Determining sea water intrusion in shallow aquifer using Chloride Bicarbonate Ratio Method

Increasing usage of ground water tends to cause sea water intrusion. Sea water intrusion often happens in coastal areas. This study was conducted to determine the spread, levels of intrusion and factors that affect sea water intrusion in the research area. The sampling technique was carried out using interval random sampling, where 30 samples were taken with a distance of 2 km each, and then chemical analysis was carried out including the parameters of Cl⁻, HCO₃ ⁻, CO₃ ⁼, and electrical conductivity (EC). The method used to determine the level of seawater intrusion was the method of Chloride Bicarbonate Ratio and then the types of water were classified based on chloride content and electrical conductivity. The determination of the distribution of seawater intrusion was done by mapping analysis using the ArcGis application program. To determine the factors that affected sea water intrusion, multivariate regression analysis with the "backward elimination" method was used. The results showed that on several sample points the shallow aquifer had been intruded by sea water with a ratio of 1.02 to 102.16 which were included in the category of intrusion of slightly to high. Chloride concentrations ranged from 2 mg/L to 31,160 mg/L and were classified as fresh water to salt water. Electrical conductivity ranged from 186 μmhos/cm to 26,400 μmhos/cm and generally above 200 μmhos/cm which meant that the sea water had been disturbed. Factors that significantly affected sea water intrusion were water usage debit and aquifer permeability with an adjusted coefficient R² of 0.797.