Estimation of in situ groundwater chemistry using geochemical modeling: A test case for saline type groundwater in argillaceous rock

Saline type groundwaters data in the Mobara area (a marine based argillaceous rock) located in the well-known “South Kanto gas field” in Japan were investigated by JNC 1 JNC (Japan Nuclear Cycle Development Institute) was merged in October 2005 with the Japan Atomic Energy Research Institute (JAERI) to form the Japan Atomic Energy Agency (JAEA). 1 as part of a natural analogue study. Most groundwaters in the field were extracted from deep gas wells ( e. g., 400–2000 m below the surface), and the all data reported previously were sampled at the wellhead, where physico-chemical parameters ( e. g., temperature, pH, Eh etc.) were also measured. In such cases, particular attention should be paid to whether the measured and/or analyzed results are consistent with the chemical and physical conditions in the in situ geological formation because air contamination, the temperature and pressure changes during sampling can affect the groundwater chemistry. The present study shows a test case to estimate the in situ groundwater chemistry in argillaceous rock of the Mobara area using geochemical model calculations. Results from thermodynamic interpretation of groundwater chemistry using the measured pH and Eh of groundwater sampled at wellhead ( e. g., pH = 7.86, Eh = −50 mV) indicate that the groundwaters are supersaturated with respect to calcite ( e. g., the saturation index; SI is 1.14). Calcite is known to equilibrate relatively quickly with aqueous solutions at low temperatures and this mineral is present in the Otadai formation, however. Therefore the values greater than 0 for SI of calcite may be due to errors in the pH measurement. Also the measured Eh is relatively oxidizing value which may be inconsistent with the in situ geochemical conditions ( e. g., pyrite and siderite coexist, CH 4(g) dominates in the groundwaters). Thus such Eh value may be disturbed by contact of the samples with atmospheric oxygen and other effects like degassing. Errors in the pH measurement might be caused by degassing during sampling of groundwaters. As a test case to estimate the groundwater considering such degassing effect, we first assume that the in situ groundwaters are saturated with respect to calcite. A back-titration geochemical model is then used to simulate the addition of CO 2(g). Regarding the redox conditions of groundwater, we also assume that pyrite–siderite equilibrium controls the Eh of Mobara groundwaters considering the mineralogy identified in the Otadai forma