Preliminary Assessment of Anomalously High Background Radioactivity in Makinit Hot Spring, El Nido, Philippines

Abstract

Hot springs in the Philippines are found mostly near volcanic regions and are the least places with possibly high background radiation. However, Makinit hot spring in El Nido, Palawan exists not only on the tectonically stable northern Palawan Microcontinental Block but also records anomalously high natural radioactivity. To quantify and determine the nature of radioactivity, this study conducted a preliminary radiological and geochemical study in the vicinity of this hot spring. The measured average specific activities of uranium and thorium were 1528.62 and 920.96 Bq/kg, and these exceed world median values on terrestrial radionuclides in the soil of 35 and 30 Bq/kg by 44 and 31 times higher, respectively. The average values of all seven calculated radiological risk parameters – namely, dose rate with an amount of 13.77 mSv/y, radium equivalent of 2862.10 Bq/kg, external and internal hazard indices of 7.73 and 11.86, respectively, absorbed gamma dose rate of 1271.42 nGy/h, annual effective dose equivalent of 1.56 mSv/y, and excess lifetime cancer risk of 5.46 x 10–3 – go beyond reference limits set by the UNSCEAR. The preliminary assessment of spring water physicochemical properties showed that the water temperature is greater than 55 °C and the pH averages around 6.62, indicating weak acid-neutral hot waters and a low to moderate temperature geothermal system. The sediments surrounding the Makinit hot spring were analyzed to be travertine, composed only of calcite, suggesting that the uranium and thorium ions are potentially bonded or are associated with the Ca-bearing mineral. This study can serve as a baseline for further research regarding the source of radioactivity and geothermal heat source and as a consideration for the development of the Makinit hot spring area.