Seawater Intrusion Assessment based on Hydrochemical Data in Gapura Sub-district of Sumenep Regency, East Java, Indonesia

Gapura is a sub-district with a large salt pond in Sumenep Regency, Madura Island, Indonesia. In this area, many residents' houses have drilled wells to meet their water needs. One factor is that Municipal Waterwork (PDAM) does not reach the area due to limited water sources and difficult accessibility. Residents in the area claimed to have found some groundwater that tasted brackish. This study aims to apply hydrochemical analysis to identify the possibility of seawater intrusion in the area. The assessment of seawater intrusion uses the total dissolved solids (TDS) value and major ion of groundwater samples. In the analysis of thirty samples, two samples have different values from the samples of freshwater. However, the value of these two samples is relatively low to be classified as seawater intrusion. It is described on the TDS values that belong to the class of brackish water, from Simpson ratio values classified as slightly contaminated, and from the piper diagram plot that is still in the mixing zone in the middle of the diagram. The presence of brackish water in these samples can be caused by a location close to salt pond activities.