The Ijen volcano has Pleistocene age (294.00 ± 0.03 Ma), and this stratovolcano was very acidic crater water with a pH of 0-1. The acidic crater water seeps into the Banyupait river flow. Asembagus is a research area located on the northern slope of the Ijen volcano, and the Banyupait River drains this. The acidic river water flows from the Ijen Crater Lake, so the pH of the water river was very acidic. This study used several different analytic methodologies with some previous

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... s, namely using the method of geological mapping, pH measurement, spectrophotometry, IRMS (Isotope Ratio Mass Spectrometer), and the technique of Induced Coupled Mass Spectrometry (ICP-MS). Besides, the petrographic analysis is used to determine the composition of rock minerals due to rocks interacting with acidic water. Banyupait River water in the Asembagus area has a pH of river water around 3-7.3, SO4 (220-683 ppm), and the type of water is meteoric water. Also, concentrations of Ca, K, Mg in the west Banyupait river irrigation water flow showed higher levels when compared to the eastern Banyupait River water flow. Likewise, REE elements from the Asembagus region showed lower concentrations compared to Ijen Crater water. This change in the level of chemical elements is caused by the acidity of the Banyupait River being diluted or mixed with water from other water. However, the spring was not affected by acidic water. The process of acidic water interaction with rocks can also be observed from rocks traversed by the Banyupait River flow. Chalcedony and hematite replace the primary minerals of basaltic rocks. This research is expected to improve the quality of water needed by the Asembagus community so that people can live healthily.