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

Abstract

Physicochemical parameters (Temperature, pH, ORP) of deep groundwater at the depth of 500m were measured at ground surface and the in-situ sampling depth of borehole to understand the redox condition and dominant redox process. The change of the pH value during pumping of the groundwater and chemical composition including dissolved gases were also analyzed to confirm the correction method of changed pH by degassing process. Additionally the sensitivity of various ORP electrodes was studied to understand the appropriate electrode type for the deep groundwater. The results show that the redox potential at the depth was approximately -100mV and was controlled by the reaction of chemical pairs of (Fe²⁺, SO₄^2-)/(FeS₂) or (SO₄^2-, FeCO₃)/(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.