The fallout radionuclide (FRN) analysis needs a reference site (RS) inventory to determine erosion and sedimentation in the study area. The investigated area is in the upstream Citarum watershed, West Java, Indonesia. Twenty-seven corings and 22 scrap samples have been prepared well and measured using HPGe gamma spectroscopy. The data below the minimum detectable activity (MDA) was found for 137Cs in RS6 cor 4 and 7 (<0.16 ± 0.08 Bq kg-1). MDA quantification implies that the inventory below MDA eroded greater than its maximum value (76.02 tons ha-1 a-1). The comparison 137Cs inventory in this study is lower than the three estimation models; however, the inventory of Mt. Papandayan is closer to the model. This study found the depth percentage of 20-30 cm using the proportion of 0-20 cm/0-30 cm ratio and predicted the portion of the existence of 137Cs and 210Pbex in the 20-30 cm in the bulk sample. The highest H0 (142.04 kg m-2), the relaxation length λ, and proportion of 20% of 137Cs in 20-30 cm depth imply that 137Cs inventory activity is possibly deeper than 30 cm. This study recommends that Mt. Papandayan could be the alternative RS for the upstream Citarum watershed.
Keywords: Citarum watershed; FRN; MDA level; West Java; cesium-137; gamma spectroscopy; lead-210; natural radioactivity; reference site.