A Notice for the measuring of physicochemical parameters (pH, ORP) of deep groundwater

Iwatsuki, Teruki  ; Morikawa, Keita*; Hosoya, Shinichi*; Yoshikawa, Hideki

Physicochemical parameters of deep groundwater at the depth of 500 m were measured at ground surface and the in-situ sampling depth of borehole to understand the redox condition and dominant redox process. The results show that the redox potential at the depth was approximately -100 mV and was controlled by the reaction of chemical pairs of (Fe, SO)/(FeS) or (SO, FeCO)/(FeS). The pH value at ground surface changed approximately 0.4 from that at the in-situ depth by degassing of dissolved carbon dioxide. The time to reach the stable ORP value in monitoring depends on the shape and surface area of Pt electrode. The columnar-shape Pt electrode is more sensitive for deep groundwater rather than a dot-shape Pt electrode. It is important to correct the pH measured at ground surface for the analysis of in-situ redox reaction process.