Determining groundwater facies and water quality index in Tanah Bumbu Regency/South Borneo Indonesia

Most people in Tanah Bumbu Regency use water from dug wells for their daily activities. Kotabaru Regency is located in Pagatan Groundwater Basin, which is located in three administrative boundaries, namely Tanah Laut Regency, Tanah Bumbu Regency, The purposes of this study are to determine the type of groundwater facies and Water Quality Index (WQI). The methods were hydrogeological mapping and hydrogeochemical analysis of groundwater samples to evaluate groundwater quality. There were 170 groundwater samples mapped in the study area. The study area has differences in groundwater quality in both confined aquifer and unconfined aquifer. The unconfined aquifer has two types of groundwater facies, namely Ca-Mg-HCO3 (Calcium Magnesium Bicarbonate) and Ca-Cl (Calcium Chloride). In comparison, the confined aquifer has two types, which are Ca-Mg-HCO3- (Calcium Magnesium Bicarbonate) and Na-HCO3- (Sodium Bicarbonate). Based on the calculation of the Water Quality Index (WQI) of unconfined aquifer samples show that 70% of groundwater samples have poor quality. Meanwhile, the confined aquifer samples indicate that 85% of groundwater samples have good to excellent, which is suitable for drinking water