With the growing population and the adverse effects of climate change, the pressure on coastal aquifers is increasing, leading to a larger risk of saltwater intrusion (SI). SI is often complex and difficult to characterize from well data only. In this context, electrical resistivity tomography (ERT) can provide high-resolution qualitative information on the lateral and vertical distribution of salinity. However, the quantitative interpretation of ERT remains difficult because of the uncertainty of petrophysical relationships, the limitations of inversion, and the heterogeneity of aquifers. In this contribution, we propose a methodology for the semiquantitative interpretation of ERT when colocated well data are not available. We first use existing wells to identify freshwater zones and characterize the resistivity response of clayey deposits. Then, we approximate the formation factor from water samples collected in the vicinity of ERT data to derive a resistivity threshold to interpret the saline boundary. We applied the methodology in the shallow aquifers of the Luy River in the Binh Thuan province, Vietnam, where water resources are under pressure due to agricultural, aquacultural, and industrial production. Twenty-one ERT profiles were collected and revealed a much larger intrusion zone, compared to the previous study. Saltwater is present in lowland areas of the left bank over almost the whole thickness of the aquifer, while the right bank is constituted of sand dunes that are filled with freshwater. At a larger distance from the sea, a complex distribution between fresh and saltwater is observed. Our methodology could be applied to other heterogeneous aquifers in the absence of a dense monitoring network.

The Vietnamese Binh Thuan province located along the Southern Central part of the coast is one of the driest in the country. The population is relying largely on groundwater for irrigation, and the groundwater resources are threatened by climate change and saltwater intrusion in river estuaries. Recent studies, however, reveal that the extension of saltwater intrusions was larger than expected, raising some doubts on the actual origin of the intrusions. In this study, we use the geochemical characterization of groundwater samples collected in both the dry and rainy seasons in the Luy river coastal area to identify hydrochemical processes responsible for the salinization and variations taking place in the shallow aquifers. The distribution of chemical components of groundwater and the indicators presenting the freshening and salinization processes, such as cation exchange code and chloride conservative element, the ionic delta, the HFE-diagram, and geochemical modelling, were studied. The results show that 65% of the samples exceed the WHO limit for drinking water and 100% of them have a degree of restriction on use from slight–moderate to severe in potential irrigation problems according to FAO. In contrast to previous expectations, freshening is the dominant process in the aquifers, and it is more advanced in the rainy season. Due to a lack of recharge and aquifer exploitation, salinization is more severe in the deeper aquifer and during the dry season. Saltwater intrusion is not limited to the zone close to the river but extends further inland and at depth, and salinity can vary quickly over short distances. Based on these new insights, we develop a new conceptual model for the evolution of salinization in the Luy river catchment involving a natural freshening process of connate water combined with anthropic influence. The conceptual model will form the basis for the development of a groundwater model of the study area and eventually lead to sustainable management scenarios for this coastal region, preventing further deterioration of the groundwater resources.

KEYWORDS: Saltwater intrusion, Geochemistry, Groundwater extraction   With an average annual rainfall of 800-1150 mm/year, the Binh Thuan province is one of the driest places in Vietnam. The quantity and quality of groundwater play a significant role in the agriculture, aquaculture development and daily life of the local communities. In 2012, the national center for water resources (Nawapi, 2012) delineated the seawater intrusion extent in Binh Thuan based on the total dissolved solids (TDS) content of water samples taken from shallow boreholes. The threshold of 3g/L and 1.5g/L were exceeded in the estuaries of the Luy, Long Song and Ca Ty rivers. In recent years, the prolonged droughts combined with the sea level rise and the over-extraction of groundwater during the dry season increased dramatically the seawater intrusion process especially in the estuaries of the province.The geochemistry of groundwater in the Luy River catchment was studied to investigate the contamination of the aquifers and identify the processes taking place. From 1991 to 2015, 98 water samples had been taken from the wells in the area in both dry and rainy seasons. 71% of the water samples were fresh while 21% and 5% were lightly saline and moderately saline respectively. In summer 2020, 110 new water samples from both shallow and deep wells were collected in the Luy river catchment in wells from 3m to 40m. The TDS values are ranging from 105 to 23080 mg/L and can be classified into 4 groups: freshwater (48%), slightly saline (40%), moderately saline (8%) and very saline (4%). The samples show that the seawater intrusion expands not only horizontally at shallow depth along the river but also deeper down the aquifer in most of the study area, what is also confirmed by geophysical data. Freshwater samples were mostly collected at a depth lower than 10m. The chemical composition of water samples were analyzed showing evidence of seawater intrusion, but also the occurrence of freshening processes within the study area. Together with the presence of saltwater at larger depths, this points towards a situation more complex than previously thought. Saltwater intrusions are likely not only related to interaction with the river estuary, but also to the presence of fossil saltwater in the aquifer, and to groundwater pumping and irrigation practices.REFERENCESNAWAPI. 2012. Hydrogeological mapping at scale 1:50000 in Ninh Thuan and Binh Thuan provinces. In Vietnamese.  *Corresponding Author. Email: Linh.PhamDieu@UGent.be