ASSESSMENT OF SEAWATER INTRUSION IN THE COASTAL PLAIN AQUIFERS OF YANG-DAI RIVER USING DC-RESISTIVITY METHODS

The distinct electrical difference between seawater and fresh water makes the Direct Current (DC) resistivity method useful for detecting and monitoring seawater intrusion (SWI). The aquifers invaded by seawater in different degrees exhibit significant resistivity difference, which could be classified into 3 groups: serious SWI (<5 Ω∙m), mild SWI (5~13 Ω∙m), and non-SWI (13~150 Ω∙m). Combined with the hydrogeological borehole data, the DC resistivity method can be used to identify the seawater intrusion affected areas after deleting the low resistance interference caused by clay layers. The results show that the seawater intrusion process in the Yang-Dai River plain has significant seasonal effects. It is relatively serious in June, the irrigation season and alleviated in March and November. The coastal shallow subsurface saltwater (brackish water) area in the study area is mainly distributed on the two sides of the river and gradually expanded into the surrounding irrigation area. Mixing of fresh and saline water from SWI and irrigation return flow is responsible for the saltwater sources. Near the coastline, sea farming could be the additive source for the saltwater input. The existing measures have played a certain role in slowing down the process of seawater intrusion, but it is still necessary to strengthen the measures to prevent and control seawater intrusion.