Genesis of an economic relevant fresh groundwater lens in Pleistocene/ Neogene aquifers in Nam Dinh province (Red River Delta, Vietnam).

Frank Wagner1, Rüdiger Ludwig1, Hoan Hoang2, Ursula Noell1, Quy Nhan Pham3, Flemming Larsen4

P 10.2 in Wasser und Umwelt in Südostasien

In Southern Red River Delta (Nam Dinh Province, Vietnam), a local lens of low saline pore water and high quality has been identified in unconsolidated Pleistocene (P) and Neogene (N) aquifers, which are regionally known to contain brackish and saline pore waters. Since the 1990ies, ongoing overexploitation of the fresh groundwater results in decreasing GW heads up to 0.6 m/a and in development of a regional abstraction cone. The presented study from 2012 has been carried out in frame of a Vietnamese-German technical cooperation project and focus on distribution and genesis of fresh and saline pore waters in frame of the regional hydrogeological context.

Observations of the geological structure and groundwater dynamics combined with hydrochemical and isotopic studies suggest adjacent Triassic hard rock aquifers as the major source for the local occurrence of fresh P and N groundwater. Salinization status in the economically most relevant P aquifer has been studied based on hydrochemical and geophysical data. Own hydrochemical field studies as well as laboratory measurements of the specific resistivity of dry sediment samples tresults in the translation of induction logging data from existing monitoring wells into vertical pore water salinity profiles. This approach suggests the regional occurrence of saline pore water in shallow Holocene sediments in the working area, as observed by pore water studies in Hoang et al. (2010).

Interpretation of induction logging and stable isotope data suggest vertical diffusion of saline pore water in shallow Holocene sediments as a source for high saline pore water in deeper aquifers. Analytical diffusion modeling for a period of 3000 years confirms that vertical diffusion of Holocene paleo-sea water can explain saline pore water in P and N aquifers in a stagnant environment. Constant influx of fresh groundwater from adjacent Triassic hard rocks results in flushing saline water which infiltrates from marine Holocene sediments. 

14C groundwater age dating suggests increasing groundwater ages from fresh to saline pore water in P and N up to 14 ka, presuming that contamination with dead carbon is neglectable. Highest 14C ages of low saline water has been observed in the center of the exploited fresh water lens reaching up to 10 ka. Due to overexploitation, the natural coastward directed groundwater flow has turned towards the centre of the abstraction cone with horizontal apparent velocities of up to 0.6 m/a. Thus, brackish and higher saline groundwater from the Red River area and offshore migrates towards the fresh water lens.

Thus, more sustainable exploitation strategies are urgently needed in order to reduce overexploitation of limited and valuable fresh groundwater resources in Nam Dinh Province. However, implementation of mitigation measurements by local authorities is restricted to intensified groundwater monitoring activities.  

HOANG, H., PHAM, Q. N., LARSEN, F., TRAN, L. V., WAGNER, F., CHRISTIANSEN, A.V. (2010): Processes Controlling High Saline Groundwater in the Nam Dinh Province, Vietnam. 2nd Asia-Pacific Coastal Aquifer Management Meeting (ACAMM), October 18-21, 2011, Jeju Island, Korea