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

Abstract

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 formation. The assumed equilibrium between pyrite and siderite implies a fixed value of P H 2 S at a given temperature and pH. A back-titration of trace levels H 2S(g) is also applied to estimate the possible effect of the in situ Eh and pH of groundwaters. The calculated result shows that pH is about 6.7 and Eh is about −190 mV, respectively. The estimated pH value for the in situ groundwater is about 1 unit lower than the measured pH value at the surface and the in situ redox potential is significantly lower than Eh value measured in surface sample. Based on a preliminary assessment of mineral-water equilibria using the mineral stability relations in the CaO–MgO–Al 2O 3–SiO 2–H 2O system, the estimated in situ groundwater composition is more consistent with the mineralogy of the Otadai formation than the measured groundwater composition. However, further consideration ( e. g., detail mineralogical investigation, reliable and consistent thermodynamic data with site mineralogy) would be needed to check the reliability of estimation technique.