Karakteristik Hidrogeokimia dan Genesa Air Tanah di Kabupaten Nganjuk dan Sekitarnya, Provinsi Jawa Timur, Indonesia - Rachmansyah, Arief; Aprilia, Faridha; Kusnaeni, Lili Retno (Fakultas Geografi, Universitas Gadjah Mada, 2026)
Abstrak. Salah satu dampak penting dari perubahan iklim adalah pengelolaan air permukaan menjadi semakin sulit akibat pola hujan yang berubah, sehingga banyak pihak memanfaatkan air tanah sebagai sumber air baku. Selain kuantitas, pemanfaatan air tanah juga tergantung pada kualitasnya yang ditentukan oleh sejarah pembentukannya, jenis batuan yang dilewati, serta pencemaran. Di Kabupaten Nganjuk Provinsi Jawa Timur yang menjadi salah satu lumbung padi nasional banyak petani memanfaatkan air tanah untuk menutup kekurangan air irigasi, namun di beberapa tempat mengalami penggaraman. Penyelidikan dilakukan pada 61 air sumur dan mata air untuk mengetahui sifat fisik dan kimia, selanjutnya dianalisis sebaran dan karakteristiknya, serta membuat model konseptualnya. Makalah ini menyajikan hasil studi karakterisasi geokimia, dan asal usul proses penggaraman air tanah. Berdasarkan karakteristik geokimianya air tanah di daerah penelitian dapat diklasifikasikan menjadi 5 fasies, yaitu fasies kalsium-magnesium-klorida (Ca2+-Mg2+-HCO32-), percampuran kalsium-natrium bikarbonat (Ca2+-Na+- HCO32-), percampuran kalsium-magnesium-klorida (Ca2+-Mg2+-Cl-), kalsium-magnesium-klorida-sulfat (Ca2+-Mg2+-Cl–SO42-), dan alkali-klorida sulfat (Na+-K+-Cl–SO42-). Hasil kajian menunjukkan, bahwa air tanah payau hanya dijumpai di bagian utara daerah penelitian yang masuk dalam Zona Perbukitan Kendeng, khususnya di Formasi Kalibeng yang merupakan sedimen asal laut. Dengan demikian penggaraman air tanah di utara daerah penelitain bukan karena intrusi air laut.Abstract. One of the significant impacts of climate change is the surface water management is becoming increasingly difficult due to changing rainfall patterns, making groundwater the primary choice for raw water. In addition to quantity, groundwater utilization also depends on its quality, which is determined by its genesis, the type of rocks it passes through, and pollution. In Nganjuk Regency, East Java Province, which is one of the nation's rice granaries, many farmers use groundwater for irrigation to cover raw water shortages. In this regency and its surrounding areas, several water sources have been salinized. An investigation was conducted by taking 61 well and spring water samples to determine their cation and anion content, then analyzing their distribution and characteristics. This paper presents the results of a groundwater characterization study and the origins of the groundwater salinization process. Based on groundwater geochemical characteristics, five groundwater facies have been identified within the study area: calcium–magnesium–bicarbonate (Ca²⁺–Mg²⁺–HCO₃⁻), Mixed calcium–sodium–bicarbonate (Ca²⁺–Na⁺–HCO₃⁻), Mixed calcium–magnesium–chloride (Ca²⁺–Mg²⁺–Cl⁻), Calcium–magnesium–chloride–sulfate (Ca²⁺–Mg²⁺–Cl⁻–SO₄²⁻), and Alkali–chloride–sulfate (Na⁺–K⁺–Cl⁻–SO₄²⁻). These facies reflect the diversity of hydrogeochemical processes and lithological influences present across the study area. The results of the study also indicate that brackish groundwater is found exclusively in the northern part of the study area, specifically within the Kendeng Hills Zone, and is primarily associated with the Kalibeng Formation which consists of marine-origin sedimentary rocks.Submitted: 2025-08-14 Revisions: 2025-10-24 Accepted: 2026-02-01 Published: 2026-02-05