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ABSTRAK Fenomena intrusi air laut di Cekungan Air Tanah (CAT) Jakarta sampai saat ini masih menjadi perdebatan baik dari segi skala, mekanisme, maupun ada atau tidaknya fenomena tersebut. Tulisan ini mengelaborasi karakter hidrokimia dan hidroisotop ( 18 O dan 2 H), serta hidrograf pisometrik untuk mengidentifikasi fenomena intrusi air laut pada CAT Jakarta. Hasil analisis menunjukkan air tanah yang bersifat asin berasal dari sistem akuifer tertekan atas dengan jarak kurang dari 3 km dari pantai, sedangkan air tanah yang bersifat sedikit asin berasal baik dari akuifer tertekan atas maupun akuifer tertekan tengah dengan jarak kurang dari 9 km dari pantai. Air tanah yang terindikasi intrusi air laut dengan TDS > 1000 mg/L, Na/Cl < 1, dan Cl/HCO3> 0,55 ditunjukkan oleh conto dari daerah Kapuk, Tongkol, dan Sunter untuk akuifer tertekan atas, serta daerah Daan Mogot, Cakung, dan Tongkol untuk akuifer tertekan tengah. Hanya di daerah Kapuk proses intrusi air laut tersebut berhubungan dengan penurunan pisometrik yang masih berlangsung hingga saat ini. Secara kuantitatif, intrusi air laut di daerah ini telah mencapai tingkat pencampuran sekitar 11 -21 % berdasarkan komposisi isotop 18 O dan 2 H.Kata kunci: hidrokimia, hidroisotop, intrusi air laut, Jakarta. ABSTRACT The seawater intrusion phenomenon in the Jakarta

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H and 18 O isotopeThis paper presents an overview of the geothermal systems on the island of Bali, Indonesia. Physicochemical data of hot springs and shallow geothermal wells were collected from four geothermal locations: Penebel, Batur, Banjar and Banyuwedang. The concentrations for the three main anions varied significantly indicating a different geothermal history. The values for Cl − ranged from 0.1 to 1000 mg/L, for HCO 3 − from 20 to 2200 mg/L and for SO 4 2− from 0.1 to 500 mg/L. Although the island of Bali is underlain by carbonate rocks, a carbonate host rock for the geothermal reservoirs could not be confirmed, because the (Ca 2+ + Mg 2+ )/HCO 3 − molar ratios were approximately 0.4, well below 1.0 and the K/Mg ratios were approaching those of a calc-alkaline rock reservoir. The HCO 3 − of the thermal waters correlated with Ca 2+ , Mg 2+ , Sr 2+ and K + indicating water-rock interaction in the presence of carbonic acid. Phase separation was inferred for the Bedugul and Banjar geothermal systems, because of relatively high B/Cl ratios. Boron isotopes were determined for selected samples with values ranging from δ 11 B of 1.3 to 22.5‰ (NBS 951). The heavy δ 11 B of +22.5‰ together with a low B/Cl ratio indicated seawater input in the Banyuwedang geothermal system. The hydrogen and oxygen isotopic composition of the thermal water plotted along the global meteoric water line (GMWL) and close to the mean annual value for precipitation in Jakarta indicating a meteoric origin of the geothermal water. Comparison of the Si, Na/K, Na/K/Ca and Na/Li geothermometers with actual reservoir temperature measurements and physicochemical considerations led to the conclusion that the Na/Li thermometer provided most reliable results for the determination of geothermal reservoir temperatures on Bali. Using this thermometer, the following reservoir temperatures were calculated:(1) Penebel (Bedugul) from 235 to 254°C, (2) Batur 240°C and (3) Banjar 255°C. Due to seawater input this thermometer could not be applied to the Banyuwedang geothermal system. There application of a SiO 2 thermometer indicated a reservoir temperature below 100°C.