Présentations Colloques

Oral Presentation
Session 8.10: Recent developments in groundwater modeling and mathematical tools in Hydrogeology
Kazmierczak Jolanta
Modeling of the geological evolution of the Red River delta, Vietnam in the context of arsenic contamination of aquifers
Contamination of aquifers by geogenic arsenic is a common problem in south-east Asia. Arsenic concentration in groundwater is controlled by the sediment age and is lower in the older deposits due to the decrease over time of the reactivity of organic matter and arsenic-bearing iron hydroxides. Development of detailed geological models of floodplains is crucial for groundwater resources management in terms of delineating areas with low arsenic concentration.**A geological model of the Red River delta was set up by combining the borehole data and 2D resistivity profiles with the results of the analysis of the satellite images. The interpreted stages of the river evolution during Holocene were validated against the sediment dating and compared with the arsenic distribution in the aquifers.**Geological and geophysical data interpreted with the use of geostatistical methods allow for a description of regional geological conditions, but rarely tell anything about the order of the river avulsions and palaeo river courses. Analysis of the satellite images provides information about palaeo fluvial structures and movement of the river, but cannot tell anything about subsurface geology. Only an approach integrating these two methods allowed to model the geological complexity of the studied fluvial system and interpret its changes over time.**The Red River migrated throughout Holocene from the mountain range bordering the floodplain at south-west towards north-east. Three major channel belts can be distinguished. The sediments are dated from 5900 to 460 years. Meandering of the river within single channel belt took place for up to 2000 years, while the abandoned meanders were filled up with the sediments within less than 100 years. The major avulsions of the Red River are related to the sea level changes, increased sediment aggradation due to the development of the agriculture and construction of the dikes. The arsenic concentrations increase from 1 Imol L in the oldest channel belt to 7 Imol L in the aquifers sands deposited by the most recent meanders of the Red River. **