in groundwater arsenic. In this case we must also consider drainage of high arsenic groundwater from young reactive aquifers into older lower less reactive aquifers. The toolbox of combined geochemical and hydrological models can be used to recognize hydrogeological/geochemical situations in the field where high arsenic groundwater can be expected, also when detailed data allowing a quantitative analysis is not available.