Abstract

The Luy River’s catchment is characterized by different types of sediment on the two sides of the river. Particularly, sandy dunes composed mainly from sands and some grits, were formed in the Pleistocene. Contrarily, the low-elevation flat plains composed by intercalated clayey sand and thin clay layers, were formed in the Pleistocene and Holocene. Our research results indicate that saltwater zones are scattered, with large variations in salinity spatially. Freshwater-bearing sandy dunes constitute a barrier against saltwater intrusion from sea. Saltwater intrusion in the whole catchment cannot be explained solely by interaction with sea and rivers, but also the result of historical transgressions, during which saline water filled in the sediment pores. Freshening is the dominant hydrochemical process in the aquifers. Saltwater trapped inside clay aquitards and lower aquifers slows down freshening processes and act as a secondary source of salinity when freshwater at shallow depth is extracted for irrigation. Groundwater simulation results indicate that over the next 50 years, saltwater will still be present, even without any pumping. If abstraction continues in the alluvial plains, Saltwater intrusion will reach about 5 km from the river mouth, and salinization will increase in shallow aquifers by upconing and lateral migration.

Therefore, we recommend a combination of both structural and non-structural solutions to prevent and mitigate saltwater intrusion as follows: 1) Solutions to change crop structure, adjust irrigation measures according to actual crop needs, increase vegetation, as well as solutions of alternative surface-water storage and use should be encouraged; 2) Groundwater exploitation in the alluvial plain on the left bank of the Luy River should be banned or at least strictly managed by local authorities; 3) Groundwater exploitation should only be carried out in the sand dune systems on the right bank of the Luy River, with pumping rates controlled according to estimated seasonal recharge; and 4) A managed artificial recharge system can be implemented in the sand dunes to replenish fresh water as well as prevent salt water intrusion.