Study of fresh-saline water interface in correlation with groundwater hydrochemistry in coastal area of cilacap, Indonesia

Interface is a contact zone between saline water from the sea and fresh groundwater in the coastal area. It is a dynamic zone influenced by the rate of groundwater extraction, seasons, and tides. This study aims to (1) analyze the dynamic depth of interface, (2) identify the chemical types of groundwater, and (3) investigate the relationship between the two. The depth of interface was measured using geoelectric sounding with a Schlumberger Array, while the groundwater's chemical type was determined based on the Stuyfzand's classification. Also, their relationship was analyzed through descriptive quantitative approach. The results showed that over time, the depth of interface in several places in Cilacap City had changed. In 1996, the interface had spread to areas where it was not detected before in 1977. Nevertheless, there were also some sites where the interface was located in 1977 and 1996 but no longer identified in 2013. Distance to shoreline determines the depth of the interface, that is, the closer the area to the shore, the shallower the depth of the interface will be. The chemical type of the groundwater is F/2/Ca²⁺/HCO₃/+, which indicates that the groundwater in the public wells remains unaffected by the dynamic of the interface zone. © 2020.