Measurement of the porewater compositions in compacted bentonite; Results of the additional experiment for spatial variations of the porewater pH

Isogai, Takeshi*; Sasamoto, Hiroshi   ; Shibata, Masahiro 

We have been developping the technique for measuring the chemical composition of porewater using low-decolorant pH test papers and high-absorbancy pads embedded in compacted bentonite. Previously, we reported that the pH of porewater near the infiltration surface slightly decreased with time in compacted bentonite contacted with distilled water. Preliminary thermodynamic calculations have suggested that the porewater pH decrease near the infiltration surface observed in experiments using distilled water may be due to partial oxidation of trace amounts of pyrite in Kunigel V1. Such interpretation has not been verified experimentally by using Kunipia F which does not include pyrite as the accessory minerals in bentonite, however. Additionally, it is also suspected that the material used in the experiments (i.e., ceramic filter) and the experimental condition (i.e., bentonite contacts with the static solution) can affect the variations of porewater pH near the infiltration surface. Thus the following experiments with distilled water were conducted to identify the reason of temporal pH variations near the infiltration surface. (1) Experiments using Kunipia F, (2) Experiments using the alternative filter (i.e., combination of plastic filter with membrane filter), (3) Experiments using the alternative experimental condition (i.e., bentonite contacts with the stirred solution) Results indicate that the decrease of porewater pH near the infiltration surface is observed, although the Kunipia F is used. Difference of variations is not observed by using the alternative filter and experimental condition. Therefore, it would be suggested that the pH decrease near the infiltration surface may not be due to these factors but be due to another factor which affects the porewater pH variations.