The hydrogeochemistry of argillaceous rock formations at the Horonobe URL site, Japan

• Published in: Physics and chemistry of the earth. Parts A/B/C Vol. 32; no. 1; pp. 170 - 180
• Main Authors: Hama, K., Kunimaru, T., Metcalfe, R., Martin, A.J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2007
• Subjects: Diagenesis; Horonobe URL; Hydrochemistry; Saline water; INW, Japan, Hokkaido, Wakkanai; INW, Japan; Horonobe URL; Diagenesis; Hydrochemistry; Saline water
• ISSN: 1474-7065; 1873-5193
• DOI: 10.1016/j.pce.2005.12.008

A hydrogeochemical investigation is being carried out as part of the Horonobe underground research laboratory (URL) project in Japan. The main aims are to: (1) investigate an actual example of a geological environment in a sedimentary rock formation in Japan; and (2) to confirm the reliability of generic technologies that may in future be applied during the geological disposal of high level radioactive waste. The main rock formations being characterized are the marine Wakkanai and Koetoi Formations (Miocene to Pliocene), consisting dominantly of siliceous shales (porcelanites) and diatomaceous shales respectively. These formations are located within the Tempoku Basin, within a back-arc tectonic setting. Rock sequences of this kind occur widely in Japan and throughout the northern Pacific region. However, prior to the present study, there was relatively little information concerning the processes controlling
in situ chemical conditions and groundwater flow in such settings. Chemical data was obtained for both pumped waters and squeezed porewaters in order to characterize the hydrogeochemistry of these argillaceous rock formations. The
in situ chemical conditions, residence time of the groundwaters and the evolution processes of the groundwaters were investigated. Generally, at each locality studied, shallower groundwaters are fresh and have Na–HCO
3 dominated chemistry. Deeper groundwaters are saline (TDS up to about 22,000
mg/l) and have Na–Cl dominated chemistry. However, lateral gradients in salinity are also recognized, with salinity contours in the Na–Cl dominated saline water (having TDS
>
10,000
mg/l) probably varying in elevation by at least 250
m. Further investigations are required to confirm the origins of the groundwater salinity, but the Na–Cl dominated groundwater chemistry is provisionally explained as a consequence of the dilution of fossil seawater, accompanied by diagenetic water–rock reactions. The vertical and lateral salinity gradients can potentially be used to test the validity of coupled groundwater flow models. A conceptual model is tentatively suggested in which the spatial distribution and frequency of fractures helps to control the spatial distribution of groundwater salinity. Future investigations will clarify the timing of flow, the flow directions and the characteristics of the flow paths.