The Stable Isotopes Approach as Tracers to Investigate the Origin of Groundwater In The Unconfined Aquifer of Denpasar, Bali

Groundwater plays an elemental role in the supply of water for numerous purposes, so the use of groundwater must also pay attention to the balance and preservation of the resource itself. Knowing the origin of the groundwater and the recharge area on an aquifer is essential to determine the conservation model. The purpose of this research is to determine the Denpasar Meteoric Water Line (DMWL), the recharge area, and the origin of groundwater in the unconfined aquifer of Denpasar. The determination of the number and location of samples, both rainwater samples, and groundwater samples, using a purposive sampling approach. A total of 5 samples of rainwater and 18 samples of groundwater were taken at several locations in the Denpasar area. The stable isotopes of δ18O and δ2H were carried out using laser spectrometry Picarro L2130-i. This research shows the Denpasar LMWL is δD = 8.5839 x δ18O + 19.876; from the distribution of oxygen-18 isotope, the low 18O value is dominated in the North and Northeast areas, so here this area can be delineated as the potential recharge area; and based on the isotope ratio, groundwater in the unconfined aquifer of Denpasar can be divided into three geneses groups: (a) Groundwater Genesis-A and B groups represent groundwater systems that come from direct rainfall as well as local recharge with low (depleted) to moderate δ18O and δ2H isotope ratios and groundwater groups Genesis-C, which has high (enriched) δ18O and δ2H isotope ratios, is located below the slope of the DMWL.