Research on the validation of As determination methods on wellbore water by using atomic absorption spectrophotometer at PT. Geoservices Balikpapan has been calculated. The validation method used is APHA-3114 C, with research stages such as linearity, detection limit, quantitative limit, accuracy, precision. The results showed that the linearity test had good result with R2 ≥ 0.9997. The detection limit value was 0,000038 µg/Land the quantization limit value was 0,00129 µg/L. The %KV value obtained is smaller than the 2/3 KVHorwitz value, indicating that this method has a good precision. The determination of As in drilled well water samples is 0.9167 µg/L, then below the standard threshold of clean water quality quality based on Permenkes. 416/Menkes/PER/IX/1990. Based on the results of the research, APHA-3114 B/C method can be used to determine As in drilled well water samples. Keywords: Validation method, arsenic, wellbore water, APHA-3114B/C, AAS-VHGA.