Domestic freshwater supply for the Mekong Delta
Salinity of rivers and canals in the Mekong Delta is increasing due to sea level rise and deepening of the river. This happens in deltas all over the world. In the monsoon climate of the Mekong Basin, this leads to salinity concentrations exceeding thresholds for intake for drinking water supply during the low flow season. This Partners for Water pilot project aims to demonstrate the use of Aquifer Storage and Recovery (ASR) to store water in the ground when there is enough fresh water, and to use it when the salinity in the surface water is too high.
People in the Mekong Delta depend on fresh water from rivers and canals for a majority of their drinking water. Groundwater is used when fresh water is not available. During the period of low flow of the Mekong River, this is happening close to the sea. This saline intrusion is expected to reach further upstream in the future due to sea level rise and deepening of the river.
Impact on groundwater and subsidence
The groundwater heads of the aquifers under the Mekong Delta are declining due to over-abstraction. This increases the risk of soil subsidence. In order to mitigate this, fresh water can be infiltrated in the ground to be abstracted when needed. This technique is call Aquifer Storage and Recovery and can protect the aquifer from over-abstraction.
Partners for Water project
This Partners for Water project aims to implement the first production scale infiltration-abstraction well in the Mekong Delta. Nga Bay has been selected as pilot location, based on the characteristics of both the surface water intake as well as the groundwater. Our partner HAWASUCO produces drinking water here with an intake from the canal. Furthermore, two abstraction wells are present that are used as back-up.
Design
Deltares has created a detailed geohydrological model of the Nga Bay area, nested in an existing model of the whole Delta. The MODFLOW model has been prepared using iMOD-Python. This model has been applied to prepare a conceptual design of the ASR system assessing different infiltration and abstraction scenarios, based on the physical characteristics of the aquifer.
Important output of the modelling exercise include the infiltration and abstraction rates, the mixing of the infiltrated water with the groundwater in the aquifer and the resulting water quality of the abstracted water. Model results show that a production scale ASR well should be possible, with infiltration and abstraction at a depth of 170m. Our partners Vitens Evides International (VEI) brought in Dutch experience of ASR to prepare the technical design of the infiltration well.
Results
The aim of the pilot is to gain knowledge of the application of ASR on a production scale in the Mekong Delta and to monitor data to make an accurate cost-benefit analysis. These results can then be used to upscale ASR as a solution for the whole Mekong Delta.
Current state
The project is on-going. A monitoring well has been drilled, samples of the aquifer have been taken and analysed and its geohydrological characteristics have been investigated. The target aquifer in Nga Bay consists of medium coarse sand, which is very suitable for ASR. The chemical composition of the aquifer also does not seem to pose risks.
However, it is always uncertain how surface water will react with groundwater and the aquifer, resulting for instance in clogging of the screen of the pipe. The infiltration well will be constructed in the coming months, after which infiltration and abstraction can start. Monitoring of the flow of water, its quality, and the use of energy will be carried out to draw lessons for upscaling.
